Bias drift. How much is acceptable?

[Blair]

I have a pair of monoblocks configured as two PP amps on each chassis with the OPT secondaries in parallel for ~ double the power. The amps work very well, and sound great.

If I fire them up and let them sit for a minute or two and then bias all the tubes to 33mA or .330V, they will hang there for a while. If I come back in a few hours, some of the pairs will stay locked in, but some of the other pairs appear to be unbalanced as much as 20-30mV. i.e. .320v and .343V or something like that.

They are Chinese 6L6GCs. Is it possible that it is just the tubes? Is it something I should worry about?

Thanks!

Blair

[kheper]

If I fire them up and let them sit for a minute or two and then bias all the tubes to 33mA or .330V, they will hang there for a while. If I come back in a few hours, some of the pairs will stay locked in, but some of the other pairs appear to be unbalanced as much as 20-30mV. i.e. .320v and .343V or something like that.

They are Chinese 6L6GCs. Is it possible that it is just the tubes? Is it something I should worry about?

Thanks!

Blair

If you do not have individual tube bias, you may wish to implement it. When my st-70 bias drifted, new caps and resistors in the bias circuit helped to stabilize it. Some tubes drift more than others. The bias on my JJ EL-34s drifted. My newer Tun-Sol KT66s do not as much. Your drifting may have multiple causes. To test, re-bias all of the tubes when the amp is hot.

[20to20]

Is it possible that it is just the tubes? Is it something I should worry about?

Thanks!

Blair

Check the actual biasing voltages for drifting, instead of just the currrent. The tubes may be throwing a little DC onto the grids.

20