Plate to Cathode Cap -- what's this??

[Buckeye Sound]

I've been seeking a better sounding overdrive for my amp and have looked at lots of other designs for suggested values etc. I've run across one design for a supposed Dumble amp that uses a 500pf cap between the plate and cathode on a 12AX7 preamp. I've not seen this in other designs, but I'm not real experienced at this, so, anyone know what this does and how it affects the tone? Here's a link to the schematic. Thanks.

http://www.schematicheaven.com/dumbleamps/distorto.pdf

[HollowState]

I'm not real sure just what this circuit is for, but I'm assuming it's some sort of distortion/overload device for a guitar amp. Correct me if I'm wrong please. However, small capacitors used this way will slightly roll off the very highest frequencies by swamping them to ground and through a tiny amount of feedback. This is most likely done to prevent oscillation. Perhaps ultrasonic. Because of their small value, I doubt if you'll actually hear much high frequency loss.

Victor

[James D.]

It does two things in a guitar pre-amp.

1) As Victor says it rolls of the HF above the audio range (way above)

2) It also introduces these frequencies back into the valve via the cathode were they intermodulate with the signal in the valve and change the sound of the distortion it produces - not a lot but enough to help give the Dumble its unique character.

James

[aletheian]

It's a pretty typical guitar preamp mod that you see to prevent oscillation and roll off some highs. You see the same thing on input stages of some Marshalls.

With the Zo of the stage being around 40K or so, the 500p cap should form a 6dB/octave single pole filter with a knee just under 8kHz should it not? That would be audible.

[Buckeye Sound]

Thanks for the replies, I can follow that, and I think I'm going to give it a try on my amp. Will adding this cap have any affect on gain? Also, I've been struggling to get tighter sounding lows, will this have any affect there as well, I suspect not?

And, yes Victor, the link to the schematic is an overdrive model for a Dumble preamp. I'm attempting to model my overdrive in my Music Man RD-50 after this design.

Thanks again

[Miles Prower]

Originally posted by Buckeye Sound
Thanks for the replies, I can follow that, and I think I'm going to give it a try on my amp. Will adding this cap have any affect on gain? Also, I've been struggling to get tighter sounding lows, will this have any affect there as well, I suspect not?

And, yes Victor, the link to the schematic is an overdrive model for a Dumble preamp. I'm attempting to model my overdrive in my Music Man RD-50 after this design.

Thanks again

Looks like a common practice from SS RF practice to prevent parasitic oscillation. works pretty good in that capacity, however, returning the bypass capacitor to the cathode (emitter, source) seems to cause less distortion than simply grounding it. However, it's essential to have the cathode resistor well bypassed, otherwise, that capacitor will cause positive feedback, and either damped or continuous oscillation.

[Wavebourn]

In RF practice it is a positive feedback to cause oscillations (or Q multiplication of tuned circuits if the feedback deepth is not enough to cause oscillations).

[AV8R]

Fi you notice that the cathode is bypassed to ground by 5 uf, then the 500pf is effectively to ground but is easier to wire directly on the tube socket? No

The effective source impedance is about 20K since plate out is about 40K plus the parallel of the plate resistor and following volume control. Approx 16KHz -3dB times 2 because of cascadeing makes that -6dB or about 12KHz -3dB.

[Brian Beck]

the cathode is bypassed to ground by 5 uf, then the 500pf is effectively to ground but is easier to wire directly on the tube socket? No

Of course, the 5uF effectively grounds the bottom end of the 500pF! The cathode only will see about 1/10,000th of the plate signal. Nothing mysterious about it.

[Merlinb]

Originally posted by aletheian
It's a pretty typical guitar preamp mod that you see to prevent oscillation and roll off some highs. You see the same thing on input stages of some Marshalls.

With the Zo of the stage being around 40K or so, the 500p cap should form a 6dB/octave single pole filter with a knee just under 8kHz should it not? That would be audible.

Aha! You are Aletheian Alex and I claim my £5!