Sunn Studio PA rectifier

[Helmholtz]

Helmholtz, from orange to ground I get 5v AC at startup, then it gradually drops to around 350mV as it warms up. I also measured from orange to green, which is steady at 3.2v AC.

While not completely clear (though likely) if the orange wire is a heater CT, it certainly has nothing to do with the missing 5V rectifier heater winding, which must have no continuity to any other PT winding.

Why do you want to convert to GZ34 rectification?

[voyager]

I wanted to convert to GZ34 mostly because I wanted the amp to be restored to stock. Partly also because I generally prefer playing amps with tube rectifiers, but mostly just looking to make it stock. If you guys think that Sunn changed the transformer, adapted the circuit to solid state rectifier, and that it’s stock as-is, then I’ll just leave it as-is, like The Dude suggested. Keeping it stock is really my primary goal.

[voyager]

Another question, unrelated to the rectifier (at least I think it’s unrelated). I’m getting a hum, 60hz I think.The volume of the hum is affected by the master volume. The treble and bass controls change the tone of it. The individual channel volume knobs do not affect it EXCEPT that the volume knob for input 2 eliminates it almost completely when it’s set at around 7-8.

Switching to standby does not immediately eliminate it (it fades) but shutting off the power switch eliminates it immediately.

Before I start replacing capacitors, I thought I’d check with you guys and see if the channel 2 volume thing gives any clues to a different fix.

Thanks again for all the help, it is so appreciated.

[loudthud]

There are two 12AX7s that handle the mic inputs. Swap them and see if the hum moves to channel 4.

[voyager]

loudthud,

Switching the 12AX7s changes the hum (better or worse depending on placement) but doesn’t change which channel affects it. However, the channel 4 volume does have a similar effect to channel 2, but not nearly as dramatic. Again, it’s not that the hum follows the channel 2 volume, but the channel 2 volume eliminates the hum when the knob is at the 7-8 range. In the 0-6 and and 9-10 ranges it has no effect on the hum.

As I said, switching placement of the 12AX7 tubes does change the intensity of the hum.

If I turn on the amp with everything at full volume, sometimes I start getting feedback as it warms up (nothing plugged in). I turn the amp down when that happens, and then when I turn it back up the feedback doesn’t come back. I’ve tapped on all the tubes and it doesn’t cause the feedback to come back, it only seems to occur at startup. I do get some sound out of the preamp tubes when I tap on them, but nothing particularly harmonic.

Thank you for your help!

[voyager]

Hey everyone, I’ve been meaning to come back here and post a follow up on the hum, in case it can help someone in the future with a similar problem. It ended up being a very simple problem - the artificial center tap resistors were dead, which was (of course) causing the terrible ground noise hum. The amp is working great now, and sounding great too.

Following up on the PT leads, the orange wire does seem to be a center tap, but seems to have been left out of the circuit in favor of the artificial center tap. It all looks to me like it was set up this way from the factory. Same with the solid state rectifier.

Thanks again to all of you for your help on this!