DIYstompboxes.com

DIY Stompboxes => Building your own stompbox => Topic started by: Paul Marossy on January 03, 2012, 01:30:09 PM

Title: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 03, 2012, 01:30:09 PM
Not technically a stompbox thing, but I need the amp to use my stompboxes!  :icon_wink:

I have a Seymour Duncan Convertible amp which has a pair of Mallory "Type CGS" 200uF 350V filter caps that are in need of replacement. These are big can type caps which are about 1-1/2" in diameter and about 3-1/2" long, with screw terminal connectors. It looks just like what is shown here: http://www.ebay.com/itm/MALLORY-CAPACITOR-TYPE-CGS-1200MFD-100VDC-/260303549551?pt=LH_DefaultDomain_0&hash=item3c9b4cf46f - these are the "main filter caps", and are the first ones right off of the tube rectifier so I assume they are the ones that "work the hardest".

I thought I had it fixed the other day, I took the whole amp apart and saw that the mechanical connections to these particular caps were corroded and apparently not making a good contact. I cleaned those up and it was working and sounding great. Used it for about four hours on Sunday, no problems. Fired it up yesterday it it was making an ugly static noise whenever I played anything under an open A in terms of frequency. Then it was "motorboating" as soon as I turned it on. I ruled out the modules as being the problem.

Do you know of a source for replacements or have any recommendations on what I could use in their place?

Any help would be much appreciated!

Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 03, 2012, 01:39:24 PM
Find the physical dimensions of the capacitors. Particularly the diameter, the length, and the distance between the screw holes. Also note how big the diameter and length could be and still fit in the amp.

The dimensions of "computer grade" caps (that's the nominal name for these typed) are highly standardized. Once you know the terminal hole spacing from center to center, you can look in any cap supplier and find on that will screw in to the same holes. The body length and diameter tell you what physical size you can stand. Then get on the net and look for caps that meet the dimensions and the cap size and rating. The rating is like any other cap replacement for power filters: get the same or more capacitance, and the same or more voltage.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 03, 2012, 01:52:20 PM
I looked online before I posted here and didn't come up with anything. The ones I did find were like ten times what I need!

I'll keep searching though....
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 03, 2012, 02:38:25 PM
if the CAP is good, but  the terminal are corroding, you could also do one of two things, you could apply solder to the plate to protect it from corroding, after cleaning first, or you could just clean them well and solder the terminal to the cap. Use a clip as heat sink while applying solder.

This could buy you lots of time to find a suitable replacement.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 03, 2012, 02:43:49 PM
I already cleaned up the corroded terminals and all that, plus I put lock washers on them so they couldn't come loose again. The caps are definitely failing now.  :icon_frown:
Title: Re: Need Help Finding These Replacement Caps!
Post by: SteveG on January 03, 2012, 02:51:01 PM
"The rating is like any other cap replacement for power filters: get the same or more capacitance, and the same or more voltage.

With the caveat that you don't want to exceed the maximum capacitance value recommended for the valve rectifier.

Steve
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 03, 2012, 02:58:25 PM
"The rating is like any other cap replacement for power filters: get the same or more capacitance, and the same or more voltage.

With the caveat that you don't want to exceed the maximum capacitance value recommended for the valve rectifier.

Steve

Good point. It's a 5U4G rectifier, powering (4) EL-34 power tubes and up to (8 ) preamp tubes depending on what modules you use.
Title: Re: Need Help Finding These Replacement Caps!
Post by: SteveG on January 03, 2012, 03:14:09 PM
I'm not familiar with that amp, but 200uF seems high for a valve-rectified design? They could be in series, I suppose, to give a rating of 700V, but even so, still high @ 100uF.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 03, 2012, 03:23:09 PM
I'm not familiar with that amp, but 200uF seems high for a valve-rectified design? They could be in series, I suppose, to give a rating of 700V, but even so, still high @ 100uF.

They are in series actually, I never noticed that! The module tube filmaments are operating on DC power, plus each "channel" has more filter caps, too. Schematic is here: http://www.diyguitarist.com/PDF_Files/SDC-Powers.pdf - they are the ones connected to "V4", center left side of page.
Title: Re: Need Help Finding These Replacement Caps!
Post by: LucifersTrip on January 03, 2012, 03:44:46 PM

Do you know of a source for replacements or have any recommendations on what I could use in their place?


I don't know what  price you're shooting for, but these guys just helped us out with replacing a can cap in a 1964 record player:
http://hayseedhamfest.com/capacitors/Capacitors.html

the other option was to actually re-stuff the large can with new cap(s). amptramp sent a cool link:
http://www.antiqueradio.org/recap.htm#replacing


good luck
Title: Re: Need Help Finding These Replacement Caps!
Post by: amptramp on January 03, 2012, 09:56:33 PM
"The rating is like any other cap replacement for power filters: get the same or more capacitance, and the same or more voltage.

With the caveat that you don't want to exceed the maximum capacitance value recommended for the valve rectifier.

Steve

Heed this caveat.  The rectifier conducts only when the voltage is above the capacitor voltage which means that the higher the capacitance gets, the shorter the available time for the rectifier to conduct.  Of the 5U4 variants, the following ratings apply:

5U4GA average current: 225 mA, peak current: 800 mA
5U4GB average current: 250 mA, peak current: 900 mA
5U4GC average current: 275 mA, peak current: 1000 mA
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 03, 2012, 10:16:38 PM
I'm not familiar with that amp, but 200uF seems high for a valve-rectified design? They could be in series, I suppose, to give a rating of 700V, but even so, still high @ 100uF.

They are in series actually, I never noticed that! The module tube filmaments are operating on DC power, plus each "channel" has more filter caps, too. Schematic is here: http://www.diyguitarist.com/PDF_Files/SDC-Powers.pdf - they are the ones connected to "V4", center left side of page.

Na the filaments are still AC.. they rectify to get 10.5 bias voltage H.
I do not see where A is going, which is your tube rectifier and the 100U (200 in series) caps..  its using silicon for the other voltages. ..Sorry it goes to your Power output center tap on the transformer for the EL34.
That is all this is feeding, so this is not a normal supply, looks like its engineered to provide SAG on the power tubes only.... interesting..


Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 04, 2012, 09:47:25 AM
Na the filaments are still AC.. they rectify to get 10.5 bias voltage H.
I do not see where A is going, which is your tube rectifier and the 100U (200 in series) caps..  its using silicon for the other voltages. ..Sorry it goes to your Power output center tap on the transformer for the EL34.
That is all this is feeding, so this is not a normal supply, looks like its engineered to provide SAG on the power tubes only.... interesting..

No, the module tube filaments are operating on DC. The three preamp tubes that are doing the variable wattage circuit and the phase inverter are on an AC filament scheme. There are non-tube (solid state) modules that can't be powered via an AC tube filament connection where the modules plug in (that's what the LM317 is for). The preamp schematic is here: http://www.diyguitarist.com/PDF_Files/SDC-Preamps.pdf

Anyway, yes, this is not your typical amp. It's a quite different animal.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 04, 2012, 10:42:14 AM
I found one that is 250uF at Mouser. Apparently a 200uF cap isn't made anymore.
http://www.mouser.com/ProductDetail/Cornell-Dubilier/CGS251T350R3C/?qs=u7ZFOgOpR2urm9VcXXMd%252bg%3d%3d

Do you think that would be OK to use as a substitute? That would make it about 25uF higher than it is currently...
Title: Re: Need Help Finding These Replacement Caps!
Post by: DavenPaget on January 04, 2012, 10:53:46 AM
http://www.mouser.com/ProductDetail/Cornell-Dubilier/381LX221M350H452/?qs=sGAEpiMZZMukHu%252bjC5l7YV8H6sZiiPP7M0UOb%2fWj3Eo%3d (http://www.mouser.com/ProductDetail/Cornell-Dubilier/381LX221M350H452/?qs=sGAEpiMZZMukHu%252bjC5l7YV8H6sZiiPP7M0UOb%2fWj3Eo%3d)
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 04, 2012, 10:56:41 AM
http://www.mouser.com/ProductDetail/Cornell-Dubilier/381LX221M350H452/?qs=sGAEpiMZZMukHu%252bjC5l7YV8H6sZiiPP7M0UOb%2fWj3Eo%3d (http://www.mouser.com/ProductDetail/Cornell-Dubilier/381LX221M350H452/?qs=sGAEpiMZZMukHu%252bjC5l7YV8H6sZiiPP7M0UOb%2fWj3Eo%3d)

It's physically the wrong size, won't work. (needs "can clamps", 1.375" diameter)

EDIT: I suppose the clamp could be changed. But I would rather have the Mallory CGS type.
Title: Re: Need Help Finding These Replacement Caps!
Post by: davent on January 04, 2012, 11:55:38 AM
http://thetubestore.com/ca-jj-250uf-500v.html

And more JJ options.

http://www.partsconnexion.com/capacitor_ele_jjcap.html
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 04, 2012, 12:02:34 PM
Paul
I do not understand, unless part of the schamatic is missing
all of your tube EL34 and 12AU7 are AC filaments. (http://dl.dropbox.com/u/14312589/SDFiliments.png)
 then is rectified to 10v through the LM317 dc which goes to first channel preamp and your effect loop buffer and recovery.


you can see that the tube rectifier tube is directly heated cathode coming off the 5v tap on the transformer --> (http://dl.dropbox.com/u/14312589/SDtuberec.png)
where once rectofied you get according to the drawing 575vdc riding on top of the 5v ac which is standard. They then do full wave rectification by having diodes pull from the plates and other taps to get other DC voltage


variably wattage is handled through changing the way the power tubes are wired to the output  transformer  where it lifts the output tube screen tap on the transformer and reduces the ratio to the plates, in effect reducing the primary impedance reflected to the output tubes.


in any case this is about capacitors, you can use this chart to find your max capacitance for 5U4 you can use this sheet to find your max capasitor value after the 5u4
(http://www.triodeel.com/5u4_p4.gif)

we need to know the PT resistance to find a good new value, the data sheet shows 40uf as max first capacitor another shows 33uf. You could replace it with one capacitor if you could get 650 - 700v cap.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 04, 2012, 02:06:20 PM
From the LM317, after the diode bridge rectifier D6-D9, there is an "H" supply which doubles as the filament supply FOR THE MODULES. This has to be DC POWERED because there are NON-TUBE modules which need a DC SUPPLY to operate. You can't operate a solid state FET or solid state IC module on an AC power supply.

Each module terminal on the main PCB has an IN, OUT, B+, GROUND, HTR GROUND and HEATER 10.5 VDC connection on it.

I'm the guy who reverse engineered the amp - http://www.diyguitarist.com/GuitarAmps/Convert.htm
Look at the modules section, at one of the layouts and maybe it will be more clear what I am talking about.
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 04, 2012, 02:36:18 PM
now I understand, thanks, I assumed the MODULES were non tube based on the the schematic, but I see you can get tube based modules which is kind of cool, thanks for the patients.

but to your caps, I would not use larger caps,  depending on the impedance of the power transformer, is already exceeding the datasheet, which is 33u or 40u depending on make. They put 100u so I assume the Independence matches that, but to go over by 25u is a lot for rectifier tubes
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 04, 2012, 03:04:57 PM
now I understand, thanks, I assumed the MODULES were non tube based on the the schematic, but I see you can get tube based modules which is kind of cool, thanks for the patients.

but to your caps, I would not use larger caps,  depending on the impedance of the power transformer, is already exceeding the datasheet, which is 33u or 40u depending on make. They put 100u so I assume the Independence matches that, but to go over by 25u is a lot for rectifier tubes

No problem. It's not your typical amp, that's for sure. Well, it's hard to say for sure about the maximum cap values you can get away with. Their power and output transformers were custom manuafactured, not sure what the specs are on them, but that power transformer is physically a little larger than a Fender 100 watt power transformer is. For a 1x12 combo amp, it's almost as heavy as a Twin Reverb is!

I think I'll have to try emailing Kevin Beller at Seymour Duncan who was one of the designers of the amp and see if I can get a little clarification on that. Geez, this should be a simple thing to just change a couple of filter caps!!!  :icon_mad:

EDIT: looking at the capacitor data sheet, maybe they used the 200uF caps because of the amps of inrush current they could handle?
Title: Re: Need Help Finding These Replacement Caps!
Post by: amptramp on January 04, 2012, 08:32:41 PM
I would dispute one thing about the schematic - it appears there is a connection through the HV winding to the 5U4GB heater supply.  The section of transformer winding between V4 pin 4 and V4 pin 2 should not exist.  There may be a hum induced by the filament connection for the other tubes as one side then the other becomes one diode drop above above ground.
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 05, 2012, 12:25:11 AM

EDIT: looking at the capacitor data sheet, maybe they used the 200uF caps because of the amps of inrush current they could handle?
it could be.. the trouble is, is you have a cap that is too big you can cause the rectifier tube to arc. you can use generic caps for testing to see if they arc, (video the tube while you play at different settings) and if not find some in that range. It would make sense that its for current handling, since it power 4 EL34. Normally you have more than one cap in line down the power section and you have drop resistors, this is using different winding.. . 
by the way I was showing this amp to a friend who own a studio, now I want to hear it.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 05, 2012, 09:55:01 AM
I would dispute one thing about the schematic - it appears there is a connection through the HV winding to the 5U4GB heater supply.  The section of transformer winding between V4 pin 4 and V4 pin 2 should not exist.  There may be a hum induced by the filament connection for the other tubes as one side then the other becomes one diode drop above above ground.

Thanks for pointing that out, I'll have to fix that. That's a mistake I made when drawing up the schematic from the original factory schematic.  :icon_redface:
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 05, 2012, 10:04:28 AM

EDIT: looking at the capacitor data sheet, maybe they used the 200uF caps because of the amps of inrush current they could handle?
it could be.. the trouble is, is you have a cap that is too big you can cause the rectifier tube to arc. you can use generic caps for testing to see if they arc, (video the tube while you play at different settings) and if not find some in that range. It would make sense that its for current handling, since it power 4 EL34. Normally you have more than one cap in line down the power section and you have drop resistors, this is using different winding.. .  
by the way I was showing this amp to a friend who own a studio, now I want to hear it.

So maybe the better thing to do is use a pair of 130uF caps instead? But I am afraid that if I do that I may have a hum problem and spent $70 for nothing. Don't know what to do exactly.

The amp is a great sounding amp when all is working properly. The various modules can be quite useful for approximating many classic amp sounds.
Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 05, 2012, 11:29:48 AM
Also note that there is little - if any - audible difference between a tube rectifier and a pair of silicon diodes with a power resistor between them and the first filter cap. However, the SS diode version is immune to any capacitor size problems.

I recommend putting two silicon diodes in series with the anodes of a tube rectifier anyway, just in case one of the tube sections shorts. The SS diodes will prevent destruction of the power transformer and filter caps in that case.
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 05, 2012, 12:14:23 PM
@R.G. Would adding the silicon stop the Rectifier from SAG?  It looks to me as that is the only reason they put the tube one in was to get SAG on the power tubes

@paul
I was thinking something like these http://www.ebay.com/itm/4pcs-NCC-450v-150uf-KMX-Aluminum-Capacitors-18x60-/320698234826?pt=LH_DefaultDomain_0&hash=item4aab1ab7ca#ht_850wt_1165 to test (http://www.ebay.com/itm/4pcs-NCC-450v-150uf-KMX-Aluminum-Capacitors-18x60-/320698234826?pt=LH_DefaultDomain_0&hash=item4aab1ab7ca#ht_850wt_1165 to test)
not a fix just a test to make sure the tube does not arc. once your sure you can pay the money to the correct sized item.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 05, 2012, 12:17:29 PM
Also note that there is little - if any - audible difference between a tube rectifier and a pair of silicon diodes with a power resistor between them and the first filter cap. However, the SS diode version is immune to any capacitor size problems.

Score one for a SS rectifier...

I recommend putting two silicon diodes in series with the anodes of a tube rectifier anyway, just in case one of the tube sections shorts. The SS diodes will prevent destruction of the power transformer and filter caps in that case.

That's a good idea. If the PT got damaged, there are NO replacements available.
Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 05, 2012, 12:30:36 PM
@R.G. Would adding the silicon stop the Rectifier from SAG?  It looks to me as that is the only reason they put the tube one in was to get SAG on the power tubes
I get a lot of flack on this issue from die-hard tubes-rool guys, so I'll blather on about it for a minute.

First, "sag" is a fairly complex thing. Like everything complex, naive users want a simple, easy to understand solution, so they pin their hopes on the first explanation they like.

Sag comes from several things, but the one most often mentioned is the power supply drooping because of high signal outputs. This happens because with heavy loading, the resistance of the power transformer and rectifiers means the filter caps can't be refilled as fast as they're being emptied, so the voltage drops some. This causes some of the characteristics of audible sag by lowering the output power of the power stage and lowering gain in the preamp tubes a little.

Compared to tube rectifiers, silicon rectifiers have negligible internal resistance and forward voltage. So they not only provide more voltage at the filter cap, they don't lower the voltage under heavy loading.

If you have tube rectifier, it will have a tens-of-volts offset and noticeable internal resistance. If you have only silicon rectifiers they will lose 0.7 to 1.2V per diode, and have maybe 10 ohms of internal resistance. In general the voltage at the filter caps will be about 40-60V higher and will not sag because of the rectifiers, although voltage ripple caused sag will be the same, being controlled by the size of the cap.

If you use only silicon rectifiers and a resistor to fake the voltage drop of a tube rectifier, you can get a near-perfect match to a tube rectifier. Weber sells this solution in its "copper cap" rectifier replacements.

If you add silicon rectifiers in series with a tube rectifier, it will make essentially zero difference to the amp - until the tube rectifier fails. Then it saves your power transformer and filter caps. Adding a silicon diode this way does not take away any sag the tube rectifier puts there. 
Title: Re: Need Help Finding These Replacement Caps!
Post by: DavenPaget on January 05, 2012, 03:03:32 PM
Also note that there is little - if any - audible difference between a tube rectifier and a pair of silicon diodes with a power resistor between them and the first filter cap. However, the SS diode version is immune to any capacitor size problems.

Score one for a SS rectifier...

I recommend putting two silicon diodes in series with the anodes of a tube rectifier anyway, just in case one of the tube sections shorts. The SS diodes will prevent destruction of the power transformer and filter caps in that case.

That's a good idea. If the PT got damaged, there are NO replacements available.
And people dare to tell me solid state is bad because when it fails , it usually shorts out ... NAH , tubes do that too .
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 05, 2012, 07:23:50 PM

If you add silicon rectifiers in series with a tube rectifier, it will make essentially zero difference to the amp - until the tube rectifier fails. Then it saves your power transformer and filter caps. Adding a silicon diode this way does not take away any sag the tube rectifier puts there. 

Thanks, that is what I was getting at.
Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 05, 2012, 07:40:36 PM
OK. I thought it might be, but also saw some other interpretations of your question. So I answered all the questions I could think of that had bearing on the issue.

No, putting silicon diodes in series with a tube rectifier does not remove any sag the tube put in there. Using ONLY silicon diodes and no "sag" resistor does decrease sag. Using an added sag resistor with only silicon diodes puts some sag back in
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 06, 2012, 12:32:21 AM
R.G. As always you did a awesome job at explaining, I was not sure how they would effect this design, and I was kind of asking if that is why they have the tube rectifier for the power tubes and diodes for the rest..
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 06, 2012, 07:48:03 AM
R.G. As always you did a awesome job at explaining, I was not sure how they would effect this design, and I was kind of asking if that is why they have the tube rectifier for the power tubes and diodes for the rest..

The diodes for the rest is so they can power the preamp modules via a DC power scheme. There were a couple of solid state modules that use the DC heater supply to power them.
Title: Re: Need Help Finding These Replacement Caps!
Post by: MikeH on January 06, 2012, 11:17:30 AM
Paul - how is the amount of room inside the amp?  It's a bit hackey, but if you can find the proper value caps in a different package you could mount them inside the chassis and cover the holes.  Not the preferred solution, but at least you'd have a working amp, until you can find the right caps.
Title: Re: Need Help Finding These Replacement Caps!
Post by: SteveG on January 06, 2012, 12:20:00 PM
Something to bear in mind when working on any amplifier with series power-supply caps is that the outer can of the 'upper' capacitor can sometimes be sitting at 1/2 B+. Not such a problem with most new caps that are fully insulated, but sure can be in some vintage units without proper insulation where the outer can is connected to the -ve terminal.

70's Selmer T&B owners take note!

Steve
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 06, 2012, 12:48:20 PM
R.G. As always you did a awesome job at explaining, I was not sure how they would effect this design, and I was kind of asking if that is why they have the tube rectifier for the power tubes and diodes for the rest..

The diodes for the rest is so they can power the preamp modules via a DC power scheme. There were a couple of solid state modules that use the DC heater supply to power them.

yep, also it looks like V1, V2, and v3 have voltage rectified by D1, D2, D4, D3 tapped off before the 5U4. to me it looks like the 5u4 is only for the EL34 (tap A)
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 06, 2012, 05:42:52 PM
Well I think I found the source of my problem, a most unexpected thing. It appears that those main filter caps are good after all from what I can tell. It seems that it was a combination of a couple of dodgy connections on one of the channel switching LEDs which was contributing to a spontaneous popping problem that happened this morning when switching channels, a dodgy solder joint on a 10uF cap on the channel switching relay and a bad ground on one of the tone controls. I surmise that the quick disconnect connector to the tone control got jarred somehow when loading it into my car (it had a broken connector that had the wire "soldered" to it - bad solder job!). I also cleaned up a few other questionable things while I had the chassis out again for the third time.

The weird thing is that is was totally motorboating like it had bad filter caps. I guess it must have been some kind of low frequency oscillation that sounded very much the same.

In any case now it sounds better than it ever has. I'm hoping that the "third time will be the charm" here...

EDIT: I have updated the schematics with a little more clarification and fixed a few minor errors on that power amp schematic. Just thought I'd pass that on to let everyone know I fixed them.  :icon_wink:
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 01:07:59 PM
OK, so now that my amp is apparently OK again now, can anyone suggest what diodes to use with that rectifier tube and how they should be oriented?
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 07, 2012, 01:30:41 PM
OK, so now that my amp is apparently OK again now, can anyone suggest what diodes to use with that rectifier tube and how they should be oriented?
1N4007

and if they are in series and this is a directly heated cathod you would have to place them before the 5u4

so right now you Pin 4 and pin 6 as input think of that as you anode side. I made this in paint from the original so you can see
(http://dl.dropbox.com/u/14312589/add%20diodes.png)
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 07, 2012, 01:42:28 PM
This is a better drawing
http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod (http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod)

They moved the connections from 4 and 6 to pin 5 and 7 as they are unused and connected back with the diodes
(http://tubes4hifi.com/TubeRectifier-DiodeMod.jpg)
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 02:10:30 PM
I think the first picture you drew is the way to do it. The picture above is for a 5AR4 type which looks like it has different pinouts than the 5U4G does...
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 07, 2012, 02:24:29 PM
same pin out,  and the drawing are the same, showing same connections, this second one is just using unused pen so that connection goes to pin 5 (unused by the tube) and goes over the diode to pin 4, just like the schematic drawing.  the diffrence between the two is that the 5AR4 is not directly heated, but that means nothing for this type of hook up. But which ever is clearer for you.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 02:27:10 PM
Oh I see. I did a quick check and it looked like they had different pinouts, but I was in a hurry to get out the door which I got to do right now.

Thanks for the help!

EDIT: One thing, though. Will a 1N4007 be OK current-wise? If I interpret the data sheet correctly, it looks like it's good for 1 amp of current. Is that enough for a 100 watt amp that has a rectifier tube, four power tubes and up to eight preamp tubes? Seems kind of on the edge to me...
Title: Re: Need Help Finding These Replacement Caps!
Post by: iccaros on January 07, 2012, 02:54:57 PM
Oh I see. I did a quick check and it looked like they had different pinouts, but I was in a hurry to get out the door which I got to do right now.

Thanks for the help!

EDIT: One thing, though. Will a 1N4007 be OK current-wise? If I interpret the data sheet correctly, it looks like it's good for 1 amp of current. Is that enough for a 100 watt amp that has a rectifier tube, four power tubes and up to eight preamp tubes? Seems kind of on the edge to me...

This is only for the 4 EL34 and not even their heaters so you are @ 300ma max, looks like D1, D2,D3 and D4, using 1N4007's for those, rectify the rest of the tubes already so that there is no load from the 5U4 to the rest of the amp... .. , so you will be fine, R.G. or Paul or amptramp would be sure to let us know if I am wrong on this, which I hold every right to be.


You can see when you chop away the schematic that the 5U4 only goes to point  A, which is the center tap of the OT, which is in all effect the load resistor for the plates of the EL34's.  and that it.. :) hope it works out, It would be cool if you could post audio clip of this amp

(http://dl.dropbox.com/u/14312589/SDtuberec.png)
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 03:52:30 PM
OK, I see what you are saying. But diodes D1 thru 4 don't look like a full wave rectifier to me (compare to D5 thru D9), they are just two diodes in series with a resistor bypassing them, which is also strange to me. I was looking at those diodes earlier this morning as was kind of puzzled by them. What they heck are they supposed to do? But I get the idea of protecting the PT from a shorted rectifier tube. That's a big deal because there are no replacement PTs available anymore.

I can try to make some audio of the amp, but I don't really quit have the right stuff to make real good recordings.
Title: Re: Need Help Finding These Replacement Caps!
Post by: SteveG on January 07, 2012, 04:54:56 PM
Diodes 1-4 are a full wave rectifier. They work with the transformer centre-tap. Diodes 6-9 are a bridge-rectifier, which does not need the transformer to be centre-tapped (although, in this case, it is).
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 07, 2012, 05:42:15 PM
OK, I see what you are saying. But diodes D1 thru 4 don't look like a full wave rectifier to me (compare to D5 thru D9), they are just two diodes in series with a resistor bypassing them, which is also strange to me. I was looking at those diodes earlier this morning as was kind of puzzled by them. What they heck are they supposed to do? But I get the idea of protecting the PT from a shorted rectifier tube. That's a big deal because there are no replacement PTs available anymore.

The tube rectifier only supplies power to the power tubes (A), so the SW3 must be the standby... The other PS taps (B, C, D, E) are rectified by the 1N4007s, off the same PT (!). They must be HT for the preamp. I guess using the tube rect for the power tubes only is a sag thing.

Resistor or cap between the SS rectifiers--either way it's intended to equalize the voltage over both rectifiers. Lot of the older Fender SS rectified PS schematics use three in series (and no resistors or caps between them), but with Fender you never know if they thought they might substitute 1N4004's if necessary...

I don't see why you couldn't use SS rectifiers to backup the tube like you plan. I'd use two 1N4007's in series like the schematic (a pair for each anode). I've personally tried using a single 1N4007 for a PS with about 400V on a 30 watt amp, and it failed.

That is one cool, strange amp!
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 06:03:30 PM
The tube rectifier only supplies power to the power tubes (A), so the SW3 must be the standby... The other PS taps (B, C, D, E) are rectified by the 1N4007s, off the same PT (!). They must be HT for the preamp. I guess using the tube rect for the power tubes only is a sag thing.

Yeah, "A" is the voltage at the standby switch. Most schematics I look at I can follow, but this one I am still trying to understand.  :icon_lol:

Resistor or cap between the SS rectifiers--either way it's intended to equalize the voltage over both rectifiers. Lot of the older Fender SS rectified PS schematics use three in series (and no resistors or caps between them), but with Fender you never know if they thought they might substitute 1N4004's if necessary...

OK, I get the idea. I don't remember seeing that on any other schematics that I can remember (ones that would apply to me anyway). Interesting.

I don't see why you couldn't use SS rectifiers to backup the tube like you plan. I'd use two 1N4007's in series like the schematic (a pair for each anode). I've personally tried using a single 1N4007 for a PS with about 400V on a 30 watt amp, and it failed.

That is one cool, strange amp!

It is a very cool amp indeed. They sound really great when all is working properly.

I was initially thinking of using some kind of "super rectifier" diode (1N5408 or similar?) that can handle much more continuous current than a 1N4007 can, which is what prompted me to ask the question. I suppose two 1N4007s in series would also work, though. It was RG's suggestion, and I don't want anything to happen to an irreplaceable PT.
Title: Re: Need Help Finding These Replacement Caps!
Post by: amptramp on January 07, 2012, 08:41:58 PM
If your standby switch, S3, is open, and the rectifier shorts from plate to filament, the 1N4007 will have its cathode connected to the peak positive voltage and the anode will go to the peak negative voltage.  If the peak in each direction exceeds 500 volts, this will overstress the diodes, which are rated for 1000 volts.  Two diodes in series connected to each plate would be good enough.  Note that the peak is greater than the average value of rectified DC.
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 07, 2012, 09:13:17 PM
Merlin writes that an SS rectifier on a fullwave, center-tapped PT setup needs to be rated at 2.8 times RMS (of one side of the PT). You gotta give some thought to that, but it makes sense.

Paul, check out the later Twins, the Showmans and even the late Bassmans. They've all got three (unspecified) SS rectifiers in series for each side of the PT.

RE: the amp's PS--The thought of two different types of rectifiers together in parallel--on the same PT, feeding different stages--is intriguing.

Were (are) there reliability issues with the design? All those module edge connectors. Man. Like an early home computer.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 07, 2012, 10:41:26 PM
If your standby switch, S3, is open, and the rectifier shorts from plate to filament, the 1N4007 will have its cathode connected to the peak positive voltage and the anode will go to the peak negative voltage.  If the peak in each direction exceeds 500 volts, this will overstress the diodes, which are rated for 1000 volts.  Two diodes in series connected to each plate would be good enough.

Aha. OK, I get the logic behind that now.

Merlin writes that an SS rectifier on a fullwave, center-tapped PT setup needs to be rated at 2.8 times RMS (of one side of the PT). You gotta give some thought to that, but it makes sense.

Paul, check out the later Twins, the Showmans and even the late Bassmans. They've all got three (unspecified) SS rectifiers in series for each side of the PT.

RE: the amp's PS--The thought of two different types of rectifiers together in parallel--on the same PT, feeding different stages--is intriguing.

Were (are) there reliability issues with the design? All those module edge connectors. Man. Like an early home computer.

The early ones had some issues, but nothing major. It was minor stuff, mostly circuitwise, but it was stuff that annoyed the average joe musician. Mine appears to be one of the first ones made - the transformers, filter caps and cooling fan all have 1984 dates, which was the year it was introduced. The main thing I remember Kevin Beller with Seymour Duncan telling me once (he was one of the designers of the amp) is that he recommended using NOS rectifier tubes because the new ones were prone to failure and sometimes it took out the power transformer. Ten years ago I wasn't too concerned about it because you could still get a replacement PT, but now you can't. I don't know why they didn't think of putting these diodes on the rectifier as a protective measure. Maybe they felt that it affected the "sag" or something? I don't really know for sure.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Earthscum on January 07, 2012, 11:02:00 PM
Hey Paul, (or anyone, for that matter), in case you're interested, I keep seeing these at my local shop and thought of you today and snapped a couple pics for you:

(http://img3.imageshack.us/img3/5626/image01072012112635.th.jpg) (http://imageshack.us/photo/my-images/3/image01072012112635.jpg/)
http://img3.imageshack.us/img3/5626/image01072012112635.jpg


(http://img214.imageshack.us/img214/5171/image01072012112705.th.jpg) (http://imageshack.us/photo/my-images/214/image01072012112705.jpg/)
http://img214.imageshack.us/img214/5171/image01072012112705.jpg

They are at Mountain States Electronics (http://www.mselectronic.com/page/page/1197965.htm)

They have a couple other types, but these are the only 2 types that there were multiples of. There are probably about 10 of the paper ones, and about 5 or 6 of the silver can ones. Just an FYI, since I came across them. He has them marked at $4 each
Title: Re: Need Help Finding These Replacement Caps!
Post by: PRR on January 08, 2012, 12:39:50 AM
The rectifiers must be rated *twice* the no-load DC.

Since they used 350V+350V worth of caps, we should assume there could be as much as 700V DC, and must use 1,400V diodes.

The cheapest "1,400V" diode is two 1N4007 in series. We see this on the preamp supply, so it makes sense.

If you must back-up the bottled rectifier with sand-state, this also must be 1,400V to hold things "if" the rectifier goes short or gassy.

Back in the old days we'd never do that. A 5U4 was clearly more reliable than silly-state diodes. However Silicon has improved some, while tube production moved to rundown factories in far corners of the world.
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 08, 2012, 09:51:53 AM
There's a lot of discussion online about various ways to "bulletproof" tube amps--fuses on the B+ HT voltage, fuses on the heaters, etc., backup rectifiers, circuits to automatically sense/adjust bias voltage, etc. But you don't see much of this on production amps. Cost vs. benefit thing, I suppose. So many old, abused tube amps survive with minimal "safeties."

The early ones had some issues, but nothing major. It was minor stuff, mostly circuitwise, but it was stuff that annoyed the average joe musician. Mine appears to be one of the first ones made - the transformers, filter caps and cooling fan all have 1984 dates, which was the year it was introduced. The main thing I remember Kevin Beller with Seymour Duncan telling me once (he was one of the designers of the amp) is that he recommended using NOS rectifier tubes because the new ones were prone to failure and sometimes it took out the power transformer. Ten years ago I wasn't too concerned about it because you could still get a replacement PT, but now you can't. I don't know why they didn't think of putting these diodes on the rectifier as a protective measure. Maybe they felt that it affected the "sag" or something? I don't really know for sure.

I doubt adding diodes would have much effect on the efficiency of the 5u4, since they'd be wired before the tube. There are faster diodes, of course, but I'm sure the 1N4007 has a shorter recovery time than the 5U4. The SS diodes might introduce some noise spikes, etc.--maybe the tube's response would smooth those out...
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 08, 2012, 10:50:22 AM
Well it's an easy mod to do. If it has negative effects with noise spikes or anything like that I can just take them out!  :icon_cool:
Title: Re: Need Help Finding These Replacement Caps!
Post by: MikeH on January 09, 2012, 11:11:44 AM
They do it on the Stray Cat too:  http://ceriatone.com/images/layoutPic/matchlessLayout/StrayCat30Ceriatone.jpg
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 09, 2012, 11:27:22 AM
Well I turned on the amp yesterday to test it out some more and suddenly the sound went away after about 15 minutes, and there was a burning smell. The fuse did not pop. Tried a different rectifier tube, same behaviour. So I pulled out the chassis again, took out the main PCB again  :icon_mad: and poked around. Checked B+ at the standby switch and it was 600V, but further on down the line in the filter section I was getting 0V.

From what I can gather, it appears that the original 150K 2W resistors I replaced at the can type caps are getting very hot. I had to replace those resistors a long time ago with a pair of NTE flame proof 150K 2 watt resistors because they broke (my own dumb mistake), and it seems that these new ones can't handle it. I guess I'll put 5W resistors in their place this time so I don't have this aggravation anymore.

I know it has something to do with the resistors overheating because when I put the amp back together again it was working again. But I bet if I leave the amp on for a while the same thing might happen again. So I think I've narrowed down the original problem to this pair of resistors. At least I fixed a whole bunch of other minor problems in the process.  :icon_neutral:

Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...
Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 09, 2012, 12:32:40 PM
The rectifiers must be rated *twice* the no-load DC.
Since they used 350V+350V worth of caps, we should assume there could be as much as 700V DC, and must use 1,400V diodes.
The cheapest "1,400V" diode is two 1N4007 in series. We see this on the preamp supply, so it makes sense.
If you must back-up the bottled rectifier with sand-state, this also must be 1,400V to hold things "if" the rectifier goes short or gassy. 
Yep. The measured B+ at 600V means that a 1200V diode is marginal. I'd like to see 2kV in an equivalent situation in my own amps if that happened.
Mouser has the 747-DSA1-18D  which is 1800V, 3.6A, but it costs $4.56 each. There are probably other choices in the industry. I didn't do a massive search. You're right, 2x1N4007 is cheapest, but this setup needs help.

It is important, but often ignored, to provide some equalizing of the reverse voltage on series devices for high voltages. That's what the resistors in parallel with series diodes do. It's a good first order fix, and forces the reverse voltage to be equal across, for instance, two series diodes, by providing a leakage path that completely swamps out the differences in reverse leakage of the two series diodes.

That works fine for DC, but what about transients? In that case, the reverse voltage divides by the inverse of the ratio of the diode capacitances (and inductances, which I'm choosing to ignore for the moment). In that case, you need to put a cap across the diodes that is greater than 10x the reverse capacitance of the diodes to equalize them for AC conditions. If you use a low-R + C snubber, often the snubber can do this for you. But it's something most people haven't run into.

There's a lot of discussion online about various ways to "bulletproof" tube amps--fuses on the B+ HT voltage, fuses on the heaters, etc., backup rectifiers, circuits to automatically sense/adjust bias voltage, etc.
Yeah. A lot of it from me.  :icon_biggrin: I like the idea of solid state minions making the world safe for tubes.

Quote
I doubt adding diodes would have much effect on the efficiency of the 5u4, since they'd be wired before the tube.
There is substantially zero effect. The tube rectifier has a 30-60V forward drop, the solid state a 0.7 - 1.4V drop, and the same current, so the power loss is in the ratio of the voltages. Call it about 2% more loss in rectifiers only for a SS diode in series with a tube.
Quote
There are faster diodes, of course, but I'm sure the 1N4007 has a shorter recovery time than the 5U4. The SS diodes might introduce some noise spikes, etc.--maybe the tube's response would smooth those out...
The 1N4007 would follow the tube down in conduction, so the spikes and ringing from a standard silicon diode slamming off would not happen when it's in series with the tube. The turn-off transient only happens when normal diodes quit conducting abruptly and excite an RF ring in the associated wiring. Snubbers can largely eliminate this; this is why you often see caps and R-C networks around rectifiers. Fast+soft turn off diodes also prevent it without snubbers. So would a series tube.  :icon_lol:

Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...
Simple enough. Open up the wires after the filter caps to the rest of the amp. Clip meter leads to the caps, get your hands out of it, then turn it on, and off. The B+ will go up to the no-load voltage, then start decaying. It will decay at the R-C rate of the caps and equalizing resistors.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 09, 2012, 12:44:08 PM
Or maybe I should take this a sign that those 200uF 350V filter caps might be failing after all? But the amp sounds great right now, none of the classic failing filter cap symptoms. Argh! I just don't know for sure...
Simple enough. Open up the wires after the filter caps to the rest of the amp. Clip meter leads to the caps, get your hands out of it, then turn it on, and off. The B+ will go up to the no-load voltage, then start decaying. It will decay at the R-C rate of the caps and equalizing resistors.

Yeah, I thought of trying to measure what kind of current might be happening at that point. But the way the amp is constructed, it makes it extremely difficult to try to test anything of this nature. All the connections are under the PCB, and to get the PCB out, you literally have to dismantle the entire amp. This is what you have to do to even get to those filter caps:

(http://www.diyguitarist.com/Images/SDC-Dismantled.jpg)
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 09, 2012, 12:47:01 PM
Burning smells are bad. Maybe one or both of the caps have failed, fully or partially. Those 150K resistors seem over-speced, even at 2 watts. Maybe the designers anticipated some failure condition I can't see...

I'm not sure where (or how) you'd be getting 0V if the amps sounds great. ??? Could there be a connector problem somewhere, when the chassis is on the bench?

They do it on the Stray Cat too:  http://ceriatone.com/images/layoutPic/matchlessLayout/StrayCat30Ceriatone.jpg

Mike, that looks like either an SS or tube rectifier, rather than both in series.
Title: Re: Need Help Finding These Replacement Caps!
Post by: R.G. on January 09, 2012, 12:48:37 PM
Wow. The Thomas Vox amps are worse, but not much.

My cut on it? If it's over 10 years old, and the work is so bad to find problems and fix them, just replace all the electros prophylactically. It removes the question and takes a shorter time than a lot of troubleshooting.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 09, 2012, 12:52:57 PM
Burning smells are bad. Maybe one or both of the caps have failed, fully or partially. Those 150K resistors seem over-speced, even at 2 watts. Maybe the designers anticipated some failure condition I can't see...

I'm not sure where (or how) you'd be getting 0V if the amps sounds great. ??? Could there be a connector problem somewhere, when the chassis is on the bench?

I had the amp all together and it was working fine when the sound suddenly cut out. I took the chassis out and started measuring B+ voltages because everything else was working, and that's when I had 0V at the end of the B+ string. I thought it was kind of weird. I am pretty sure it's not a connector thing. I already had the chassis out once to take care of some corroded connections on those caps, which led to a whole set of frustrating problems!  :icon_lol:

Wow. The Thomas Vox amps are worse, but not much.

My cut on it? If it's over 10 years old, and the work is so bad to find problems and fix them, just replace all the electros prophylactically. It removes the question and takes a shorter time than a lot of troubleshooting.

Which goes back to where I began! I can't get exact replacement caps, they apparently don't exist anymore.  :icon_cry:
(For the ones I really suspect are the problem here)
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 09, 2012, 12:56:58 PM
Thanks for that info, R.G.

There's a lot of discussion online about various ways to "bulletproof" tube amps--fuses on the B+ HT voltage, fuses on the heaters, etc., backup rectifiers, circuits to automatically sense/adjust bias voltage, etc.
Yeah. A lot of it from me.  :icon_biggrin: I like the idea of solid state minions making the world safe for tubes.

And thanks for all (I mean ALL) that info, too  :)

Did the Workhorse have those safeguards integrated? I seem to recall a discussion about an automatic digital bias thingie, and you having at least tried it...
Title: Re: Need Help Finding These Replacement Caps!
Post by: gmoon on January 09, 2012, 02:28:36 PM
Which goes back to where I began! I can't get exact replacement caps, they apparently don't exist anymore.  :icon_cry:
(For the ones I really suspect are the problem here)

Just a wild thought--Paul, could you "rebuild" the old caps by reusing just the base, and retrofitting new electrolytics?

Not as good a solution as finding new ones, but...
Title: Re: Need Help Finding These Replacement Caps!
Post by: amptramp on January 09, 2012, 09:50:20 PM
You can always restuff the existing capacitor shells as shown most of the way down in this article and its links:

http://www.antiqueradio.org/recap.htm

This is done where you want to preserve the old design but new capacitors of the same type are not available.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 10, 2012, 09:54:37 AM
You can always restuff the existing capacitor shells as shown most of the way down in this article and its links:

http://www.antiqueradio.org/recap.htm

This is done where you want to preserve the old design but new capacitors of the same type are not available.

Yeah, someone else had mentioned that, too. That would be probably a good option.

You know this is just weird. I had the amp on for 40 minutes last night and no problems. I was playing it about the same level as the other day when the B+ went wonky after maybe ten minutes. But I had reflowed the solder joints on those cap connections, too. So maybe that is a factor here.

Quite a while ago, I remember reading somewhere about "forming caps" and why it's important, blah blah. I don't think I've turned the amp on except for once or twice in the last five years until recently when I decided I want to start using this amp more. It's always been in an air conditioned environment since I got the amp in 2002 so I really doubt they "dried out". Point is I wonder if the caps just needed some time time get used to having high voltage on them after sitting dormant for so long? In any case, I'm going to keep testing the amp over the next week or two and see what happens. Got to do that rectifier tube diode mod too...
Title: Re: Need Help Finding These Replacement Caps!
Post by: amptramp on January 10, 2012, 10:37:09 AM
I'm the one who mentioned restuffing caps before.

You can waste your time trying to reform electrolytics.  I have a power supply for battery tube portable radios that I picked up at the London Vintage Radio Club that had a rather interesting characteristic.  It used back-to-back filament transformers for isolation and when I measured the open circuit voltage for the high voltage (which is rectified 117 VAC, so open circuit, it would be about 162 volts).  The voltage started at 87 and rose by one volt every three seconds as the caps formed and leakage current declined.  But the problem with reformed electrolytics is that they are still old with proven defect sites and they may revert to their high-leakage state.  New caps would solve everything.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 10, 2012, 10:43:07 AM
I'm the one who mentioned restuffing caps before.

You can waste your time trying to reform electrolytics.  I have a power supply for battery tube portable radios that I picked up at the London Vintage Radio Club that had a rather interesting characteristic.  It used back-to-back filament transformers for isolation and when I measured the open circuit voltage for the high voltage (which is rectified 117 VAC, so open circuit, it would be about 162 volts).  The voltage started at 87 and rose by one volt every three seconds as the caps formed and leakage current declined.  But the problem with reformed electrolytics is that they are still old with proven defect sites and they may revert to their high-leakage state.  New caps would solve everything.

I agree. I wasn't suggesting that I try reforming them, just that maybe they have to go through a similar process until they stabilize after you suddenly start using an amp that hasn't been used for several years.
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on January 12, 2012, 10:07:40 AM
Just an update...

So I've been testing this amp for the last few nights between 30 minutes and 60 minutes each evening. So far it's been working fine. I'm really baffled by what happened a few days ago when the B+ voltage at the end of the string was at 0V. Maybe it works more reliably now because I reflowed the solder joints on those connectors where the screws attach to those Mallory CGS caps? In any case I'm happy that so far I am not having any problems.

Kevin Beller from Seymour Duncan (one the designers of this amp) has rounded up some caps for me which are for the Convertible 2000 but will work in their place if I should need to replace these caps. I'll probably have to mount them differently, but at least I'll have some backups in case I should need them.

EDIT: This is the style of cap that is in the amp now just for the record.
(http://www.diyguitarist.com/Images/SDC-MalloryCGS2.jpg)
Title: Re: Need Help Finding These Replacement Caps!
Post by: Paul Marossy on June 16, 2012, 11:29:24 AM
Just thought I'd give an update on this. I went out and got the diodes to put on my rectifier tube like five months ago and then forgot about adding them (had a lot of personal stuff going on at the time), but I remembered about it yesterday when answering an email regarding the Convertible. So I put them in this morning, some 1N5399s from the local RadioShack (same ratings as a 1N4007 but can handle more current). The amp still works and nothing melted, so off I go to play with it now. So far no noise problems (that I can hear anyway). It was about a 1/2 hour project, but well worth the effort knowing that it should prevent any damage to the power transformer by a shorted rectifier tube.

So now for a dumb question: What happens now if a rectifier tube shorts? Does the tube do internal fireworks?

(http://www.diyguitarist.com/Images/SDC-FCapDiodes.jpg)