1973 Twin Reverb...Reverb Circiut. Running Out Of Ideas

[Chuck H]

Something else to check would be that both grids are paralleled, both cathodes are paralleled and both plates are paralleled on the 12at7 reverb driver V3. If anything is wrong here you'll have less than half your drive to the tank. Also check the 500pf reverb circuit coupling cap and it's connections. Since you have some reverb the connections may be fine, the cap may be bad. Though I've found it to be rare.

[Tom Phillips]

And what is the cathode voltage on the 12AT7 reverb drive tube?

[g1]

Can we get an exact schematic or model so we can point to components and all be on the same page?
Master? pull boost? ultra-linear?

[keithb7]

Thanks for your assistance. The Amp is a 1973 master volume version Twin Reverb. AA270 circuit. Non-ultra linear.

I am learning by studying, reading, trying and servicing my own vintage Fender amps. I have not formally studied electronics.
I am learning as I go. It's a hobby. My full time job has no relation and pulls most of my time away from amps.

Having said that I get by pretty good, and come here for advice when I need help and more info. I recently inherited
a Tektronix 2215A scope. I am not having much luck finding any "How To" instruction or video for a beginner on the internet.
However I think I am starting to understand it a little. I think I know what do with it. Clarification on my questions is appreciated.

If I run a signal line through the amp input, it travels through the amp circuit as an AC sine wave. Is this right?
The tube and amp run mainly on DC voltage. The guitar or signal AC wave rides on top, or shares it's travel path with
part of the DC voltage running the tubes. Correct? If Irun some sort of signal through the amp, on the bench, then hook up
the Scope on AC setting, I can then trace the line signal through the amp circuit. Where it stops in the reverb section, is where my
problem is?

I bought a cheap signal generator, does all kinds of wave forms and frequencies. I need to adapt it I guess to go into the
guitar amp. Maybe clip it to the connections on the back of the 1/4" input jack. Then I have a signal I can control.

Am I on the right path here? Thanks for being patient with me. - Keith

[Jazz P Bass]

twin_reverb_aa270_schem.zip

[Chuck H]

A dummy load is a good thing to have as well. Otherwise, since tube amps need a load to operate, you have to listen to the test procedures at volume. Which isn't always possible in my house My dummy load includes an adjustable low level speaker output for audible monitoring at low volume. I almost never use that feature.

Signal tracing can be done with your DMM. Just read AC volts where signal should be present. Knowing how much you should see at any given point in the signal path depends on the circuit and some ability to speculate. Unless you have the benefit of a modern schematic that indicates specific test parameters. Otherwise you can post results here and we may be able to interpret them, maybe

[pdf64]

If I run a signal line through the amp input, it travels through the amp circuit as an AC sine wave. Is this right?
The tube and amp run mainly on DC voltage. The guitar or signal AC wave rides on top, or shares it's travel path with
part of the DC voltage running the tubes. Correct? If Irun some sort of signal through the amp, on the bench, then hook up
the Scope on AC setting, I can then trace the line signal through the amp circuit. Where it stops in the reverb section, is where my
problem is?

I bought a cheap signal generator, does all kinds of wave forms and frequencies. I need to adapt it I guess to go into the
guitar amp. Maybe clip it to the connections on the back of the 1/4" input jack. Then I have a signal I can control.

Yes, that's the basic method.

I find a useful test signal to be ~0.1Vrms sine 400Hz-1kHz.
For the purposes of fault finding the reverb though, it would be hard to beat connecting a speaker to the output of the reverb drive transformer.

[Chuck H]

We can at least guess that the problem has been narrowed to the input side, I think. And we know the transformer and tank are good to go, so...

Either not enough signal is hitting the tube, the tube is not conducting sufficiently, or both. I think we need to see the voltage readings at every tube pin.

[Zipslack]

I'll put in my $0.02 (which is worth...$0.02)...have you swapped the V3 12AT7 reverb driver? Is it a 12AT7 and not some other tube that has been transplanted in the past? Swap in a known-good tube to eliminate that option...leaves the few resistors and caps and the reverb transformer. Of course, could be bad contacts (tube socket) on a good tube, so why not hit the socket with some cleaner while you're at it?

[Chuck H]

I'll put in my $0.02 (which is worth...$0.02)...have you swapped the V3 12AT7 reverb driver? Is it a 12AT7 and not some other tube that has been transplanted in the past? Swap in a known-good tube to eliminate that option...leaves the few resistors and caps and the reverb transformer. Of course, could be bad contacts (tube socket) on a good tube, so why not hit the socket with some cleaner while you're at it?

In post one Keith said:

I pulled both reverb send and recovery tubes and tested them in my tube tester. No problem. Still proceeded to swap in known good tubes. No improvement.

He never did indicate the tube number though. But since the 12at7 driver has been mentioned repeatedly in the thread I would have thought that if Keith were using the wrong tube he might have mentioned it by now. Still, I suppose we can't know if we don't ask.

[keithb7]

Thanks folks. I have 12AX7 and 12AT7 tubes that I have attempted to swap in and out with no change. I don't have an actual 12AT7. I will proceed acquire one though as soon as I can.
My understanding is either the 12AX7 or 12AU7 will work for reverb testing purposes. No?

I'm just getting set up here with a 1,000 hz sine wave signal now. I plan to run it into the amp input jack. Then I'll start measuring and reporting voltages.

[Enzo]

Doesn't the Twin have two 12AT7s? One for the reverb and one for the phase inverter? or are both of those missing here? If you have a 12AT7 PI tube, pull it and use it for reverb during tests. We don't need the PI to test the reverb drive.

[loudthud]

Check the cathode resistor for the drive tube. They've been know to go bad in a high resistance way that kills the drive.

[keithb7]

I found something interesting. Below here is a photo of how I received the amp. Notice the small white Mallory 25uF/25V cathode caps for the pre-amp tubes. 2 at right, then another 1 to the left. There should be two cathode caps there, just left of the first pair.
One if these is the cathode cap for the reverb driver tube. The missing one. The 25uF/25v cap is missing from the circuit. Just the resistor is sitting there. Supposed to be 2.2K. It measures in at 13.82K. I'll change that resistor and put in a cap and fire it up.
I suspect someone went in there to fix something or other at some point I guess. Forgot to re-install the cathode cap? Then the resistor proceeded to burn up? Guessing here as I acquired the amp in Dec. I replaced all those caps in Dec and overlooked that one missing cap. I pulled and replaced what was on the board. Rookie eyes... Another good lesson for me.

Good chance this is it?

[Chuck H]

The 25uF/25v cap is missing from the circuit. Just the resistor is sitting there. Supposed to be 2.2K. It measures in at 13.82K...

Good chance this is it?

Yes.