Biasing EL34's

[daz]

Right now my 50w marshall style build is at 412V/40Ma. Would you go lower on the current? At 35Ma it's 418V. Where do you thinkn the best longevity/tone balance is?

[pdf64]

That's your call really! With plate voltages that low, assuming that you are using 1ohm cathode resistors to measure current and so will be including screen grid current, with 30 watt tubes 70% would give ~55mA. So go twiddle and compare - if you can't hear the difference between 35 and 55mA, then leave it 35, or try lower. Most people seem to prefer between 50 and 70%. See Aiken for a mine of great info -
http://www.aikenamps.com/Why70percent.html
Peter

[Steve Conner]

There's a lot to be said for the old radio ham rule of thumb: "Crank it until you smell burning, then back off a quarter turn".

I put 6550s in one of my homebuilt amps a while back. They sounded great biased to 60mA each at 475V, but they gave off so much heat, the whole amp seemed in danger of cooking. I ended up turning them down to 45mA, more out of concern for the other components than the tubes themselves.

[daz]

Yeah, i pretty much already knew what to do..bias them till they start to redplate then back off a bit. Its what i've seen said many times and i suppose it makes sense. Tho i'll take it a bit further and say back off till you begin to hear a tonal degradation then go back up a bit. guess it's not all that critical, but i just wanted to see if anyone had anything new to say. I guess I'm getting so happy with this build i'm just a bit anal about making sure everything is right.

[jrfrond]

The difference with 6550's in Steve's case is that they can take the abuse. EL34's are known for thermal runaway issues, so it is always best to run them a bit cooler. Even at normal levels, you can often smell them cooking and see the plates going cherry at the bends in the plate structure in a dark room. So, as was said, this is your call. EL34's are known for their impromptu light shows!

[daz]

I have a pair of 6550's by the way, and in fact a old NOS pair of RCA'a that are great sounding. Can i just swap them with a bias change? Will they not be optimal with a 4k primary?

[jrfrond]

It would still work with that transformer.

If you really want to experiment and get the best sound out of 6550's, try running them cathode-bias. 6550's sound really sweet in that mode. They overdrive and compress more.

[R.G.]

Tubes age and run away because of heating.

You can cope with heat two ways: make less of it, and remove it better.

You're on the "make less of it" side. You could also put a low power, low speed quiet fan in there blowing over the power tubes. My experience has been that even a little air flow will run the temps on the power tubes down to where you will be able to remove them with your naked hand. Glass is not all that great a thermal conductor, so there is not a concomitant reduction in the internal temps, but it may be enough to get the tone you like at the same time as lengthening tube life.

[daz]

Thanks all !