Plate resistor hiss bypass cap: Value?

JJman

Member
Messages
994
I want to experiment with bypassing the plate resistor on the 1st stage of my hi-gain amp. It's a 100k with a 1500ohm cathode. Plate has 160V on it. PS node is 240V.

What value range of caps should I try? Any success stories?
 

Ronsonic

Member
Messages
3,303
You can put 10-ish pF across it - just rolls off highs.

Not uncommon to see in high gain amps. Peavey does a gimmick with PCB traces on some models.
 

hamfist

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1,594
Bypassing the plate resistor is usually a quick fix used in amps that have a rediculous amount of gain. It helps to control high frequency oscillation. If the amp isnt squealing, or doing anything else funky, I recomend you don't add one. If it is freaking out, and you're trying to control oscillation problems, I recomend you try lead dress changes first, shielded wire second, and if that fails, the plate resistor bypass as a last option. If in fact you need to use a bypass cap to calm it down, .001uf would be the maximum value I'd consider, preferably lower. The Ideal value will be the smallest value that eliminates your problem, but as I said, this should be your last resort.

Cheers!!
Jade

A sensible answer. If you are going to try it, start at about 100pF, and only go higher if you are still having problems.
 

JJman

Member
Messages
994
This is the last resort since I've already selected the lowest hiss tube from 10 (NOS Mullard) and the lowest hiss resistor (3watt MF.) No oscillation problems nor squeals. I'll try my 10, 50, and 100 and see what happens.

Ignoring harmonics, what is the highest frequency an electric guitar puts out? I want to scope it to see the results on that. Maybe finding a happy medium.
 

Blue Strat

Member
Messages
30,686
This is the last resort since I've already selected the lowest hiss tube from 10 (NOS Mullard) and the lowest hiss resistor (3watt MF.) No oscillation problems nor squeals. I'll try my 10, 50, and 100 and see what happens.

Ignoring harmonics, what is the highest frequency an electric guitar puts out? I want to scope it to see the results on that. Maybe finding a happy medium.

If you ignore harmonics and cut off at the highest fundamental frequency, the amp will sound like "ass".

Low E so multiply X 2 for each E above that.

Using a cap to cut noise is a really bad idea if tone means anything.
 

SatelliteAmps

Member
Messages
6,168
This is the last resort since I've already selected the lowest hiss tube from 10 (NOS Mullard) and the lowest hiss resistor (3watt MF.) No oscillation problems nor squeals. I'll try my 10, 50, and 100 and see what happens.

Ignoring harmonics, what is the highest frequency an electric guitar puts out? I want to scope it to see the results on that. Maybe finding a happy medium.


Are you doing this just to remove hiss? If so, then there are much better places to go after hiss and noise than using a bypass cap on the plate resistor.

For that matter, are you sure the hiss is coming from the plate of one tube?

Fundamental frequency range hits a little over 1k hZ depending on the actual instrument. Harmonics go way higher.
 

donnyjaguar

Member
Messages
4,194
JJMan, are you sure the solution may be just to reduce the gain? In my experience, "high gain" amps have too much gain for playing even the heaviest of music.
 




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