AB763 trem question

[Randall]

Deluxe Reverb AB763 schematic shows 270v on the first plate of the trem 12AX7A, with only a 220K 1/2 watt resistor between it and the [B] screen node. How can this be? I have over 400v on the one I just built, and even more than that on a AB763 Super Reverb I checked it against. Both trems are working. I also have 0v on both trem tube cathodes on both amps, with the second plate voltage on the neon within spec. I don't understand the theory here.

Davidson Amplifier Repair, Nashville, TN : Schematics: Fender: Deluxe Reverb Amp AB763 Schematic

[loudthud]

Was the tremolo enabled by a short at the footswitch jack when you took that reading?

That amp biases the oscillator off unless there is a short at the footswitch jack. The drop from the 'B' node is your meters load on the 220K resistor. What voltage do you get with the tube removed?

[Enzo]

See that 2.2M resistor from the bias supply to the trem tube grid circuit? That puts a substantial negative voltage on tht grid and so turns the tube off. That is why there is zero on your cathode - zero tube current means zero voltas dropped across the cathode resistor. And that is why you plate voltage is the same as the B+ supply voltage. Again, zero current through the tube means zero volts dropped across the plate resistor.

As LT said, you have to have the trem running to get the working voltages.

[Randall]

The part I understand is that the tube forms a LFO and powers the neon on and off which causes the photo-resistor too conduct and pull the signal after the second gain stage to ground. I don't get how the LFO actually works, or why the schematic gets from the 415v [B] node to 270v at the first plate.

Here is what I have on a Fluke 77 with trem off:

tube in
1. 414v
2. -33v
3. 0v

6. 366v
7. -33v
8. 0.023v

tube out

1. 415v
2. -33v
3. 0v

6. 420v
7. -33v
8. 0.023v

trem does seem to work well, except for a bit of a ticking.

edit: DOH! Enzo got there before I did. Thank you for explaining that. I read LT's switch question the opposite of what he meant, maybe because the schematic shows the switch open. I'll go back and see what it looks like with switch closed.

[Enzo]

When it is off, the tube is off. You have to short the FS jack to turn on the LFO.

The LFO is pretty simple really. From the plate of the tube, there are three caps in series, with a resistor to ground between each. Whatever is at the plate gets fed back to the grid through them. The phase shifting through those RC stages sets the frequency. Making one of the resistors variable allows you to adjust the speed, that is the panel control. When the footswitch is open, it leaves a cap unterminated, so it can't "time out". And in this one the FS not only completes the RC circuit, it also grounds off the negative bias voltage that shuts the tube off.

[Randall]

Well as always Enzo, that makes sense. Thank you for that. I went back and measured again with trem switched on, but all voltages are swinging with the oscillation. My Fluke has an average function, and with it the averages fell right into about 10% of the schematic. I would have thought the schematic would have directed these measurements, especially by not showing the foot switch open. But thank you guys for helping me to learn something. And by chance, I am told a $5K Fender Vibro King is coming to my bench soon, and guess what the problem is? Trem doesn't work! Bring it on!