bias for mesa .50 caliber

[JC@]

Should I bias this amp as a real class A amp?
Or should I keep it in AB class bias range...
Thanks

[Wild Bill]

"You'll be sor-ry!!"

Should I bias this amp as a real class A amp?
Or should I keep it in AB class bias range...
Thanks

Have you looked at your plate voltage? When we bias an amp true class A we have to pay attention to the max plate dissipation figure. Class A amps run much lower plate voltages so they don't cook the tubes at idle currents high enough for class A.

I just looked at the schematic and I see 436 vdc on the plates and 366 vdc on the screens. Boogie obviously ran a lower screen voltage to run the tubes a little cooler so they would survive.

To run class A I would guess you should be looking at about 300 vdc on those plates.

http://www.aikenamps.com has some great FAQs on class A.

Wild Bill

[JC@]

Wild Bill,
Lowering the screen voltage must help a lot, because this amp came to me with a bias of -17.5 volts (the schem calls for -16 volts on the grids), and that indeed push the tubes to dissipate 12 watts at iddle.

[JC@]

Ok, so should I?

Should I then just follow mesa's guide?
That is, keep them at approximately -16 volts on the grids while trying not to exceed a 12 watt dissipation per tube?
Or should modify the bias circuit and forget that pseudo-class A

[JC@]

Funny little amp, just to call it something nicer of what I really feel about the mesas from that era.

If I try to bias a bit cooler, the voltage on the plates rockets to 490 volts.
So there's no way to avoid cooking the poor tubes.

Plus you can't tweak the bias circuit much at all, because as you do you may also vary the bias supply for the phase inverter!