Marshall JCM900 model 4100 bias missing

[Bigdrums]

This amp in for repair has no bias voltage present. On page 3 of the attached I find where the -50 v is supposed to appear between R26 & R27, but nothing there. 500 v are present at R22 & R23, but
C9 & C10 prevent DC from getting to R26 & R27, I believe. So where does the voltage needed for the bias come from? jcm900_dualrev_100w_4100.pdf

[Enzo]

You found the spot between R26 and R27, so follow the line down to the bias supply.

High voltage AC comes from the transformer through C12 and then rectified by the diode D1. There is then a negative DC on the cap . Then R29, R28, VR2 all form a voltage divider from that point, from which your -50v. Track it back to teh source.

[Jon Snell]

D1 = 1N4007. C15 = 47n Class X 250volts AC
If the bridge rectifier has no load or is faulty, there will be no HT or low bias voltage as there is no route for the high voltage to return to ground.

[g1]

Make sure your circuit board matches the schematic. There are several versions and the component numbers are different. The cap that feeds the diode is called C12 some schematics, but C15 in other revisions, just for one example. (pg.3 vs pg.4 of posted schematic)

[The Dude]

FWIW: I have seen that coupling cap fail often enough. I'm hesitant to call it a common failure, but............

[Bigdrums]

Dude: That's exactly what it was. No output yet, but bias all there!

[Bigdrums]

C15/C12 called out as .047uf has now been changed to .1uf. Bias is -60v measured at pin 3 on the EL34's, set at the lowest on the bias trim pot, too much I know.
There is virtually no plate dissipation shown on the bias probe meters.
And the line current draw is low too.
I wouldn't think changing the cap value would change the negative bias voltage but maybe I'm mistaken?

[Jazz P Bass]

Changing that capacitor value changes everything in that circuit.

[Jon Snell]

Changing that capacitor value changes everything in that circuit.

1/2πFC comes instanly to mind!
Bigdrums, please read up on basics.
In the old days of mechanical watt meters in the home, we used to call the capacitor fed power supply, 'Wattless Power Supply' because the electricity meter wouldn't go round!

To increase the capacitance will increases the reactance which increases the cuurent and increases the voltage.

[Enzo]

-60v on pin 3 of the EL34? I hope you mean pin 5?

Turning the bias adjust end to end, what range of bias voltages results at the tubes?

[g1]

To increase the capacitance will increases the reactance which increases the cuurent and increases the voltage.

I think you meant decreases the reactance. Which in turn allows more 60Hz through to the bias diode which makes the bias voltagew more negative and makes the power tubes run colder.