OT - My Spitfire Clone

[Paul Marossy]

Just for those of you thinking about building an amp someday...

Got my Matchless Spitfire clone chassis completed last night. Aside from a hum problem I have with the tube heaters (a simple fix), it's done. Turned out pretty good for my second tube amp build. I have a page for it that's still a work in progress, but when I'm done, it will become an official page at my website. Check it out:

http://www.diyguitarist.com/GuitarAmps/Spitfire.htm

I did the chassis preparation, wiring, graphics and final pre-power up double checking in about 12 hours. Now, I need to get busy on making an enclosure for it and covering it with some kind of cool covering. (BTW, it's a very nice sounding amp for clean or slightly dirty tones. I don't really know it's capabitilities at this point, the hum was too loud)

[petemoore]

Some of the Pic at the bottom of the page I coudn't see, but the amp part looks PRO !!!
 Beautiful looking unit. [Looks Expensive !!].
 Drooool...I'm interested to hear about the heater hum problem/fix, I hope you get that figured out soon.

[Paul Marossy]

pete- Yeah, they aren't there because they don't exist.  :wink:
It's still under construction, the amp and the page.

Thanks for the compliment.  

[u1061810]

Ultra-Fine work!! I wish my work area was that neat.

[Paul Marossy]

Thanks u1061810. I find that I get a lot less frustrated when I can find what I need, when I want it. So, that's why I am so organized.  
Well, I also have a very limited amount of space to work in, so it's really a necessity for my sanity.

[Phorhas]

Waht a beautful piece...

[Paul Marossy]

Well, after several long nights of dealing with a HUMMMMMMM from hell, I think I've got this thing whipped. The wiring diagram that was sent with my kit had an error, which I reproduced... anyhow, if I reverse the wires to the plates on the second preamp tube, I think my hum problem will be solved! Apparently, by having these two connections reversed, it allows HUMMMMM to be injected directly into the signal path! It only took me about four days to figure this out.  :oops:
But, in the process of trying to exorcise this amp of the HUMMMMMM demons, I now have paralleled heater supplies, a thought-out ground bus connecting to the chassis at one point, and some neater wiring inside now. I sure have learned A LOT during this process, though. I'm going to make the correction tonight and hopefully I will be in 15 watt heaven... and I can get started on building my cabinet. It's going to be a head instead of a combo.

I guess the moral of this story is that you should always double check wiring diagrams created by someone other than yourself.  

[Lonestarjohnny]

Paul, My Wife invited me on a shopping trip several years ago now that I'm thinking about it, I always hated going on those, but would never say as much, anyway we had been through about every store on the So.Side of town and I thought I was gonna get to go home, when she wanted to stop at this Fabric Shop, Boy was I glad she invited me, I been buying cool lookin grill cloth, pedal board covering and some really good Tolex type stuff that I've covered many amps with, and it's a lot cheaper than buying it from the amp supply store's.
JD

[gtrmac]

Hey, nice job. I've made two amps so far and it sure is a lot of fun. I used a kit for the first, a Fender Tweed Deluxe clone, and a chassis kit for the second, a Marshall 18 watt clone. Actually, the Marshall 18 is very similar to the Spitfire I believe.

It's great to have the sound of these "Holy Grail" amps for a fraction of the price of an original. I actually got into DIY FX after the amps. I couldn't afford any more amp builds! Not within the context of my present marriage anyway! :lol:

[Paul Marossy]

Thanks guys.  

Yeah, fabric stores can have some cool upholstery. I found some cool stuff to cover my Firefly with that looks pretty snazzy. Check this page out: http://www.diyguitarist.com/GuitarAmps/FireFly.htm

Back my present problem at hand. While it looks nice, it's still a HUMMMMMINGGGG pile of crap. I don't know what else to do at this point. I have tried all the textbook things to do, none of them have helped. I switched the two wires on the phase inverter, which I thought would fix it, but it still hums just as loud with the controls turned up. I guess I'm going to have to get a new pair of power tubes. I know for sure something funky is going on in one of them. It still hums with the funky EL84 pulled out of the socket, though.

[csj]

For starters I'd dump that parallel wiring coming off the screen filter supply junction. You've got the rectified current from the PI and the preamp both cycling through the screen supply. Try disconnecting the preamp supply from the screen supply and just tag it on after the PI supply...if anything you'll at least get a cleaner DC current downstream. Check your preamp voltages and see if you're still close to where you want to be. The R value here shouldn't be that critical... the voltage drop here isn't going to be drastic. Though you might want to keep that RC combination since, presumably, that's what was determined to have the "right" time constant for the Spitfire sound.

[Paul Marossy]

OK, so if I follow what you are saying, I want to disconnect Point D (329V) from the junction of Points B (349V), C (320V) & D (329V) and instead connect it only to point C, at the junction of the 22K resistor and the 22uF/450V cap? I would still keep the 22K resistor, but put it in series with the other 22K?

I did some more testing last night. If I pull the preamp tube, all the controls have an effect on the hum that is still there even with that tube pulled. If I pull either one the EL84s, I still get the same hum, controls have the same effect on the hum. If I pull the phase inverter tube, then there is no hum at all, or any sound. Should I interpret that as the problem being in the phase inverter stage somewhere? I've went over that section about 20 times now...

I'm also wondering if my filter caps are any good. I tried measuring them with a DMM that can measure capacitance, but they read in the very low nF range. Is that because they are in a circuit, so I am not getting true readings? Do I have to physically disconnect them and then measure for capicitance?

Also, how do I know if my choke is working as it should be? I was thinking that maybe the rectifier tube was the culprit, but even if it had a problem, the filter cap/choke combination in theory should filter out the power supply ripple, right?

[csj]

Edited...
Skip this post...had a bad schem copy.
Let me do a little DIY lawnmower work and then a little DIY sprinkler system work then a little DIY bar-b-q first and I'll be back...

[Paul Marossy]

Gee, you're not a do-it-yourselfer, are you?  :lol:

[csj]

Hey Paul,
Yeah, it's always something you know ...if it's not a noisy amp or an nonworking pedal it's the lawnmower's solenoid or the plants not getting enough water or me burning the chicken on the grill.

How did you wire your heater supply?  Is it floating like the schem shows?

As for the choke...why they put them in these smaller amps is questionable anyway. They're meant to better regulate the B+ during larger load currents but this isn't usually that necessary for an amp of this size. What's your PT's current rating? That looks like a Westlab transformer set...correct? As I recall that PT's something like 280-0-280 at 100 or so ma. which I guess could cause a problem if you really push the el84s. You might want to check the ac drop across the choke. It shares the filtering duties at this point with that second filter cap as a sort of voltage divider. Most of the ac drop should be across the choke and then whats left will get filtered in accordance to whatever the Xc of the cap is (which isn't much - compared to the choke). If it comes down to it you can figure out what resistance the choke represents based on it rating in henries and then sub a power resistor just to see if it is, in fact, a matter of the choke not doing it's job. A good filter requires that there is a high resistance to AC while allowing the DC to get through easily (chokes, of course, are great at this). Remember that the resistor now will present exactly the same resistance to AC as is does to DC so along with reducing the AC (actually I guess I should call the fluctuating DC) it will also drop the DC rail voltage too. This might not be a bad thing since it seems to me that your DC voltages down the line...over 300...seem a little high to me anyway.

[Paul Marossy]

No, the heater isn't floating. The West Labs transformer has a center tap on the 6.3V heater supply. I have it currently connected directly to the power tube cathodes. It didn't make any audible difference if it was connected directly to ground or on the cathodes.

I really don't know what the PT current rating is. It's a 15 watt model, which is designed for (2) EL84 power tubes and a few preamp tubes. The center tapped secondary is supposed to put out ~280-0-280. I measured pretty close to that, 285V I believe. The B+ voltages that I measured are real close to what the schematic indicates. There is supposed to be ~350VDC at the power tubes.

The West Labs transformers are replacements for Vox amps and OEM for Matchless amps.

I have wondered if that choke was working or not. I'll have to get some specs on it. How do I check for AC drop? I've never had to do that before...

[csj]

Hey Paul,
I just now got to read that post concerning "more and more OT".  :?  

I guess it's good that we had already moved this into the PM world.
Let me know how it goes...
you'll get it.

Clay

[Paul Marossy]

csj-

I finally got rid of the hum! I tried the metal plate between the power tubes and PT, but that had no effect on the hum. I changed the connection to ground at the phase inverter section and that was more of an improvemnet. I also I tried grounding the volume and tone controls a little bit different. I got a few gator clips and started seeing if one ground point was better than another. The volume control didn't care where it was grounded, but the tone control sure as heck did. So I have a wire running to ground just for the tone control. Oddly enough, I still had a little bit of hum with everything maxxed. The tone control still had an effect on the amount of hum. Then I noticed where one of the leads from the OT primaries was a little close to a ground connection. I moved that over a little bit, and voila!, no more hum. Apparently, a wire to a star ground point can pick up a hum if a HV wire with really rough DC is too close to it. I did not know this was possible before. Boy did I learn a lot on this build!

Anyhow, thanks for all for your help and suggestions (and patience).

PS: I have some pics of the chassis if interested.

http://www.diyguitarist.com/Images/SF-ChassisLeft.JPG
http://www.diyguitarist.com/Images/SF-ChassisRight.JPG
http://www.diyguitarist.com/Images/SF-ChassisTurret.JPG
http://www.diyguitarist.com/Images/SF-ChassisPots.JPG
http://www.diyguitarist.com/Images/SF-ChassisSockets.JPG
http://www.diyguitarist.com/Images/SF-ChassisStar.JPG

Sorry, the pics are a little fuzzy at a large viewing size. Anyhow, the layout isn't the neatest, but it works fine. I got the idea for the screw terminal strips from looking at a DC30 chassis pic.