Transformer hum?

[txstrat]

After rewiring I the preamp power supply to TUT3 specs (left the power amp as it was) I tinkered around again with the grounding of each stage. When I disconnect the 2nd stage's cathode from ground (left it floating) the amp is quiet. It's NOT when I disconnect the cathode of the first stage.

[txstrat]

I've rewired the amp almost exactly to the TUT3 layout. I'd say the buzz is even more than before.
Here's the differences to the TUT3 layout:

1. My amp has two inputs and the input (grid stopper) resistors are soldered directly to the input jacks with a shielded wire to the grid.
2. I've added an ultra low switch after the second stage (before the volume pot) just like in the B-15N '68 schematic I posted.
3. The grid stopper before the PI input is 220K NOT 270K as in the schematic.
4. The power amp is fixed bias
5. My 1K dropping resistor after the first filter cap is 5W instead of 10W
6. Bridge rectifier because PT has no center tap.
7. My amps missing the 100nF and the two 10 ohms resistors before the first filter.
8. I'm using two 100uF (first filters) instead of four 22uF

Still the buzz increases with the volume pot. It's dead quiet when the vol is all the way down.
The deep frequency hum is not worth mentioning. Very low level I'd consider normal.

[kg]

tx, you can forget everything after the volume pot.

working "upstream" from the volume pot, at which stage does the hum stop when you short its grid to cathode through a cap?

[txstrat]

I've interchanged the triodes with each other (i.e. first triode is pin 4 on both tubes). The buzz went away and made room for the low frequency hum again, which is not dependable on the volume pot unless I turn it all the way up.
Think I'm moving in a circle.
When I short the grids to their respective cathodes (two clip leads with a 100nF cap in between) nothing changes, except triode two (pin 1) on the PI tube which pulls the hum down a little.
When I pull both preamp tubes the amps dead quiet. Pulling tube 1 only makes no difference.

[Tom Phillips]

...Think I'm moving in a circle

I know what you mean. I went back through the postings looking for things that may have been missed but I have not come up with anything new to suggest yet. It seems that all ideas have been covered by the contributors. Several have been repeated over the course of the discussion contributing to the “moving in a circle” feeling. My current thoughts are:
1) Are you expecting more than is possible / practical? Without being there in person it’s hard for me to evaluate the actual level of hum/buzz left.
2) Some key thing has been missed, such as a wiring dress or layout topology that is allowing the remaining hum/buzz.

I’ll keep thinking.
Tom

[txstrat]

Thank you Tom

I know what you mean. I went back through the postings looking for things that may have been missed but I have not come up with anything new to suggest yet.

Yeah, I thought there must be some obvious thing which I haven't been thinking about yet, i.e. defective part or whatever.

1) Are you expecting more than is possible / practical? Without being there in person it’s hard for me to evaluate the actual level of hum/buzz left.

All my other amps being almost dead quiet don't mean this one has to be too. When I put my ear close to each of my other amps speakers, I can hear a slight amount of hum which I consider normal. When I stand up in front of those amps I can't hear it anymore (almost).

When I stand up in front of the B-15 copy, I can still hear the hum approximately as loud or even a bit louder as with the other amps while putting the ear to the speaker.

Don't know how to describe it any better. It's hard to put sound level into words. The level of hum/buzz is comparable to a microwave oven you hear in the next room.

If you happen to be in Germany some day I could show you in person.

2) Some key thing has been missed, such as a wiring dress or layout topology that is allowing the remaining hum/buzz.

That's why I rewired the preamp completely to TUT3 specs - which led to a slightly louder buzz than before - which changed to the better when I interchanged the triodes.
Funny thing is the change from buzz to hum with changing the triodes. The buzz was quite loud and it might have drowned out the hum - and when I interchanged the triodes only the hum was left.

I’ll keep thinking.
Tom

Thanks again.
Matt