Grid to plate bypass caps

[radiocaster]

I'm not sure what they're really called, but that is how they are connected. Usually 10-20 pF and not too common in amps, but I've seen them on quite a few schematics.

I once asked in a thread about the Ramparte if they can be disconnected, and someone said the amp wouldn't work anymore.

Of course it didn't make sense looking at the schematic, but I didn't want to really try it on the Ramparte because it uses pc boards.

I did eventually try it on another amp that had it soldered straight on the socket.

It does seem to affect the gain as far as I could tell, but in a very subtle way and not much at all.

Can anyone give me a more technical explanation about them?

 

[Tom Kamphuys]

Chapter 1.22 discusses the interelectrode capacitances.

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

I think what you mean is depicted as Cga over there. By adding a capacitor there, you increase Cga. "This input capacitance will form an RC low-pass filter with the source impedance, thereby cutting the high frequencies. This can be a hindrance or a help, as will be seen throughout this book."

 

[tubegeek]

It's there as the quote above says - to limit bandwidth. If the caps are small enough you won't hear any difference, they are there to prevent ultrasonic oscillations which can create a sort of runaway effect that causes many different kinds of trouble.

 

[radiocaster]

Chapter 1.22 discusses the interelectrode capacitances.

http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

I think what you mean is depicted as Cga over there. By adding a capacitor there, you increase Cga. "This input capacitance will form an RC low-pass filter with the source impedance, thereby cutting the high frequencies. This can be a hindrance or a help, as will be seen throughout this book."

That makes sense because capacitance adds in parallel, but I think it's more complicated because the signal takes two paths then comes together.

Also you can use a simple plate bypass cap if you want to cut some high end.

There are also plate to cathode "stability" caps, they've been briefly discussed on the forum.

 

[Tom Kamphuys]

What you intuitively feel is called Miller effect/capacitance and is also explained in 1.22.

The tube itself already has these interelectrode capacitances. You only make the capacitance larger by adding a capacitor. No new effects/components are introduced, only a value changes (after combining of the capacitances as you stated yourself).

There are more ways to achieve less bandwidth. I lower the input grid resistor to 10k to reduce noise and add a capacitor from grid to ground to still get 20kHz bandwidth as advised in the same book (Designing Tube Preamps for Guitar and Bass) in chapter 4.6 on page 88.

 

[clydethecat]

It's there as the quote above says - to limit bandwidth. If the caps are small enough you won't hear any difference, they are there to prevent ultrasonic oscillations which can create a sort of runaway effect that causes many different kinds of trouble.

See a lot of those in older Audio Research preamps, yeah it's for stability and RF protection. The 5-20pF usually seen there is well out of the audio range isn't it?