Capacitor between screen and control grids

[aiyiadam]

The photo is from a Mesa Mark III Blue stripe.. There is this one 30pf capacitor between the screen and control grids of one of the EL34. That confused me, because I don't see that on the schematics. Does anybody know, what the cap is doing there?

[pdf64]

I guess it may cause a slight compression effect on very high frequencies. ie a lower signal levels it won’t do much, whereas at high power output, when g2 current draw increases, it will have more effect.

[Helmholtz]

I doubt that a 30p cap will have an effect in the audio range

They probably needed the cap to avoid RF oscillation.

[Enzo]

Agree^^^, I suspect it is for stability.

[aiyiadam]

Thank you for the replies.. The interesting thing is that this type of cap is only on this tube socket, which is the closest one to the OT. Maybe the longer OT lead (~15cm longer) might already have a capacitative effect on the other EL34. I don't know.

Anyways, the cap looks damaged. And since this one isn't in the schematics, I think I could just remove it.

[Mick Bailey]

Quite a few MESA amps have disc ceramic caps to prevent oscillation of some sort and in many cases they can be removed without ill effect, and sometimes removal can improve the top-end, depending on their size and location. I wonder if the designers had experienced some borderline instability with some amp examples and decided to fit the caps as a blanket remedy just in case.

[Helmholtz]

And since this one isn't in the schematics, I think I could just remove it.

IF that wiring is OEM, I would replace the cap.

[loudthud]

I remember a Mesa amp (Mark something or other) that had a wire from one of the screen grids of the output tubes that was wrapped around (one or two turns) the outside of a coax that went from the input jack to the preamp board. WTF colon left parenthesis.

[Helmholtz]

I remember a Mesa amp (Mark something or other) that had a wire from one of the screen grids of the output tubes that was wrapped around (one or two turns) the outside of a coax that went from the input jack to the preamp board..

That makes an individually tunable capacitor of some pF.

Seems some Mesa amps have RF instability problems.

[Leo_Gnardo]

I remember a Mesa amp (Mark something or other) that had a wire from one of the screen grids of the output tubes that was wrapped around (one or two turns) the outside of a coax that went from the input jack to the preamp board. WTF colon left parenthesis.

And some had a wire just sticking up from the PC board. These things are called "gimmicks" and are employed mostly in RF work - which I know little about. I expect they're trying to quell some pesky ultrasonic oscillation.

Do we have any ham DIY'ers, what say you about gimmicks ?

[Helmholtz]

I know a bit about RF effects, though I wouldn't consider myself an expert.

That wire sticking up might produce a capacitance in the order of 1pF toward nearby components/wires/traces.
I don't think it has a useful effect by itself as we're are not talking GHz here.

But it might be there in case an amp needs wrapping it around a close-by wire or component to achieve stability.

[g1]

I think some of them also had a little spiral dead end trace on the board that acted as a gimmick cap.

[Leo_Gnardo]

I think some of them also had a little spiral dead end trace on the board that acted as a gimmick cap.

Sneaky, that.

[Enzo]

Gimmicks were more common in the earlier days. I recall some Mesa model with the little inch of wire sticking up. It mattered which way the thing leaned. It quelled some hum. I never bothered to find out why. When amps go into RF, they often still work, but the sound is weak and hummy/noisy. Cure the RF and that hum goes away. The other possibility was that the little antenna picked up enough hum signal from an out of phase point nearby that it cancelled the overall resulting hum.

On th Classic 30, there is a gimmick at the input tube. From plate to grid of V1 pins 1 and 2. Look on the board layout drawing for the trace pattern there. It looks like Lombard street..

[Mick Bailey]

The first time I saw the Mesa wire thing was when I had one for repair that had a piece of purple wire soldered to the PCB that didn't go anywhere and wasn't on the schematic. I thought it was some kludge and removed it. Years later I had another amp with the same piece of wire in the same position, so left it alone. Only then did I realize it was factory fitted.