Vox AC50CP guitar head

[Bucket Head]

Hi guys, just wondering if somone can give me a bit of advice. ive changed the valves on a AC50CP head and am looking to bias them. i Cannot find the schematic for this amp but have found the 100 watt version.

It says that the bias voltage is 75mV and that the current should be 38mA ish

Im just a bit puzzled as the test points on the 50watt and 100 watt appear to be exactly the same. im just wondering if anyone has any experience with this amp.

do the biasing test point need to be 75mV per side?

there is 3 pins with the middle pin being common and the right point controlled by the right point and the left by the left pot.

below is a link to the schematic

http://www.ampix.org/albums/userpics/10003/Vox_AC100CPH.pdf

Thanks in advance
Bucket

[GOM]

First the easy part - yes, both test points need to be the same as the pots provide the grid bias voltage to the EL34s.
Now the guess work ... there's a good chance that the 50W version only has a pair of EL34s instead of the 4 in the 100W. If that's the case and the cathode resistor is the same then the voltage would be half of the 75mV as you only have a standing current flowing through one valve rather than two. In a class A or AB amp the bias defines the point in the valve transfer function at which you switch from one side (of the EL34 pair) to the other. Basically if the bias voltage is too low you'll get distortion and if the bias is too high the EL34s will run hot(er) than they normally would. Would be nice to know what resistor value you have for R120 and R127 and what the HT voltage is (488v on the 100W).

P.S. that 38mA is the standing current through EACH EL34.

Edited March 6, 2016 by GOM