Express bias

[Tubetwang]

should i re-bias when using different pre-amp tubes in V1-V2-V3?

Amp is now biased for 3x12AX7...

Popping in 12AY7 in V1, 12AT7 in V2, 12AX7 in V3...re-bias or not?:shock:

Thanks!

[Firestorm]

That's a matter of taste. There's a fairly broad range of "safe" bias settings for your output tubes. So if you find that changing the preamp tube complement makes the output too thin or too clean (or too dirty), you could experiment with changing the output tube bias. One caveat: if you already have the output tubes biased at 70%, you do NOT want to bias them any hotter.

[Tubetwang]

Thank you for fast reply Firestorm!!

[doctord02]

Power tube changes need rebiasing. Preamp tubes do not - with the caveat, that the Ck/Rk for a 12ax7 may not be the ideal choice for a 12at7, 12au7 or 12ay7, tho they generally can be swapped willy-nilly...

Preamp tubes are self-biasing (ie cathode biased) unlike the power tubes.

[Tubetwang]

Thanks Doc!

Exactly what i wanted to know...

I like the fullness and authority of my Express and Liverpool clones but...find them a little...huh... loud in my camper...

[Firestorm]

I like the fullness and authority of my Express and Liverpool clones but...find them a little...huh... loud in my camper...

You'll be hard-pressed to do much about the loud part. Extreme bias settings do impact output volume, but usually at the expense of tone or tube life. The power output is largely a function of the power supply itself.

A deliberate impedance mismatch (a larger load) will usually reduce output power, but you can only do it safely if you're sure the OT is up to the task. It was typically safe in Fenders, but not safe in Marshalls (whose transformers were barely sufficient for the rated load.) I can't speak to the robustness of any of the transformers used in Trainwreck-style amps. Anybody know?

[Firestorm]

Double post.

[Tubetwang]

Thanks!

[Allynmey]

FWIW, most of the real 'wrecks I've seen were biased between 40-45mA and one was set at 48mA right from Ken's shop!

[Tubetwang]

Thanks Allyn!

Mine is set at 45.

I had read that Glen had is at 45...

Sounds great to me...