Great forum find-
Spent most of my electronics life in digital and solid state design and build. Got a wild hair and bought a kit tube amp to do something different- not having dabbled in the art since the 5 tube superhet AM radio I built as kid.
Decided to go PP, class AB- So the attached is what arrived-
All works well- except the feedback loop arrowed causes about 1khz squeal after 3-4 seconds. Researched on this forum, tried changing primary and secondary windings to no avail... Admit didn't try all combinations.
So main Q is-
Amp is stable and works fine with feedback loop disconnected.... but I've read some here that a working loop will accentuate bass response of PP design. This conclusion correct? Is it worth fixing?
Thanks in advance,
Jim
ps. changed out the 75R in Pi filter to 1mh choke- if that matters
Spent most of my electronics life in digital and solid state design and build. Got a wild hair and bought a kit tube amp to do something different- not having dabbled in the art since the 5 tube superhet AM radio I built as kid.
Decided to go PP, class AB- So the attached is what arrived-
All works well- except the feedback loop arrowed causes about 1khz squeal after 3-4 seconds. Researched on this forum, tried changing primary and secondary windings to no avail... Admit didn't try all combinations.
So main Q is-
Amp is stable and works fine with feedback loop disconnected.... but I've read some here that a working loop will accentuate bass response of PP design. This conclusion correct? Is it worth fixing?
Thanks in advance,
Jim
ps. changed out the 75R in Pi filter to 1mh choke- if that matters
Attachments
The squeal is indeed usually a winding that's out of phase on the OT. Generally it's easiest to switch the secondary winding around. That should absolutely either take care of the problem or make it a million times worse.
Without the feedback loop the amp as designed will have very high THD and poor damping. You could triode strap the output tubes and leave the feedback loop off if you get annoyed and don't want to deal with this.
The 75R resistor might benefit from a 1H choke. 1mH offer 0.75 ohms of impedance at 120Hz, so you've taken a big step backwards in power supply filtering.
Without the feedback loop the amp as designed will have very high THD and poor damping. You could triode strap the output tubes and leave the feedback loop off if you get annoyed and don't want to deal with this.
The 75R resistor might benefit from a 1H choke. 1mH offer 0.75 ohms of impedance at 120Hz, so you've taken a big step backwards in power supply filtering.
Thanks,
got it straightened out.
So is it better to put the 75 ohm resistor back in place? Or opt for a 1H choke? I've got room for the latter, if a cleaner DC.