Tube Works RT-4250 BK Butler era

[65ampman]

Anyone have a schematic for this or know something about the idle current when operating normally? After replacing the shorted power transformer, everything is up and running. B+ is approx 470v and bias voltage is -45v and I am measuring 63mA per tube at idle which is way too hot for class A/B. There are no bias trim pots and the bias supply appears to be zenor regulated. Any help is appreciated.

[g1]

Should be a 47V zener there, that may be close enough but check for AC ripple on the bias filter cap.
A couple errors on the schematic, the circled connection and the bias filter cap polarity, maybe more errors:

[The Dude]

I'm not sure -45V is going to get you where you need to be in regards to safe bias. You may have to do some modifying. BUT FIRST, is that measured at the tubes or before the 10K resistor?

[vintagekiki]

The idle current value depends from tube to tube. The guaranteed value of idle current is only when using original factory tubes (Part no ... ...)
To reduce idle current, a negative bias for the output tubes should be increased.
Replace your existing 1N5261 47V zener diode with one from the 1N526x series that has a higher zener voltage to get smaller idle currents.

https://html.alldatasheet.com/html-pdf/41785/SEME-LAB/1N5261/645/2/1N5261.html

[glebert]

How far negative can that supply even go? That is a voltage doubler after the rectifier, correct? With a 15VAC transformer it seems to me like it may be struggling to get to -47V, but doublers are not something I have much experience with.

[vintagekiki]

You're right. Now I find that voltage doubler is powered by 15VAC transformer.
Definitely no escape from horrors. The voltage doubler should be converted to a voltage tripler, and with the choice of the zener diode set idle current to the desired value.

[glebert]

Don't know if this is a good or terrible idea, but could this be converted to a hybrid bias by putting a cathode resistor in place? Seems like it wouldn't need to be too big to get a bit of positive cathode voltage to reduce the idle current.

[nickb]

You're right. Now I find that voltage doubler is powered by 15VAC transformer.
Definitely no escape from horrors. The voltage doubler should be converted to a voltage tripler, and with the choice of the zener diode set idle current to the desired value.

It is a sort of tripler as the clamp diode of the "doubler" is referenced to the negative rail of about 19V. This makes the peak voltage at the junction of the 47uF and the clamp diode 60V or so. Therefore you could simply install a higher voltage zener to get you a more negative grid bias but it could take a while to charge up as the voltage gets higher. It would work much better ( less ripple and faster charge) if you were to add a 47uF 80V from the rectifying diode to ground.

[vintagekiki]

Fixed bias is fixed bias. Hybrid bias with cathode resistor hm hm. Bias is not very stable, knows depends from tube to tube.
Perhaps the winning combination is reduce voltage g2 to value between 1/2 to 2/3 by means of parallel resistor from Uc = +450 to gnd (similar as Fender 70 series).

https://el34world.com/charts/Schematics/files/Fender/Fender_cbs_70w_mstrvol_pullsw_super_pro_bmstr_rev.pdf