Fender Pro Tube Reverb: no workie (UPDATE: Fixed!)

[markszabo]

Greetings Amp Garage people. First time poster, long time lurker.

I'm at my wit's end with a 2002 Fender Pro Tube Pro Reverb model. This is their pre-Super Sonic amp with a high gain 2nd channel. It's actually quite a nice amp with big trannies, lots of tubes, and that classic Fender 6L6GC wallop. Only the reverb doesn't work.

I've tried everything I can think of. It appears to be a problem with the supply side of the pan, as tapping the pan with the reverb control up results in spring crash. I have:

- Replaced the pan (twice, don't ask)
- Replaced the 12AT7
- Replaced the transformer
- Benched the amp with a dummy load and checked voltages, all check out within reason (although I did find a bad tube in channel 2, bonus!)
- Run signal through the amp and scoped it, seeing signal all the way to the pan input

So if this was your amp, and you'd done all that, what would be your next move? For reference, here's a link to the schem:

http://ampwares.com/schematics/Pro_Reverb_Schematic.pdf

Thanks, and nice to meet you -

[NickC]

You mention one 12AT7 tube. Isn't there a reverb send tube and a recovery tube in the circuit? (I can't see sideways schematics, makes my vertigo go wacko. Plus, schematic is labled a Bassman 100 and not a Pro Reverb Series amp).

Check continuity of the reverb send and return cables to/from the pan?

[pops]

This may sound dumb, but easy to do. reverse the wires going to the pan, easy to get them backwards.

[markszabo]

Ah, yes, thanks Nick: There is indeed a recovery tube, V5, which is a 12AX7WA. I swapped it with a replacement with no change. And yes, I did do a continuity check of the reverb cables.

The schematic may be labeled "BASSMAN 100" but it's indeed the amp in question.

Pops: I've checked and rechecked the send and return from the pan. The new pans actually have color coded surrounds on the input and output, so if you can tell red from white, it's pretty easy. That said it can't hurt to check it again!

[Firestorm]

First, I would check the pan. I've seen a lot of bad input transducers even on new units. Make sure the transducer isn't open. If the pan is good, check the control voltages at the reverb mute test points to make sure the amp isn't muting the reverb all the time.

[markszabo]

Firestorm, I'll check those control voltages. Great idea. Pretty sure it's not the pan as I have not just replaced the pan, but tested it too. Thanks, I'll report my findings!

[markszabo]

Update: it's fixed. The cabling was reversed.

I feel pretty stupid, but to my credit, the input on new Accutronics pans have a white surround, and the output has a red surround....the opposite of the way the amp has its send and return marked.

The amp sounds fantastic now. Channel 2 is surprisingly high gain, even more than a first-generation Super Sonic.

[Stevem]

Now that you have it working take a preemtive measure a grab a tube of whatever color silicone sealer you want a place a dab on each wire soldered to its RCA jack and a good blob where the wires go into each little transducer.
This will keep them from snapping off as they always do with time and enough travel in a car.
While you are in there also make sure that each transducer is gripping its iron core tight, as many times even on new pans they rock around and should not.
To fix this get some medium thickness super glue and dribble some down inbetween the transducer and the core.
After these mods the only thing that can fail is the thin termpered wire that grabs each spring end and there is no way to cure that maylay but for a pan lock like used on some vintage 70s Kustom amps.