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

[keithb7]

Hi folks. I have been having trouble with my reverb circuit on my 1973 Twin Reverb. I have read through several posts here and have tried many suggestions. I have attempted to troubleshoot by measuring several specs vs the schematic, as well as I have swapped in new parts in hopes of shot gunning the repair. So far no improvement. I have read many times, where Enzo has said that swapping in new parts most often, does not lead to a repair. I agree. I''m just out of options and now I am humbly turning to the group here for advice. Here are the details, symptoms and what I have tried so far.

When I first acquired the amp, with the reverb on 10, sounds like it is on 2. It works, it just does not seem to go beyond, what sounds like a #2 setting on the dial.

First of all I did a full cap job on the amp as it was overdue. All original. I changed all cathode caps too, and bias caps. Set amp bias and adjusted bias balance. 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. I tested voltages on the tube pins and compared them to the schematic. Nothing that stuck out as abnormal to me. I measured the resistors in the reverb circuit. Nothing abnormal found. All my reverb circuit tube cathode caps are new and good I assume. I do not know of a sure fire way to test the reverb transformer. A new one was $10, so I put one in. No difference. I got to thinking and scratching my head. Maybe it's the 100K reverb pot? Measured it. Within spec. Went ahead and changed the pot anyway, it was under $2. I swapped in some good known working RCA send and return cables. No improvement. I tried swapping send and return cable positions, incase of a mix up. No improvement. I pulled the reverb tank and inspected inside the tank. Nothing obvious. I have a friend with a 1971 Twin Reverb. I pulled out his known well working reverb tank and put it in mine. No improvement. Exact same original symptoms in my 73 Twin when I put his tank in my amp. I put my tank in his 71 Twin, works perfect.

I'm out of ideas. I have done all I can think of. Anyone have any more suggestions? Any help appreciated. Thanks, Keith

[g1]

Figure out whether the problem is in the send or return part.
Shake the amp around a bit with the reverb turned up, do you hear the springs crashing? As loud as with your friends '71?
If so, the return side is good.
For the send side, you can drive a speaker with the cable that goes into the tank. Again, you can compare volume to the '71.

[nosaj]

How does the amp sound minus the reverb?

What tubes are you using in v3 and v4?

Retouch those solder joints since a lot has been done. Your probably seeing the issue but are blinded to it.

nosaj

[pdf64]

Try loosening off and retightening all the RCA sockets, to ensure a solid connection to the chassis 0V.
Are you sure it's not a bad footswitch cable, eg tried it without the footswitch connected?

[Enzo]

What g1 said^^^

Isolate the problem. You have thrown a million parts at it, now sit down and TROUBLESHOOT. That does not mean guess "maybe it is this", it means finding where the problem lies.

You have two RCA cables. Turn the amp on, and have the reverb up half way. Now with the RCAs unplugged from the pan, touch the tip of each with your finger. One should make hum and the other does not. Is that hum from the one that does a strong hum, or is it diminished like the reverb? That tests the return circuit. And since it is a Fender tube drive reverb, the other RCA cable will drive a small speaker. Connect it to a speaker of some sort, and play through the amp, preferably into a dummy load so you can hear the result from the extra speaker. Does that drive cable make sound out the little speaker?

I'd expect only one of those to have a problem.

g1's rocking the amp to make the springs crash is similar to touching the plug tip.


The RCA that hums goes into the OUTPUT jack on the reverb pan.


Once we know which half of the system has the problem, we can troubleshoot further.

[keithb7]

I know, I'm guilty. My reverb circuit troubleshooting skills suck. I know little about the circuit and have not had much trouble with the circuit before. Therefore no experience with it other than a tube swap here and there.

Ok so to answer a few questions:

Reverb know set at about 1/2, shaking tank, yes lots of groovy spring "boing" noises that you hear from a healthy circuit when amp is bumped.
Touching the 2 RCA tips that I pulled from the tank. Input buzzes when I touch it. It is a strong hum. Output RCA tips stays quiet when touched.

I am pretty confident the symptoms are not from a bad footswitch cable. The footswitch cable only grounds the reverb circuit to shut it off.
The design of the reverb circuit is that it is active full time when no footswitch is attached. I am troubleshooting here with not footswitch attached.

The amp sounds great 100%, except for the reverb.

Fixing to hook up a speaker now to tank input cable. Will report back shortly. Thanks.

[keithb7]

Hooked a speaker up. Pulled the wire marked input from reverb tank. Soldered a female RCA tip to send of 2 speaker wires. Plugged wire braided reverb cable into speaker. Other end of wire
plugged into amp chassis rear, marked "Reverb Send". Turned on amp. Reverb knob on zero all quiet. Turning up reverb knob quickly produces loud pitched squeal from added speaker.

As seen here:

[Jazz P Bass]

Touching the 2 RCA tips that I pulled from the tank. Input buzzes when I touch it. It is a strong hum. Output RCA tips stays quiet when touched.

Is this setup backwards?

There is an Input & Output jack on the amp & on the Reverb Pan.

The wire From the amp that hums when touched goes To the Output side of the Reverb Pan.

[keithb7]

Must have mixed up a wire somewhere during all this. Yes output wire buzzes. Sorry. Hooked speaker back up to proper reverb input wire. Turned amp on. No noise at all coming from the speaker. Turning reverb pot up does nothing.

[Chuck H]

I was thinking the same thing as Jazz. If you've been fastidious about how things are plugged in, is it possible you've consistently had the tank plugged in backwards?

[keithb7]

I've plugged the tank in as many ways as possible. No change. With the speaker hooked up to input at amp chassis, (aka input of tank) I get no sound. Reverb knob up or not.
I assume this most likely means the send section of the reverb circuit is suspect? Thx.

It is confusing. Does signal travel like this?
Out from circuit at chassis RCA jack labeled reverb input. Wired to reverb tank input jack. Signal travels through springs to tank output jack. Wire from there goes to RCA on chassis rear labeled Reverb input.

[Chuck H]

Must have mixed up a wire somewhere during all this. Yes output wire buzzes. Sorry. Hooked speaker back up to proper reverb input wire. Turned amp on. No noise at all coming from the speaker. Turning reverb pot up does nothing.

In case you didn't do this, you need to send a signal through the amp to get sound from the speaker connected to the reverb transformer. You're actually using the reverb transformer to drive a speaker instead of the reverb pan. Just like the amps output transformer is connected to the amps speakers. If you don't put a signal through it, nothing comes out of it. The reverb pot won't be part of the reverb transformer/speaker test circuit. So adjusting it won't make a difference.

[Chuck H]

First you said:

When I first acquired the amp, with the reverb on 10, sounds like it is on 2. It works, it just does not seem to go beyond, what sounds like a #2 setting on the dial.

Then you said:

I know little about the circuit and have not had much trouble with the circuit before. Therefore no experience with it other than a tube swap here and there.

Did the reverb on this amp ever work properly or not? Did the problem just pop up one day with no suspected cause? Did the problem start after some work you performed?

[keithb7]

Thanks Chuck. The amp was acquired in December. The reverb has acted the same, from the day I got it. The amp was all stock when I got it, the reverb was weak. I did the cap job, and got through the rest of the items the amp needed, and am now down to the last remaining problem. The reverb. When I said "I have had not much trouble with the circuit before", I should have been more clear in saying, I have not had any trouble with any reverb circuits on any amps I currently own, or have owned in the past, therefore I have little to no experience troubleshooting the reverb circuit. Sorry about that.

When I was testing the reverb sound through the speaker I hooked up to the reverb circuit, I had my guitar plugged into the amp and I was strumming to put a signal through the preamp tubes. I placed my hand on additional speaker cone to also confirm there was no cone movement. It was dead silent.

[Enzo]

On the amp end, let's call it the drive and the return (or recovery). If we use in and out for both the amp and the pan, it gets confusing. The return cable, the one that hums when touched, plugs into the OUTPUT on the pan.

When you connected a small speaker to the return cable, it acted as a microphone, so that is why you got feedback.

The small speaker is connected to the drive cable, then you play guitar into the normal amp input up front, and in a working amp, that little speaker now should be driven by the reverb drive.

Instead of strumming, play music through the amp. use a CD player or MP3 player. I use a line from my shop stereo FM receiver. This frees your hands.

Since it sounds like the drive side is not working, we play music through, then use a scope or a signal trace or an AC volt meter to follow the signal through the drive circuit.

There is a 12AT7 driving the small transformer. Is ther high voltage on the plates of that tube? And if so, is ther signal there? No? Then how about at the grid of the tube? If not there, go back further along the path until you get to the point the reverb signal branches off the main signal path.