Crate Vintage Club VC3112B Modifications, Questions of

[hewo]

On channel high gain, there is an ecc83 stage in class A mode with a cathode bias resistor 4.75K without a cathode bypass cap.
Question: In order to augment this stage with a cathode bypass cap, you need to know the working voltage between the cathode and earth.
Is it permissible to ignore grid current and calc the triode current merely by calc the plate resistor current and applying this same current to the cathode resistor value to obtain the cathode working voltage?
I calc 233-189=44volts, current thru 100K plate resistor=44/100,000=.00044ampere, so voltage cathode=.00044*4750=2.09volt
Thus doubling voltage, the cap must be rated minimally 4volts?
I recall small capacitance must be padded with extra working voltage to alleviate failure.
If this is so, selecting 474 capacitance requires minimum working voltage 50wvdc?
Is 100wvdc overkill for .47uf?
there is hardly any motherboard room to augment the cap, maybe underside solder landings.
last, is it sound conscience prudent to use nonelectrolytic here, or does it matter at all?
the prior stage utilizes factory 16wvdc tantalum as its cathode bypass cap, tantalum electrolytic

[g1]

Yes, you can use your voltage drop across plate resistor to calculate voltage across cathode resistor.
If it's 2V on the cathode, then 100V is way overkill. 16V like the other stage should be fine.

[Jazz P Bass]

I would think that your main concern is the fact that the working voltage minimum must be met.
Going overboard will only affect the size of the capacitor.

0.47 capacitors are available in electrolytic.

Why are we attempting to bypass the cathode of that stage?

[loudthud]

In a high gain channel you can get into trouble when you bypass the cathode resistor in a cold clipper stage. What happends is a sustained note or power cord will shift the operating point and the charge on the cathode cap will drift. It won't have enough time to recover before the next note. That's the beauty of the cold clipper, no recovery time. If you want more gain, just reduce the value of the cathode resistor.

[J M Fahey]

Agree and add: there's also another 2 very important functions for the so called cold bias triode:
1) it clips very unsymmetrically and that is good.
2) it protects the next stage, which is usually overdriven to death, from horrible blocking distortion.

Yet all armchir designers/modders start by bypassing it and feeling very clever.

Never think that Marshall/Soldano/MB/Peavey and in general all high gain monsters "waste" a tube in that function ..... maybe they know something after all