Swapping Reverb tanks for testing?

[Mutato]

Hi,

I have a '77 Princeton Reverb with a weak reverb. Have to turn it up to ten to get some decent sound out of it. I replaced the preamp tubes (12AX7's) with some from another amp (Blues Junior, which works). Also bought a new 12at7 driver and that didn't change the sound.

Can I swap out the reverb from my Fender Blues Junior amp to see if it's a bad reverb pan versus it needing a cap job?

[mac1amps]

'sounds like it's highly likely that it's a spring-tray ("pan") issue. Unfortunately the reverb trays from the newer model Fender's aren't compatible with the older ones (the newer ones use op-amps to drive/pick-up the signal around the tank). If you have an Ohm-meter, you should take resistance readings from the ends of the tank cables. In an older model tray, one should read about 1 Ohm, and the other should read around 170-200 Ohms.

[52 Bill]

Can I swap out the reverb from my Fender Blues Junior amp to see if it's a bad reverb pan versus it needing a cap job?

No, the small tank has the wrong input transducer for the Princeton. Are you sure the tank is plugged in correctly? Try reversing the RCA plugs at the back of the amp and see what happens.

If the reverb is weak the problem could be the tank, or it could be the drive circuit and sometimes but not often it could be the return circuit.

When you shake the tank itself, does it crash really loudly through the speaker? If it does, then the problem could be in the tank or the drive circuit. You've already tried replacing the tubes, so see if you can get a friend with another Fender tube amp to get your amps together and swap reverb tanks. You can put the two amps together back to back and just swap RCA plugs.

[Mutato]

Hi,

I get crashing reverb splash when I jar the amp with the Reverb knob turned up. If the send and return were somehow reversed, would there be ANY sound?

I will explore either way. Not that I am an amp expert, I do have a multimeter and could open up the tank to inspect.

[52 Bill]

Hi,

I get crashing reverb splash when I jar the amp with the Reverb knob turned up. If the send and return were somehow reversed, would there be ANY sound?

I will explore either way. Not that I am an amp expert, I do have a multimeter and could open up the tank to inspect.

If the tank leads are reversed, the impedances will be mis-matched to the point that the reverb will be greatly reduced, but will usually still work. That is why I suggested trying to reverse them.

Take your multimeter and set it to read resistance/ohms. Pull out the red RCA plug from the rightmost rear panel jack and read the resistance from the inner tip to the outer shell of the plug. On your amp this should read somewhere around 1-2 ohms. Next do the same for the second lead from the tank. This time you should get a reading somewhere around 180 ohms.

From your readings you will be able to tell if the tank is plugged in correctly or if it has been broken or replaced with a wrong tank.

[Mutato]

Hi,

I somehow missed that part of your original post! I will check first thing when I get home tonight.

[Mutato]

I checked the connections. They are correct. I reversed them and tested. Results were NO volume on reverb. So I switched back. I will take tank out Thursday to inspect.

[Mutato]

Opened up reverb tank

Please take a look at my tank I pulled from the '77 Fender Princeton Reverb. I finally opened it up to see that they had done a hack job "repair", if you can even call it that! More like a non functioning spring holder!

Does this look like a vintage reverb unit from the 70's? Also, if I was to repair it, it would look like a transducer assembly would be needed (The brass tube with the attaching wire). It seems an easy thing to swap out if it was available as a replacement part. I emailed Accutronics to see if they can help. Obviously, if it's not worth it, I'll end up getting a replacement. It's a shame that these aren't serviceable.

Please let me know your thoughts.

Thanks,
Martin

[Mutato]

Heard from Accutronics

Hi,

This is the response from them:

Martin,
The reverb does look like one from the seventies because of the Geneva
Stamp. The reverb, if it was working, was only working at half of it
capabilities because of the bad repair. If you were to repair this unit,
you would need an input transducer assembly and a spring set, due to the
stretching of the spring at the repair. You can purchase these parts
directly from us, if you wish. The cost for a input transducer is $11.95
usd and the spring set is $9.95 usd plus $7.00 usd for shipping and
handling within the continental USA. If you wanted to you can purchase a
new reverb from any of our distributors or directly from us. The cost
for a reverb unit is $32.00 usd plus $11.95 usd for shipping and
handling within the continental USA. If you wish to order the reverb
unit you can do so on line at http://stores.ebay.com/SEPI-GROUP. You can
then make your payment via paypal. If that doesn't work for you, you can
call me direct at 800-284-5172 ext 17 or 847-639-4646 ext 17 between the
hours of 7:00-2:30, central time, Monday-Friday. You can then make your
payment via a credit card, Visa, MasterCard, or American Express. You
will also have to call me direct if you wish to order just the parts.
Regards,
Cal Shuett - Accutronics Reverb
A Division of SEPI-GROUP

Not sure if it's worth it to repair. I might be looking for a new one. The thing is, is this current tank the right one? Pulled from the tank: Accutronics model number: 4AB3C1B 17”

[52 Bill]

Not sure if it's worth it to repair. I might be looking for a new one. The thing is, is this current tank the right one? Pulled from the tank: Accutronics model number: 4AB3C1B 17?

Yes, I think that is the correct model for your amp. Repairing the tank is hardly worth the effort, as there is a lot of labor involved.

If you do replace the tank, you need to remove the foam weather strip from your old tank and apply it to the new one as well as the cardboard cover panel.

Whoever "repaired" the old tank must have thought that having the second spring just mounted in there would be better that if he had just removed it.

[Mutato]

Quick repair to reverb

I did a repair that would've made "MacGyver" proud!

I took out the brass transducer assembly. Heated up the solder to expose/clean the pinhole at the end of it. I clipped the ball end of a .13 guitar string to about 1" in length (D'Addario strings with the end hole) and jammed that through the front. It was stiff enough to push through the soft damper material and through the pinhole. That was bent and soldered to secure it. I removed the tiny brass ring at the guitar string end, so it was just a nice loop. Perfect for hanging the spring onto! Now I needed a magnet. I had a roll of soft rubber magnet material. Almost like magnetized rubber. You can cut it with an Exacto knife. I clipped a tiny bit off and used some doublestick tape to place it where it needed to go (landing right in between the coils).

Fired up the amp and got way more volume from the reverb. I don't think it's 100% original signal, but probably about 80% full sounding reverb! It will never replicate the delicate metal spring holder of the original. But this will do until I get around to buying a replacement tank.

Sorry no pictures! I did this and rushed to get it back into the amp.

[52 Bill]

Congrats, the "MacGyver" award goes to you!

[scole]

what does the foam weather strip do?

[Mutato]

Hi,

I bought a new tank and installed it last week. I've got a lot of reverb now. Way more than my half fixed tank. I did transfer the foam strip to the new one. Also ditto for the cardboard bottom.

I think the foam strip must be a dampener for the tank part, I assume. Cut down on any vibrations?

My reverb, although a lot better, still is a little weak. I think it's the caps. I did put new preamp tubes in and I'm finding I have to turn it way up to get heavy reverb. Like up to ten!

[jrfrond]

You could have a bad reverb drive transformer, an open 300pF coupling cap, a partially-shorted cathode bypass cap on the 12AT7 (both sections are paralleled for current delivery). In any case, it seems like the pan is not being driven hard enough.

Here's a little down and dirty trick for troubleshooting reverb drive: substitute a small 8-ohm speaker for the reverb pan input. The drive circuit is just a small Class A amplifier driving a transformer that is going to drive the 8-ohm reverb pan input transducer. It doesn't care what load it drives, as long as the impedance is correct. It's a quick, audible test. This works ONLY on transformer-coupled reverb circuits like the BF/SF amps. It might toast a BJ's drive circuit because the load would look almost like a short.