Music Man 2100-65 (12AX7 driver) HT voltage doubler output reduction

[Alex R]

This nice old Music man came in with a bodged nonstandard mains transformer fitted messily, not producing anything like the 725 volts these things like to run on their 6CA7s. I have fitted the TAD mains transformer which seem to be a compromise type to fit a variety of models, and took the PSU wiring back to stock with new parts.

The HT supply is via a voltage doubler as you can see.

My problem is the HT is now 840vDC. Everything works fine but the output tubes - JJ 6CA7s - don't like it. One shorted. Any ideas for reducing this to something more like 700vDC?

[TimmyP1955]

Is the heater supply at 6.3VAC, or is it high as well? If the latter, you could put a bucking transformer in series with the primary of the PT (if you can find a place for it).

[g1]

Is that PT for a 100W amp? I don't see what 50W model it could be designed for with that high of an output voltage?
Can you link to the PT you used?

[J M Fahey]

Just checked their xxx-65 model, it claims 285 VAC secondary which translates to 403V straight or 806V doubled.Add the typical 250V plus UK mains and there you have your extra voltage.
Which is deadly.

Specs claim
0V Black
220V Yellow
240V Brown
300V White
not sure whether you are using the 220V or the 240V tap.

[nickb]

You could use a FET dropper but you'd have to manage the heat. Or a bucking transformer on the secondary. The bottom like here is that the TAD transformer isn't really suitable for the advertised application. The secondary should be more like 250V to give 700V on the plates.