Reverb tank puzzle

[Stratfordade]

I've fixed up a Fender Performer 650 so all is working perfectly, but having reassembled the amp and done a final test playing guitar into it realised there was no reverb, so took it apart again to find out why.

The tank is marked S2AB2C1B so I believe it has 8 ohm input impedance and 2250 ohm output. DC resistance with multimeter measure 1.1 ohm and 180 ohm output which seems correct according to tables I've seen of DC resistance.

When I connect the output to the amp and touch the springs, you get the expected noise and can control with Reverb pot. If I reverse it and plug the input transducer into the 'output' plug, I also get the characteristic sound albeit quieter.

But feeding a signal into the amp right at the input I don't hear any reverb effect, and there's no output from the Reverb recovery op-amp, not even the 10mVAC TP14 indicated on the schematic (attached).

With the input to the tank disconnected I get the 1.2VAC TP13 expected from the schematic. With the scope still connected the trace goes to zero as soon as I connect the tank input plug.

So there tank shows expected resistance readings and I can make it sound the right noises when I wobble the springs, but it kills any input signal from the amp. I'd have normally thought the input transducer was somehow shorted out yet it can create an output so it's very puzzling.

Could anyone please point me to any further tests to decide if the tank is scrap or not, and/or explain the behaviour I'm seeing? It's frustrating with the rest off the amp performing so well. Thanks.

[52 Bill]

That is the wrong tank for that amp. The drive circuit is part of the feedback loop of the opamp and needs to see a much higher load.

[dmeek]

Instead of AB it should probably be EB. The input coil needs to be higher impedance with an op-amp drive.

[Stratfordade]

I was just coming back here to answer my own query but the members here are so smart that two of you already spotted the problem!

I looked at the parts list for the reverb tank number as the one in the amp looked not original (and the screened input lead had clearly been desoldered and was not terminated with a red plug). Yes the parts list has 8EB2C1B and that's 800 ohm impedance not 8 ohm!

I'll look into sourcing the correct tank. Someone had clearly been here before me. I see the 8EB2C1B is also a 3 spring pan whereas the one I've got in front of me is only 2 springs.

Thanks for such fast and helpful answers though. Feel very close to resolving this now.

[g1]

Not sure about the S, but the 2AB2C1B would be a drop in replacement for 4AB2C1B, so that tank you have should be usable in any classic style Fender tube amp.

[Stratfordade]

Thanks. I’m bound to need it in future when the right amp comes along.

[glebert]

The 3 spring tanks are shorter than the 2 spring, you can probably use either. Sometimes the shorter tanks are easier to mount in the cabinet far enough away from the transformer (to avoid hum). Some people say the 3 spring tanks are more complex sounding, some say they are harsher sounding, but I think it also varies from one manufacturer to the next.

[Stratfordade]

The 2 spring tank (S2AB2C1B) that was in the amp when I acquired it is actually pretty short (just over 9”) and though it’s not original nor the flying leads connecting it, it was mounted in the original cabinet holes. Whether the new 3 spring tank sounds harsher or not is less of a worry for me as there was no reverb at all up till now!

[glebert]

The 2 spring tank (S2AB2C1B) that was in the amp when I acquired it is actually pretty short (just over 9”) and though it’s not original nor the flying leads connecting it, it was mounted in the original cabinet holes. Whether the new 3 spring tank sounds harsher or not is less of a worry for me as there was no reverb at all up till now!

My bad, I was thinking of the 4xxx two spring tanks which are longer. BTW, I am a big fan of these amps, at least for cleans. I've had probably five Performers or Roc Pros. Great bang for the buck amp and pretty lightweight too.

[Stratfordade]

Yes I’m very impressed by the clean tone and it’s seriously loud - I can barely move the clean vol off 1 unless I use control on guitar to keep volume down to domestic friendly levels. The overdrive sound nice and tight and versatile too.