philbard wrote: ↑Sat Dec 25, 2021 12:20 amSo, do you have an opinion on I should be looking for the source? I've checked lead dress and chopsticked most of the wires. Also, do you think this could be causing the overall weakened output condition of the amp, or is that likely to be something else?
A parasitic oscillation can definitely be the cause of these symptoms. It is most likely a lead dress issue that is creating some unwanted feedback. Maybe try moving the PI output leads around.
Moved the plate wire away from the OT secondaries, no change, the oscillation is still there. Also, watching on the scope while moving any of the PI wires, there is no variance in the oscillation. One thing that is different on this build from the 2 I"ve done before, this time I didn't coil the OT secondary wires, as Rob Rob mentions one should not do. Could this have any effect?
Plate Wire Moved.jpg
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OT Primary and secondary leads are considered "high current loops" and sometimes twisted to minimize the field radiation (like heaters) but I've always just bundled them like you have them. Although the sec. is considered low impedance I've always tried to keep them away from the PI anyway (phase issues)
BTW..Though you moved the wires a bit apart but you still have the sec leads running directly under the trimmer. Not saying this is definitely your problem as oscillations are tricky to track down and can come from anywhere and spread throughout the amplifier.
Good Luck!
Tony
" The psychics on my bench is the same as Dumble'"
You could pull the powertubes and run the amp without them to see if this changes anything for the PI.
Oscillation caused bij the OT primary or secondary wires can't occur without the powertubes so this way you can eliminate that.
Richard1001 wrote: ↑Sun Dec 26, 2021 9:38 am
You could pull the powertubes and run the amp without them to see if this changes anything for the PI.
Oscillation caused bij the OT primary or secondary wires can't occur without the powertubes so this way you can eliminate that.
Yup that did eliminate the oscillation. Thanks! I had already moved the OT secondary wires away from the trimmer, and twisted them to see if that made any difference and even lifted the preamp board to get more space, none of that had an effect so I'm wondering if there is another source for the oscillation, somewhere in the power section perhaps?
One thing that is different on this build is that I'm using teflon coated wire for a fair bit of the low voltage connections, but I haven't seen mention on the forum that this could contribute to this sort of problem.
Great, now you can fix the problem.
I can't Imagine how teflon isolation could cause this problem.
Maybe look at places where wires with high signal voltage run parallel sensitive low signal wires.
For example: the red gridwire of the last two powertubes runs parallel to the brown anode lead of the same tubes.
I don't know how wires are running under the supplyboard but maybe reroute the powertube gridwires and alle other signal wires away from the transformer primary leads.
Richard1001 wrote: ↑Sun Dec 26, 2021 6:25 pm
Great, now you can fix the problem.
I can't Imagine how teflon isolation could cause this problem.
Maybe look at places where wires with high signal voltage run parallel sensitive low signal wires.
For example: the red gridwire of the last two powertubes runs parallel to the brown anode lead of the same tubes.
I don't know how wires are running under the supplyboard but maybe reroute the powertube gridwires and alle other signal wires away from the transformer primary leads.
I bored a hole in order to route the OT secondary wires beneath the chassis past the trimmer to eliminate that possibility. At this point there are no low voltage signal wires beneath the power board. The issue remains. I did some AC voltage measurements after the PI coupling caps with a .6v 5KHz signal sent to the input jack and at moderate master and preamp volume settings I get about 5v off the right CC, and zero volts off the left. The left side is where the oscillation is showing up, so I would guess this is related.
Here is the current state of things...
Wires rerouted.jpg
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Test for loop instability by disconnecting the nfb from the output transformer like Sluckey suggested. I'd expect oscillations on both halves of the PI, but you never know. Heck, could even be a faulty tube socket of the PI.
sluckey wrote: ↑Sun Dec 26, 2021 7:12 pm
Have you tried just disconnecting the NFB wire?
Early on I disconnected the presence pot. No effect. Just now disconnected the NFB wire at the impedance switch and from what I can tell the oscillation disappears. The amp doesn't function with a guitar plugged in in this condition, I suppose that is normal. I have repositioned the wire from the impedance selector to the 4.7K resistor many times and this has never had any effect, currently it is raised up so as not to be near the PI plate wire for V3A. Should I be looking at the wires to the Presence pot?
