R/C values for cathode bias EL34's?

[iknowjohnny]

i decided to try CB on my otherwise marshall 50 stype back end. I like the tone so i decided i'll probably leave it like that. However, while it would seem the values don't need to be high, the matchless i was looking at with EL34 shows the caps as 150v. Any reason i need anything even remotely close to that? I'm using 10 watt resistors and 25 v caps. What would you all suggest?

[Jana]

Are your transformers rated to handle the current levels that class A will require?

[iknowjohnny]

Yes. By the way, the amp started out CBiased. I changed it to fixed because i was having other issues, and fixed bias seemed to help a bit. Much later the preamp was changed to the point that the issues i thought fixed bias would help were cured elsewhere, so i thought i'd try CB again and this time it sounds better than fixed.

[iknowjohnny]

Along with the original question, could anyone also answer this....what are the disadvantages of using no bypass caps? I seem to prefer the tone that way, but if i recall they are there to stabilize the bias. Any issues if i don't use them?

[Jana]

if everything is matched, tubes, signal to the tubes, etc. that bypass cap is not needed or doesn't matter since one side is always in 180 degree opposition to the other. If one tube is slightly mismatched from the other or the signals from the PI are slightly off, then that cap bypasses any differences.

[iknowjohnny]

Thanks. now any idea about cap voltage value in case i find another uf value that i feel sounds better than no cap at all? how bout R value....think 10 watts is enough?

[Jana]

measure the dc voltage on the cathode resistor. The cap voltage is going to have to be higher than that. I would go twice as high to allow for some wiggle room.

Do the math on the resistor, what's the ohms? what's the volts dropped across it? Figure out the watts being dissipated and go at least two or three times higher than that to allow for a cooler and happier part.

[iknowjohnny]

OK, thanks. I guess i'll just go with the big finned 30 watt resistors i first used. I wanted to use smaller because theres less room than when i first built it. But i'll fit em in. At this point i'm pretty sure i won't be using any caps, but if i do i'll go 50v.

[modern]

Have a look at some datasheets.

The Philips Datasheet from 1969 for the EL34 seems like a good place to start and it's available at the Dr Tube website:

http://www.drtube.com/tubedata.htm

[Zippy]

Are your transformers rated to handle the current levels that class A will require?

"Cathode bias" does not put the amp into Class A.

Not all cathode bias amps are Class A.