Stereomour OG Power Transformer (and my mods)

[Doc B.]

Peebs preaches this all the time, and he is right on. Deoxit bad. When I was starting to restore my old Mercedes I did a lot of pulling apart of electrcal connectors, cleaning them and I hit a few with a little Deoxit. The car then sat in a dry, heated garage for about 18 months. When I finally got the car ready to test drive it was a rainy day. Turned on the windshield wiper that I had completely overhauled, and nothing happened. Turned out the connector from the harness to the wiper motor was one of the connectors I had used Deoxit on and a couple pins had turned green. Cleaned up with generic spray electronic cleaner and lightly wiped with dielectric grease and all was good.

They should probably change the name to Reoxit.

[tumble2k]

Thank you for your replies. I do have the 5670 internal shield pin tied to ground. The heater pins are connected to a ganged secondary of a transformer with nothing tied to earth.

Ugh maybe the Dexoit is the problem now. I may have to replace the socket. I'll try regular cleaner like you recommend first.

The problem is not related to Wi-Fi. I turned off my router and the problem didn't go away.

[tumble2k]

Am I supposed to ground one of the heater pins? Grounding one of the heater pins makes the scratching more continuous, less intermittent, if that makes sense. Maybe I should use a 6.3V transformer with center tap and ground the center tap?

[Paul Birkeland]

If the heater winding for the driver has no reference to ground/earth, then you will get noise. 

[tumble2k]

I soldered one of the heater leads to ground ... and blackness ... wow nice! Thank you gents!! TIL I need to reference my heaters to ground. Makes perfect sense in retrospect that if the heaters are floating they might have high voltages with respect to the signal. It's not perfect. I'm still hearing occasional ticks and stuff. Could now be Wi-Fi.

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.

I'm still thinking about putting a single shunt regulator (I have one from Vacuum State) to drive the C4S on both channels. I would pull the power directly from the output of the doubler and filter it with an RC.

Your amp is so much fun!

[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.