Topic review

On the advice of another tech, I put all the ground connections together on three solder lugs firmly bolted to the chassis at the same place, adjacent to where the ground for the power transformer was originally connected to the ground lug of a terminal strip. So now instead of a star ground at the location of the quad cap, and instead of the factory layout where some grounds were far removed from the power transformer ground, now all of the grounds are together, near the terminal strip. The improvement is noticeable. Subtle, but definitely there, and the difference is audible on a Speakerlab K-Horn clone. I don't really know why this helped, but it did.

That's the first progress thus far. I've not been able to get a good measurement of S/N on the bench due to noise in the test setup (now that I've retired from active repair work, I'm on a makeshift bench in my garage). But I'm suspecting that what I've got now is getting close enough to the Mark II specification, so I'll call it that, for now.

Wieslaw Lipowski wrote:Are you sure the output tubes are really well matched? If you have a mismatched pair you can set the cathode voltages equal, but that doesn't mean they become matched. A mismatched pair of output tubes can produce hum too.

Also, are the cathode resistors common just like in a stock model? Or have you wired them individually? You'd better wire them common as they can cancel each other's hum and distortion a little too.

A couple of detailed photos would be very helpful in further diagnosis.

Dan

Are you sure the output tubes are really well matched? If you have a mismatched pair you can set the cathode voltages equal, but that doesn't mean they become matched. A mismatched pair of output tubes can produce hum too.

Also, are the cathode resistors common just like in a stock model? Or have you wired them individually? You'd better wire them common as they can cancel each other's hum and distortion a little too.

Don't think so.

Note that I mentioned the hum is present even with the input jack shorted; i.e., no outside signal coming into the amplifier

AND that the hum is present with the coupling capacitors to the output tube grids disconnected, so the grids see no signal from the 6AN8 stages, only the bias voltage...which section I also essentially rebuilt with a new rectifier and new filter capacitors and separate bias adjustments for each output tube.

I suppose I should next try MEASURING the S/N to see how far off spec it really is.