Capacitor on Powertube Socket - Mesa Mark III green stripe

BesaMoogie

BesaMoogie

Well-known member
Hi everyone,
I recently acquired my first Mark Series amp, a Mark III green stripe long head that sound amazing!

While cleaning all the socket and pots I found a little plastic bag attached to one of the cable inside the amp. Inside was a 250pf ceramic cap and a note saying "250pf 1kV cap across pins 4-5 of EL34 valve".

So assuming the cap was removed I searched for schematics of the Mark III but I have not found this cap in any of them. However, I was able to find pictures of another green stripe showing this cap one the mentioned position above. Oddly enough, this cap only seems to appear on one of the EL34 sockets, the one closer to the OT.

Does anyone know, what`s the purpose of this cap?
Thanks
 

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vertigo said:
part of a snubber circuit to prevent ringing?
Click to expand...
Never heard of that before.
Why do you think this could be one of these? Have you seen it on other amps before? Why is it only on one of the EL34 sockets and what is the reason to remove it as it was done in my amp by a previous service?

Sry for bugging you with this much questions, I just want to understand, whats going on here 😅
 
Revi San said:
Never heard of that before.
Why do you think this could be one of these? Have you seen it on other amps before? Why is it only on one of the EL34 sockets and what is the reason to remove it as it was done in my amp by a previous service?

Sry for bugging you with this much questions, I just want to understand, whats going on here 😅
Click to expand...
I don't know enough about it to give you a full technical breakdown of how it works, but my understanding is that there are cap/resistor combinations that can be put on the plates of the output tubes near the OT leads and they attenuate high frequency oscillations that might otherwise lead to instability.
 
That's exactly what that cap does - high frequency negative feedback for that tube to stabilise it's gain.
Mesa tweaked the circuit almost constantly so not surprising that it's not in a schematic. It could have been related to the tubes they were using at the time, and that end tube being susceptible to oscillation - likely as mentioned something to do with the leaddress.

You dont need it if there's no problem with the amp. It's probably not audible to put it in though so not hurting anything.
 
Thanks for the explanations guys!
The amp had four TAD 5881wxt installed, so probably the cap was removed while changing the tube type.
How do those high frequency oscillations sound when they appear?
 
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