Low (hot) negative bias voltage.

[GuyNoir]

Now that you mention it, I think I read after the fact that the 'LPS tubes could actually worsen noise in certain circumstances. A happy discovery, in any event! I had to put my ears right up to the woofers because I didn't think the amp was on. It was only a slight hiss of the tweeters that told me it was on. After nulling with the balance pots, I'm seeing -17.5v, -17.7v, -18.0v, and -18.2v. Before it was -17.6 or -17.7 across the board.

 

[GuyNoir]

And now the slight hum is back. :-( That's it...I'm packing it in and going SS.

 

[gadget73]

If the output tubes are not properly balanced, it will hum. Have you balaced them per the manual?

 

[GuyNoir]

Are you referring to the balance pot, or is that something else entirely? I have two quads each of "matched" 6p14p-ev and 6p14b-eb. I've most likely lost track of which tubes had which numbers/ratings marked on the boxes. However, the hum is present with both sets of tubes. Thankfully, the little bit of hum I hear now dissipates a few feet from the speakers and is not volume dependent. My ceiling fan motor and the air conditioner compressor outside of my bedroom window are the worst HUM offenders. On the plus side, this amp has just about the cleanest, unadulterated rendering of mid and high frequency content (vocals, cymbals, high-hats, brushes, bells, etc.) that I've ever encountered. The diodes in there now are 1N4 without caps/snubbers. I've ordered UF4007s and caps to shunt noise to ground just for something to try.

 

[gadget73]

Its the concentric hum balance control above the input jacks. Put the switch to "test" and adjust the pot for minimum hum on each channel. Not sure which is for left and right but it should be marked on the panel.

If it has any missing tube shields, that could also be a source of hum, same if the bottom plate is not installed.

 

[GuyNoir]

Ages ago, I had a Hallicrafters SX-101 with a hum pot, so I figured that one out early on.

 

[gadget73]

The hum pots on these work a little different though. Its used to balance the output tube current. Most things the hum pot is used to balance the heater string relative to ground. Either way, if its not set right it hums more than it ought to.

 

[GuyNoir]

Thanks again gadget. As always, your sage advice is welcome and appreciated.

 

[sp58]

And now the slight hum is back. :-( That's it...I'm packing it in and going SS.

I did it, changed to Nakamichi SS-Receiver.
Now the Pioneer 21 Tubemonster sits upstairs in my bedroom - i am waiting for the winter - listeing radio and get heated

 

[GuyNoir]

A Stasis? I did in fact go out to the garage a few days ago to retrieve the first new receiver I bought as an adult - a 30+ year old Harman Kardon HK495i. My tech went through it a few years ago, but it's been unused. It sounds much better than I remember.

I may take a cue from you and use the SS in summer and the tube amp in winter. With my bedroom door closed at night, between my PC and amp, it gets entirely too warm in here.