Biasing an old Lucky 7 Mk I

[Caevan O'Shite]

Hey, e'erbuddy!

Biasing my Fuchs Lucky 7 (old, original, 'Mk I', Gain/High/Mid/Low controls ONLY) with a DMM, nominally looking for -50mV at pin 8 of the EL34, as recommended. Using the same JJ tube as the amp shipped stock with when new.

I found it had drifted to -43mV; under-biased/hot, correct? Just for experimentation's sake, I re-set it to -52mV, a little bit over-biased/cold, right? I think I liked it better before.

What's a safe bias range here with this amp, minimum/under-biased to maximum/over-biased? I want the amp to sound warm and fat and full, with the Gain (its sole volume-control) set to about "3:00 O'Clock" (most of the way up) using the guitar's volume-controls and my "touch" to "play the amp" and vary from clean to overdriven and distorted.

Or... do you recommend that I set it to -50mV and ONLY -50mV, no higher, no lower?

And... if I score a nice NOS or "ANOS" EL34... ?

 

[FUCHSAUDIO]

I would use the bias calculator on the Weber site: http://www.webervst.com/tubes1/calcbias.htm .

This allows you to plug in plate voltage and currents, and make sure the amp is running in a safe zone. 56-Ma on an EL-34 is true class A with a 400 V b+ (which is what is in that amp). The supply might sag a little, but it's a safe operating point. You can go down in bias until you start to hear distortion (at which point it's entering class AB). Slightly above that is Class-A operation, which is required when an amp is single ended.

Have fun !

 

[Caevan O'Shite]

Thanks, sounds like a good idea! And sag- when pushed hard- is plenty alright by me; I want a little sag and compression on single-notes and double-stops when I push it hard with the guitar's volume wide-open.