what's the role of 47pF between plate and cathode

[reroe]

hi there

though i'm not good at electonic things or tube stuffs
luckily i was able to finish some projects

now i'm planning to build laney GH50L clone
(i always like IA and paul gilbert tone)

while analyzing schem., i found small cap between 1st stage plate and cathode. it doesn't seem normal cap like decoupling, bypass..
exactly what's the role of this 47pF cap? (i attached picture of it)

and another question...
from an attached pdf, in the schem (page 3), that cap places on V1a but in the main PCB layout(page 4) that one connected to V1b.
it's so confused, where is the right place for that?

please help me
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[tubeswell]

HF rolloff

[JamesHealey]

Rolls off a high end, 47p will be well out of guitar sounds. It will be there for stability in the circuit

[reroe]

thanks for reply.

one more question...
are there any differences whether rolloff cap is placed on V1a or V1b?

[PCollen]

AS another twist on this, Mesa Boogie is 'famous' for shunting plate resistors with very low value caps . The Nomad 55 has 120p caps shunting the PI plate resistors, and other amps have .001uF's either across their pre-amp tube plate resistors, or from plate to ground.

[labb]

I have always been under the impression that that cap was to prevent high frequency oscillation which is generally caused by poor lead dressing.

[David Root]

That is correct. Another way is to attach it across the two plates as shown in some older Fender schematics. Sometimes it is 100 pF.

It works by interacting with the plates' resistance and plate load resistors to roll off highs above the audio range caused by parasitic oscillations.

[Structo]

Sometimes called fizz caps as well.
As in "anti-fizz" cap.

A good way to tap off some high end where you don't want it.

[PCollen]

That is correct. Another way is to attach it across the two plates as shown in some older Fender schematics. Sometimes it is 100 pF.

It works by interacting with the plates' resistance and plate load resistors to roll off highs above the audio range caused by parasitic oscillations.

There is a whole cult surrounding that cap, it's value, and it's contribution to EVH's 'brown sound'.

[David Root]

I didn't know that. I have heard everything else from totally stock to running the wall voltage down thru a Variac to the amp.

47 pF isn't going to do anything in the audio range, would have to be much bigger.

Can't recall the specific formula right now for the cap across the plates, maybe someone else can.

The other one, cap from plate to ground is C=1/(2pi*R*f) where f is the desired 3dB down rolloff frequency.