Reverb Issues on a Fender Twin Reverb

[womantone]

I am having some issues with the reverb on my 1967 Fender Twin Reverb. Which sucks =( I started to debug by going to RG Keen's page. When on 10 the reverb sounds like its on 2, and anything less than 10 on the dial and its nonexistent. I have tried a known good pan, a known good tube replacement for both the driver and the recovery. But still no avail. According to RG Keen the bias on the tube could be off. Could this be due to a failing reverb transformer? And how would I test to verify this?

[52 Bill]

I am having some issues with the reverb on my 1967 Fender Twin Reverb. When on 10 the reverb sounds like its on 2, and anything less than 10 on the dial and its nonexistent. I have tried a known good pan, a known good tube replacement for both the driver and the recovery. But still no avail. According to RG Keen the bias on the tube could be off. Could this be due to a failing reverb transformer? And how would I test to verify this?

There are many reasons that can cause the problem that you describe. I know that you've tried some of these, but in a nutshell here are a list of things to try/check.

1-Check the tank. Make sure that the tank is plugged in correctly, reversing the input and output will cause severe drop in reverb.

2-Check to see if the problem is in the drive side or the return side. If you shake the tank do you hear a loud crash? If you do, then you can assume that the return circuit is ok. If not, then check the return tube and circuitry.

3-Check the driver tube and circuitry. What are the voltages on the plate and cathode? Do they come close to the values on the published schematic? Try attaching an 8 ohm speaker to the reverb send jack on the back of the amp. If the circuit is working you will hear what the amp is sending to the tank input.

4-Test the reverb drive transformer. Swap it out with a known good one, or build the shorted turns tester on R.G.'s site.

Hope this helps.

[cbarrow7625]

Also Check the cathode resistor.

You should get "normal" voltage drops across both the cathode resistor & the transformer. The schematic for that amp should be readily avaialble. See if your voltages match the schematic (or are at least close). If you see less than 100-200 volts at the plate I would suspect the transformer. If you measure very high voltage at the cathode (above 25 volts) I would suspect a bad cathode resistor.

Do the same check on the plate & cathode of the recovery tube.

[olddawg]

Without looking at the schematic I would suspect a bad coupling cap.

[Enzo]

Without looking at anything, I would first suspect the cables running down to the pan from the chassis.

Then do what Bill said.

Turn the amp on and the reverb up some. Pull the two cables from the pan. Now touch the tip of each cable plug with your finger. One of them should make hum out the speaker. That plug goes in the jack on the pan marked OUTPUT.

Is that hum strong or weak? if strong, the reverb recovery is likely OK. If weak, it is your problem.

Check the drive side by looking for signal at the cable to the pan INPUT jack. You can scope it, read signal with a voltmeter on AC, or connect it to a small speaker or another amp. Voltages on the schematics are only ball parks anyway. Is there signal on th plates of the drive tube?

[womantone]

So did a little reverb testing. Checked the cable using a 9v battery and multimeter. Made sure the right inputs and outputs were in the respective spots. Then I did the recovery test by banging on the pan and we got some nice loud crashes. Then we test the driver, using a speaker from a Fender Champ. Dead silence with any reverb. So I went to work measuring voltages on the 12at7. 0v on the cathode, and 0v on the plate. We also checked voltages with a knownn good 12at7, So I checked the incoming voltage from the filter section to the reverb transformer. That was in the ball park range schematic gives. So I figured its a bad transformer So I took out the transformer, went to radio shack built RG keen's transformer short tester in 10 min. And low and behold we have our culprit. So a new one is on order from triode electronics, it arrives Monday or Tuesday will let you know the verdict. Thanks for everyones help. I am new to this diy thing, but it looks like I am getting the hang of it. Rebuilt a 1970 Fender Champ, a 1967 Twin Reverb. Not a bad resume for just out of the box.

[womantone]

So that was the ticket. I replaced the transformer and the reverb works. At first it was slightly weak, but as we played it more it seemed like the cathode cap rebuilt itself, and the reverb got to its normal level. I am in tonal heaven right now.

[gil gillian]

Hi, I'm a newbey to this forum. Have just picked up Fender Pro Reverb circa 75 (SF) that has almost no reverb, (and probably needs all the usual valves replacements and new caps). Followed the thread and checked the tank return is good and checked the the feed by connecting to 8ohm speaker. Also swapped the 12AT7's over and this made no difference.The speaker is quite low but audible. Question is I don't know how loud it should be, Is there a meter test that can be run at the send to check level? I've also checked inside inside the tank and no obvious wires off. Any thoughts as to what next? Great Forum, wish I'd found years ago! Thanks.

[52 Bill]

Hi, I'm a newbey to this forum. Have just picked up Fender Pro Reverb circa 75 (SF) that has almost no reverb, (and probably needs all the usual valves replacements and new caps). Followed the thread and checked the tank return is good and checked the the feed by connecting to 8ohm speaker. Also swapped the 12AT7's over and this made no difference.The speaker is quite low but audible. Question is I don't know how loud it should be, Is there a meter test that can be run at the send to check level? I've also checked inside inside the tank and no obvious wires off. Any thoughts as to what next? Great Forum, wish I'd found years ago! Thanks.

Gil:
Welcome to the forum. As many people here will tell you, if you start a new thread for your problem, you will probably get more response.

As for your questions, there is no definitive level to check for from the reverb drive output. Generally, I use the speaker test to see if there is any output at all.

Can you tell if the tank is original to the amp?

If you have an ohm meter check the resistance of each transducer coil. If you check these readings at the RCA plugs located at the back of the amp, you can check the cables and connectors as well as the coils at the same time. Set your meter to the lowest ohm setting. Facing the back of the amp, pull out the cable that is furthest to the right and hold you meter leads to the tip and the shell. Your meter should read somewhere around 2 ohms. If you get no reading (infinity) or a short (zero ohms) either the cable is bad or not making a connection, or the coil is open.

Next pull the second plug and test the same way. This time you should get a reading of about 200 ohms.

In your case, you can assume that the cables are ok, because you have reverb, just not enough. If the resistances of the coils is off, then you will need to check the tank.

[womantone]

Gil, you addressed that caps might need replaced. Have you checked the voltages on the tube? If not, grab the schematic and test each point to the ground and see if the voltages match the schematic (keep in mind +/- 20%). Anything out of the ordinary let us know. My guess is the cathode cap or the reverb transformer is history. Both are inexpensive fixes.

[Enzo]

And make sure teh cables to the reverb pan are not reversed.

[gil gillian]

Thansk for all the feedback.

Bill's q's; the output from Reverb Send with channel gain @ max and MV @ min is just audible on an 8 ohm speaker. As far as I know the rev tank is original, it is an Accutronics series 4 (4AB3C1B) which has 8 ohm input and 2250 ohm output. When shaken there is output to the amp. Ohm meter readings are 2 and 180 respectively DC not inductive, so that seems about right.

Womantone;I've not dug inside yet and checked any voltages. I like to do all the no voltage stuff first; I'm a wus when it comes to HT rail stuff! If the transformer, where can it be obtained, I'm in UK? I'm assuming that the caps will be on their way out because of the age of the amp. It was fairly well maintained in a project studio up to about 10 years ago and then put into storage, where its been ever since.

Enzo; I'm assuming that because when shaken it outputs the cables are right.

I have a circuit diagram which appears to be 74/75 45 W CBS MV (no mid control) drg no 012108. Is the cathode cap the 560pf between anode and cathode?

Thanks again you guys.

[Enzo]

DOn't assume, know.

Pull the cables from the pan, turn the amp on and the reverb up some. Now touch the tips of the two cables. One will hum out the speaker. MAke sure that one goes to the OUTPUT jack on the pan.

The input end of the pan will make a weak output end. The output end will make a weak drive end. If the cables are reversed, you can still get sound from it, but it wil be weak.

When you shake the amp, is the reverb noise godawful loud? SHould be.

[gil gillian]

Cheers Enzo. Yes they are correct, a lead connected to Reverb Output on back of the amp makes a good buzz, and this is connected to the pan output, ie the high impedence side. Although why Leo/CBS name Reverb Return as Reverb Output is a mystery to me. Guess it's one of those sidewalk/pavement issues! And yes the tank is outputting a row when shaken. I'll start checking through the circuit around the 12at7 reverb driver and transformer next.

[52 Bill]

Gil:
To me, is seems that if the sound from the speaker is barely audible, the output to the tank is on the low side. Time to check the circuit related to the driver tube.

Do as Womantone suggested, and check all the voltages per the schematic. Note that not all versions of this amp have cathode bypass caps installed. This is the cap that connects the cathode to ground.

The 560 pf cap that you metioned earlier is to cut the high frequency response of the circuit, to eliminate any possible parasitic oscillations in the circuit.

There is one other possibility, and that is a bad tank. I've seen it happen when a tank is just weak. If you have access to another tank, try swapping them out as a test.