Premier 100r cutting out

[pontiacpete]

This amp is doing something really strange. It works great sometimes then it doesn’t pass signal at all. I’m having a very similar problem that this thread had: https://music-electronics-forum.com/...-have-any-info.
The foot switch is not original. Someone spliced in a new cable and added a home made pedal.
when I turn the amp on everything works perfectly. When I touch my probe on pin 5 of the 6eu7 the sound cuts out completely. Sometimes the same would happen when pressing on the reverb stomp switch.
All electrolytic caps have been changed and even the socket of the 6eu7.
The cap on pin 5, .01uf, has been changed but still is microphonic.
And looking at John Snells hand drawn schematic, like his there is no voltage on the last node (F).
What is causing this? Oscillation?

[Chuck H]

Almost surely oscillation. You could try paralleling the 6eu7 pin 5 grid load (470k) with a 220p cap. I don't think you'll hear a difference but I haven't researched this idea yet.

[pontiacpete]

Almost surely oscillation. You could try paralleling the 6eu7 pin 5 grid load (470k) with a 220p cap. I don't think you'll hear a difference but I haven't researched this idea yet.

Just gave this a try. It held up for a couple of taps with the probe then cut out. I also went and tried a 470p cap, same thing.

[Chuck H]

Ok. That's probably indicative of the oscillation being somehow associated with the reverb recovery input, but not only there and that grid may not be the best place to attack it. I have to ask... What happens if you tap that grid pin with something non conductive? If it breaks into oscillation anyway then you may have a microphonic tube and/or an intermittent connection on some tube pins at the sockets. I say this partly because you called out the .01 capacitor as being microphonic and then the replacement also being microphonic. That seems unlikely even with a ceramic cap and I've never seen a microphonic film cap. So there may be something/s else in the amp that is microphonic.

[g1]

If it's related to the reverb, I would suspect the cobbled-up footswitch.

[pontiacpete]

Ok. That's probably indicative of the oscillation being somehow associated with the reverb recovery input, but not only there and that grid may not be the best place to attack it. I have to ask... What happens if you tap that grid pin with something non conductive? If it breaks into oscillation anyway then you may have a microphonic tube and/or an intermittent connection on some tube pins at the sockets. I say this partly because you called out the .01 capacitor as being microphonic and then the replacement also being microphonic. That seems unlikely even with a ceramic cap and I've never seen a microphonic film cap. So there may be something/s else in the amp that is microphonic.

The .01 cap was one of those small turquoise type. But interesting you brought this up. I did find that when I tapped a lot with a chopstick the amp would turn off. This is when I changed the socket. I hate changing sockets on these amps. My 6eu7s might not be the greatest, I have only three and they are a bit noisy.
Anyway, I’m starting to think as G1, that the footswitch is possibly the culprit. Here’s picture of what that looks like

[pontiacpete]

I figured out that eliminating the reverb switch the amp works, no cutting out. What I did was first disconnect the switch and applied voltage to both of the 5uf caps. I don’t understand this but when using the switch, voltage is switched from one 5uf cap to the other. One of those caps is connected to pin 6 of the 6eu7 and the other is connected to the 12AX7 pin 6. If you look at this schematic that Snell posted of the 120r this is the input tube that the low input jack is connected to at pin 7. How does the Lo input jack even work without voltage on it’s plate?

premier_120r_sch.pdf

[Chuck H]

Unfortunately "note A" seems to be omitted from the schematic posted. I suspect the question mark could represent a resistor. You could test for this yourself.

EDIT: And you are correct that the channel would not work without plate voltage. Ergo,.. There is plate voltage. (<period)

[pontiacpete]

Oh hell the oscillation is still there, it’s just different. As I turn the reverb up it feeds back,,, a lot. This is such a weird design, about 100v on the foot switch. This was also mentioned in the thread that I linked in my first post. Any thoughts on this?
I’ve been trying to go by the premier 90 scheme where the switch is on the output RCA of the tank and trying different combinations of dropping resistors with no luck.

[Helmholtz]

EDIT: And you are correct that the channel would not work without plate voltage. Ergo,.. There is plate voltage. (<period)

It is possible that V3b doesn't work at all (the pin 6 plate voltage would give a clue).
As the channel inputs 1-L and 1-H are linked, the reverb channel input (pin 2) always gets signal.

[Chuck H]

Well sure. But I would think a purely wet signal would have been recognized. I could be wrong.

[pontiacpete]

It is possible that V3b doesn't work at all (the pin 6 plate voltage would give a clue).
As the channel inputs 1-L and 1-H are linked, the reverb channel input (pin 2) always gets signal.

You are correct about the H and L ch being connected, I admit I didn’t notice that. I still don’t understand why there are connections to V3b in the first place, and a filter cap node F to boot. And the whole thing of there being 100vdc on the footswitch, toggling between the plates of V2 and V3b. The wiring at this end of the amp looks original as far as I can tell. The footswitch that was replaced had been spliced in from outside of the chassis.

[Helmholtz]

Well sure. But I would think a purely wet signal would have been recognized. I could be wrong.

Look at the schematic. The reverb channel mixes dry and wet signals via the bypass between V2a and V2b cathodes.

[Helmholtz]

Please post a drawing that shows how the reverb footswitch is wired.
I assume it either kills either the output of the reverb channel or the output of the dry channel, maybe by shorting plate signals to ground via the caps.

[pontiacpete]

Does this help?