Need Help Finding These Replacement Caps!

[Paul Marossy]

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!!!  

EDIT: looking at the capacitor data sheet, maybe they used the 200uF caps because of the amps of inrush current they could handle?

[amptramp]

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.

[iccaros]

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.

[Paul Marossy]

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. 

[Paul Marossy]

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.

[R.G.]

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.

[iccaros]

@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
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.

[Paul Marossy]

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.

[R.G.]

@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. 

[DavenPaget]

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 .

[iccaros]

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.

[R.G.]

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

[iccaros]

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..

[Paul Marossy]

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.

[MikeH]

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.

[SteveG]

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

[iccaros]

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)

[Paul Marossy]

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.  

[Paul Marossy]

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?

[iccaros]

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