Question about a lil hummmmm...

[tubedood]

Hi my electronics wizarding friends!
I have a quick question about a tweed Princeton type circuit and the negative feedback resistor (yes again hahaa)!

The amplifier is quiet as a mouse with the neg feedback not hooked up... (but bright and a lot of spank to chords ) but if I land the wire for the feedback on the feedback resistor I get a hum. No squeal or loud noise just a slight low volume hum. I have tried reversing the output transformer secondary's in case I have the primaries hooked up with the wrong phase (blue is going to the plate and brown to the B+) but the hum is the same regardless of any wiring changes I try. Its not real loud but I am not sure if it is normal.

I do prefer the tone with the feedback resistor but I hope I can tame the little hum it induces!

Dale

[eschertron]

with the NFB connection, you are adding some circuitry to a point in the signal path with relatively low amplitude. any hum induced will be amplified by the LTP phase inverter. I'll suggest checking lead dress from the OT connection back to the PI.

[tubedood]

Thanks eschertron for the advice! I have poked with a stick to see if any wires seem to be the culprit but no difference... The amp is single ended so no LTP. I tried flipping the output transformer secondary's (its easier at this point than the primary but I will do that if needed). Its not too loud the hum but it isn't there until I add in the negative feedback hmmph. It seems great other than that!

[g1]

Is the speaker jack an insulated type or connected to chassis? If not insulated, maybe try with it disconnected from chassis as a test.

[The Dude]

Does the amp's output level go up, down, or stay the same when you hook up NFB? Check with test tone and scope if you have one.

[tubedood]

The hum dropped a lot when I added a 1K resistor across my first 2 filter capacitors, dropping the plate voltage from 360 to about 330.

Progress not perfection as the saying goes!
Thanks as always for the suggestions and guidance!

Dale