Marshall w/ melted caps next to tube sockets

[VintageGrooves]

Hi. My name is drew, and I fix (mostly solid state) amps at a shop called Vintage Grooves. Anyway, the other day a Marshall TSL602 comes in with the ht fuse blown. The tubes tested fine, but one of the el34s was visibly arcing at high volume, so I replaced both of them. The amp still popped the ht fuse, so I popped the hood and saw that two of the contacts on the back of one of the el34 sockets had arced. I cleaned the arc as best I could, hooked it all up, and voila - all is well. Well, for a few minutes, anyway. However, after a few minutes at high volume the amp began to have that "something is burning on the pcb" smell. I popped the hood again, and noticed that the little ceramic disc caps right next to the el34 sockets (bottom left and right of pic 2) had black oooze under them, like they were getting way hot. (see pic three - the cap is under the grid resistor) So, I'm wondering - do I just replace the caps (as though they were originally damaged by the bad tube) and hope for the best, or do I consider that possibly there is another problem somewhere? As I mentioned, we mostly do solid state stuff here, so I'm really stumped on this one - any help at all would be greatly appreciated!
Sincerely,
drew

[Enzo]

C26 and C27 are little 22pf 500v disc ceramic caps. When they burn up, replace them with 1000v caps. Or leave them out if the amp seems stable without them. They are wired between plate and screen on each tube - pins 3 to 4. They fail short, and then they short the plate and screen together, making it a triode essentially. Hum noises and low gain result.

Usually we get tube guys who are afraid of solid state. it is refreshing to have a SS guy wondering about tubes. In either case, troubleshooting is troubleshooting, and you seem to be doing OK.

[VintageGrooves]

Thank you!

Thanx, Enzo - you rock! I finally found a schematic on the drtube site, and see what you're saying. Two more quick Qs, if you don't mind: 1) by "stable", do you mean that the amp doesn't oscillate? Also, if it's stable w/ the tubes I just put in , will it necessarily be stable w/ others? 2) I've never seen these caps on older tube amp schematics I've looked at - do you know why they're there?
Again, many thanx.
drew

[Enzo]

They are there for... stability.

Yes, not oscillating, not making parasitics on peaks. WHich is also oscillating, really.

WOuld other tubes be less stable? Perhaps. No guarantees in this life.

You can operate the amp withiout them, and in fact I would, just to verify that it works OK otherwise. I do replace them, I have a bag of suitable parts for it. If I didn't have tham, I would be seeing if they were necessary or not by testing.

[Jasonp]

Do you think this cap effects tone or dampens the feel or dynamics of the amp to gain stability?

[Enzo]

Not that I can detect. I certainly would not remove it if it were not bad. Not replacing it is more a matter of expedience than tonal nirvana.

[Jasonp]

Not that I can detect. I certainly would not remove it if it were not bad. Not replacing it is more a matter of expedience than tonal nirvana.

Can you please explain to me what that cap is techinally doing for the circuit in that spot? .. voltage wise? Is there signal runing through it?

[Enzo]

it couples the plate and screen together at really high frequencies. This reduces the available gain at those freqs. This is way above anything coming out of the guitar.

On the screen side the boltage is more or less the B+. On the plate side, the very large plate signal is added to that. In my view, the 500v cap is not enough for the occasional large spike that comes flying back from the transformer

[Chris G]

it couples the plate and screen together at really high frequencies. This reduces the available gain at those freqs. This is way above anything coming out of the guitar.

Well, I'm not so sure ... I remember doing some corner-frequency math involving the caps in my ancient CBS-era Twin, which had them, and I seem to recall they rolled off everything above 3K or so.

I mean, guitar amp speakers don't put out too much above 5K, but still ... clipping them out made everything noticeably less dull.

[52 Bill]

Well, I'm not so sure ... I remember doing some corner-frequency math involving the caps in my ancient CBS-era Twin, which had them, and I seem to recall they rolled off everything above 3K or so.

Those caps in the Twin were used differently and were much higher in value than the ones that are being discussed here.

[Enzo]

Yes. Chris if you look at the Fender, they are MUCH larger caps and are wired from grid to ground. These are small 22pf caps wired screen to plate.

[Jasonp]

Very intersting! Thanks for the explain guys.
I would bring my amp to you guys in a heart beat and pay you guys top dollar for what ever my amp needed or fixed.

[Jasonp]

Yes. Chris if you look at the Fender, they are MUCH larger caps and are wired from grid to ground. These are small 22pf caps wired screen to plate.

what does pf stand for? I take it this value isn't very large..?

[Enzo]

pf mean picofarad.

A 0.001uf (microfarad) cap is also a 1000pf cap. WHich is also a 1nf cap (nanofarad)

SO the 22pf cap we speak of here is if you prefer, a 0.000022uf cap. Yes, very small.

In the old days when I was learning, we never used "pico" for anything, the term then would have been 22uuf or 22mmf, either of which meant "micro-micro farad." If you see mmf or uuf on an older schematic, that is what it means, it means picofarad.

[Jasonp]

pf mean picofarad.

A 0.001uf (microfarad) cap is also a 1000pf cap. WHich is also a 1nf cap (nanofarad)

SO the 22pf cap we speak of here is if you prefer, a 0.000022uf cap. Yes, very small.

In the old days when I was learning, we never used "pico" for anything, the term then would have been 22uuf or 22mmf, either of which meant "micro-micro farad." If you see mmf or uuf on an older schematic, that is what it means, it means picofarad.

wow, amazing small value for sure! Now does this mean that a very small (current?) is allowed by the cap?