Phase Inverter Voltage seems off

[philbard]

Here are the waveforms at the left PI CC with NFB connected and disconnected. 1v/div, 2us/div

Connected

Left CC, signal.png

Disconnected

Right CC, signal.png

[Richard1001]

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

Nice job on the secondary wires.
You have a scope so i would suggest tracing the signal. It is ODD to have a signal comming of the inverting side of the PI and nothing from the non-inverting side. You would think its a bad component, bad connection or wiring problem. You did not by any chance mix up the red wires to the PI plates and B+ right? (Accedently connecting B+ to the non inverting capacitor side where the anode should go and connecting the anode to the middle of the AC balance pot) This would explain your problems of not having signal on one side, 40 volt difference on the plates and the voltage dropping on the inverting side when measuring.

[philbard]

Nice job on the secondary wires.
You have a scope so i would suggest tracing the signal. It is ODD to have a signal comming of the inverting side of the PI and nothing from the non-inverting side. You would think its a bad component, bad connection or wiring problem. You did not by any chance mix up the red wires to the PI plates and B+ right? (Accedently connecting B+ to the non inverting capacitor side where the anode should go and connecting the anode to the middle of the AC balance pot) This would explain your problems of not having signal on one side, 40 volt difference on the plates and the voltage dropping on the inverting side when measuring.

No, the amp is wired correctly and has been checked several times. Just to reiterate, it works great, sounds normal, it just seems underpowered. I have to set the preamp and master volumes at noon to get up to bedroom levels. When I route the signal out of the send jack to another amp's (same 102 build) return jack I get normal volume at these settings (loud). I suppose I could check that setup to see if there is the same PI imbalance if you think that's worth it...

[Richard1001]

Well you wrote you get 5V AC out of one side of the PI while the other side shows no signal at all. This is odd and suggests either a wiring or component problem. You should be able to measure this.

Just for the PI
What are the DC voltages on both the board eyelets and the tube pins? And if you tracé a signal on the PI where do you lose it?

[martin manning]

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.

That's not normal. Is the PI tail ground connection good? At any point did you try swapping the OT primary leads?

[rootz]

With the nfb wire disconnected you should gain at least 6dB of output volume. I'd follow Martin's suggestion. Might wanna check all connections around the PI again: test for continuity.

[philbard]

Have you tried just disconnecting the NFB wire?

That was it!!! The end of the wire from the 2.2 uf cap to the effects loop jack was stuck up under the lip of the chassis so I didn't see it wasn't soldered on. I checked all the grounds before the first startup but obviously missed this one. You're truly a genius Martin!

So does this mean only half the PI was functioning?

Thanks to everyone who contributed time and comments to this thread. Bonehead mistake on my part but I owe a lot to the forum members for guiding me along. Cheers everyone and have a great holiday!

[martin manning]

Great! Everybody learns something from these little adventures. The PI was getting its tail grounded through the NFB loop and OT secondary ground, and you had another feedback loop there.

[philbard]

As I've heard mentioned on this forum, the best part of troubleshooting is when its over...

Thanks to Martin, Richard, Sluckey, Tony, Rootz and anyone else I've forgotten to mention.

[talbany]

As I've heard mentioned on this forum, the best part of troubleshooting is when its over...

Thanks to Martin, Richard, Sluckey, Tony, Rootz and anyone else I've forgotten to mention.

Phil
Glad you got her goin! Congrats

Tony