JCM900 2100 Power Tube Bias question

[oc disorder]

This should clear the air a bit These parts of the schematic were a bit scattered to say the least.
The bias supply comes from the 40volt CT which also provides the I.C. supply + & - 15v.
It is capacitively coupled to the ac via two 22uF 50v capacitors.
Stupidly it shares 2 fuses with the IC supply so if that shorts it takes the bias and the tubes with it !

[J M Fahey]

Ocdisorder, thanks for the detective work
So they turn a 20+20VAC center tapped winding into a "floating" (from the bias circuit's point of view) 40 VAC single winding, by capacitively coupling both legs.
Interesting.
It does not allow for much current consumption ... which is no big problem for a bias circuit.
Now, it's being only 40VAC instead of traditional Fender/Mesa 50/60VAC means that it *barely* meets bias voltage needs ... which by the way was the idea behind this thread.
As seen, I can very much doubt it can bias 6L6, which need around -52V DC.

[g1]

Thanks OC. I would be a little concerned about those 22uf 50V (C11 & C29) caps feeding the bias circuit. For sure I would replace them before modifying the circuit for a low bias condition. Cap failure in capacitor coupled bias circuits like this can do a lot of damage to power tubes and even output transformers, have seen it more than once in JCM900 and newer Marshall's with cap coupled bias circuits.
Suggest that the OP replace C11 & C29 if only to err on the side of caution.