1975 Fender Twin Reverb - No Reverb

[forwardmarch]

Long time reader - First time poster (as I can usually find my answers by reading).

I am at my wits end with the reverb in my 1975 Fender Twin.

Symptoms :

- Reverb is not working, I only get a hum when the signal is turned up.
- Tank is fully responsive to "crashes".
- No signal output when a small speaker is hooked up to the "Reverb Input" jack.

What I have tried so far :

- I have checked the tank, no disconnections.
- Replaced the Output Transformer.
- Replaced all the bypass, bias & filter caps.
- Ran a wire direct from the master volume to the "Reverb Input" jack - (only slightly boosts the dry signal volume)

Still no reverb !

Any help is appreciated, as I am completely out of ideas for this.

[Tom Phillips]

Good information in your post.

...No signal output when a small speaker is hooked up to the "Reverb Input" jack....

Based on that you have isolated the problem to the reverb drive circuit. Do you have a voltmeter (DVM / DMM) ? If so, would you report the DC voltages on pin 1 and pin 3 of the 12AT7 that drives the reverb transformer?

[The Dude]

When you say "no disconnections" is that from a visible inspection or via meter. The first thing I would suspect from your description is that the little input transformer inside the tank itself is possibly open or one of the wires from the RCA jack to the transformer is broken. If you haven't already, pull the two wires from the back of the amp and check each of them from hot to ground (basically across the tank transformers). This will verify both the cable and the transformer inside the tank. You should have some resistance measurement (how much will depend on the I/O impedances of the tank). If you read infinity, the problem is either the transformer/coil inside the tank or the connections to it.

[The Dude]

Oh, by the way,......welcome to the forum!

[g1]

What I have tried so far :

- I have checked the tank, no disconnections.
- Replaced the Output Transformer.

Do you mean the output transformer that connects to the speaker or the reverb transformer that connects to the 12AT7 reverb driver tube?

[forwardmarch]

If so can you report the DC voltages on pin 1 and pin 3 of the 12AT7 that drives the reverb transformer?

Pin 1 is reading at 440V

Pin 3 reads 7mV

[forwardmarch]

Do you mean the output transformer that connects to the speaker or the reverb transformer that connects to the 12AT7 reverb driver tube?

I have replaced the transformer that connects the the 12AT7.

[forwardmarch]

When you say "no disconnections" is that from a visible inspection or via meter.

Based strictly off of visual inspection & since the reverb has been out of commission on the amp, I have had the tank set up for use with an aux-send on my tape machine & the tank functions great.

[Tom Phillips]

Pin 1 is reading at 440V, Pin 3 reads 7mV

Based on your readings at the 12AT7 reverb driver tube it appears that the tube is not conducting. The telling point is that the voltage at the cathode(s) (pin 3 & 8 too) is very low. This indicates that no (or very little) current is flowing through the tube.
Possible causes of the problem could be:
1) Bad tube
2) Open cathode resistor or solder connection in that path (Pin 3 & 8 to ground)
3) Bad heater connection causing the tube to be off
4) Bad tube socket (less likely)

Check those things and let us know what you find.

Tom

[forwardmarch]

Based on your readings at the 12AT7 reverb driver tube it appears that the tube is not conducting. The telling point is that the voltage at the cathode(s) (pin 3 & 8 too) is very low. This indicates that no (or very little) current is flowing through the tube.
Possible causes of the problem could be:
1) Bad tube
2) Open cathode resistor or solder connection in that path (Pin 3 & 8 to ground)
3) Bad heater connection causing the tube to be off
4) Bad tube socket (less likely)

Check those things and let us know what you find.

Tom

Thanks a lot for the suggestions ! I really appreciate you taking the time.

Unfortunately I'm still without any reverb.

- First I grounded pin 3/8 as you said.
- Checked each heater connection along every tube, resoldered them in case of a cold solder.
- Replaced the tube socket.
- Lastly, tried another new tube.

As an interesting turn - In lieu of reverb, I now have an 11V reading on pin 3.

[Tom Phillips]

First suggestion is that you should restrain yourself from just changing things unless they are verified to be bad by the troubleshooting process. Changing out a tube would be the exception because that is easy as is part of the troubleshooting process. In your initial post you did a good job of explaining what you already checked. We are now in the discovery phase doing troubleshooting to isolate the problem. We are looking for things that could be wrong and also rulling out things that are right so we can eliminate them as problems.

… First I grounded pin 3/8 as you said….

What I said was "[possible] Open cathode resistor or solder connection in that path (Pin 3 & 8 to ground)." I can see how my words could be misinterpreted but I meant the “path to ground through the cathode resistor.” When you trace the connection from pins 3&8 (which are connected together at the tube socket) where does it go? It should go to a resistor (The “cathode resistor”) and the other end of that resistor would be connected to ground. Fender kept changing the value of that resistor in the 1970s. In addition, since people often modified those amps the value could be anything from 470Ω to 2200Ω and it may or may not have a capacitor in parallel with the resistor. Don’t operate the amp until we get a cathode resistor back in the circuit. The cathode resistor sets the bias point and is a required component in the reverb drive circuit.

There are other basic things that we would do next. It’s possible that the signal from the pre-amp is not making it to the grid of the 12AT7 reverb drive tube.

The 1975 Twin Reverb had a master volume. Does your amp have the pull out boost switch circuit? If so is it intact? I.e. do you know if the amp has been modified? For that matter are you sure it is a 1975 model?

Do you have the schematic and layout service information? Can you post photos of the wiring in your amp so we can see the overall condition?

- Replaced the tube socket. .

Hold off on changing out the parts until we verify they are bad. As I said a bad socket is unlikely and there are ways to inspect, clean and re-tension them rather than replacing.

[forwardmarch]

the value could be anything from 470Ω to 2200Ω

Thank you so much !

I was looking past something very obvious.

The only resistor that appeared to be replaced from the stock-brown was the bright blue cathode resistor.
My first mistake was assuming the person who put that in knew what they were doing...

I added up the stripes... measured it with a multimeter & sure enough it was FAR too high.
After changing it for a resistor within the specs you gave, my tank is singing sweetly !

Thanks again !

[Tom Phillips]

...After changing it for a resistor within the specs you gave, my tank is singing sweetly !

Thanks for the report. Glad to hear that you were able to fix the problem.
Cheers,
Tom