Stereomour OG Power Transformer (and my mods)

[Paul Birkeland]

Now I'm going to work on a new set of mods. I was thinking about setting up a -60V power supply with my free 12.6V winding on the power transformer and fixed biasing the grids of the 2A3s. I'll need to adjust the bias so that I get 50 mA of current and readjust the bias as the tubes age.

You'll need to reduce the B+ sufficiently so that you aren't running the 2A3 over maximum voltage.  You will also need to lower the grid leak resistor to the appropriate rating for fixed bias in the datasheet, then adjust the coupling cap to restore the desired time constant, then evaluate whether the driver will put up with these changes.

 

[tumble2k]

Thank you for the advice. Very much appreciated.

An alternative is to put an adjustable shunt regulator on the 2A3 cathode as you do with the 12AT7 on the Stereomour, but it probably won't sound any better than the cap.

 

[Paul Birkeland]

This is something that I certainly see people do, but you'd want to use a current regulator so that the bias voltage can vary a little bit to attain the desired current.  You'll also still need the big bypass cap across it. 

The big downsides to doing this are that you'll have some kind of solid state device cooking its brains out that needs a big heatsink, and these kinds of devices tend to short out when the fail, which is the opposite of what's deisred.