phase inverter and negative feedback problem (?)

[dscottguitars]

Hi,  I am opening a new thread to an older hum problem I've been having.  I have some questions about a strange problem that I'm not sure is correct.

I have a PP 6BQ5 stereo amp I built.  I have been having hum issues and traced part of the problem to the power supply rail.  Not sure why it helped but I switched the B and C points by accident and the major hum problem went away.  I only found this out by noticing it was backwards, made it to the schematic and the hum came back so I back tracked all the changes I made and this is the one that helped.  (this is just a little background and is not the main question)

I thought the remaining hum could be tied to the negative feedback.  I disconnected it and got nothing.  No signal.  The voltages on the plates however are very strange.  They don't come down at all.  I get the same voltage-360v on the plates which is the same as on the power supply.

Why would the negative feedback line be doing this?  And, how????  I checked and rechecked my connections a dozen times, re-soldered and still the same thing.  

[sluckey]

I thought the remaining hum could be tied to the negative feedback.  I disconnected it and got nothing.  No signal.  The voltages on the plates however are very strange.  They don't come down at all.  I get the same voltage-360v on the plates which is the same as on the power supply.

Why would the negative feedback line be doing this?  And, how???? 

That sounds like that when you disconnect the NFB you are breaking the ground path for the PI cathodes. That would explain the high plate voltage because no current is flowing thru the tube. And it would explain no sound.

So, exactly where are you disconnecting the NFB? Is that 820Ω resistor really 820Ω? Is it even in the circuit?

Try this... Reconnect the NFB. With the amp turned off use your meter to measure the resistance from pins 3/8 to ground. One meter lead directly on the tube socket pin and the other on chassis. You should get 470 + 33,000 + 820 = 34,290Ω. What do you really get? Now disconnect the NFB exactly as you did before. What is the cathode resistance now?

[dscottguitars]

I would also like to point out that the output transformers are from the amp in the schematic below T1 and T3.  I did not use the 3 secondary taps, just put shrink wrap around them.  This was a suggestion by Hot Blue Plates.  Is this something that could be related?

The amp sounds great with guitar or my computer- iTunes...  Just a bit of hum I'd like to tame.

[dscottguitars]

Thanks Sluckey!  That's a big DUH!!!!

I never disconnected the negative feedback before and forgot about the ground link...

[dscottguitars]

That worked! (of course it did...)

I don't think it helped much with the hum or not.  I did do some tweaks to the tone stack and coupling caps and it sounds better and the hum at low volume is pretty low.  I think I'm at the point where it's as good as I'm going to get it.  It won't be used as a cranking guitar amp much at all, just a stereo and practice amp.

Thanks for your help...

Daniel