Need Help Finding These Replacement Caps!

[gmoon]

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.

[R.G.]

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.

[Paul Marossy]

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!  

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.  
(For the ones I really suspect are the problem here)

[gmoon]

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.  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...

[gmoon]

Which goes back to where I began! I can't get exact replacement caps, they apparently don't exist anymore.  
(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...

[amptramp]

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.

[Paul Marossy]

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...

[amptramp]

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.

[Paul Marossy]

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.

[Paul Marossy]

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.

[Paul Marossy]

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?