twin reverb problems

[tubelovin]

My '73 silverface twin reverb has no reverb. Tried new cables tubes etc. I do get the crashing sound when I bump the reverb unit but thats all. How do I check the pan to make shure thats not the problem. It did work when I got it but just failed some time without notice.

I checked voltages on the preamp tubes and they are all very high when checked against the AA270 schematic. I know thats not the right one but shouldn't it be a little closer than about 40v to 60v high at the plates? I suspect that the dropping resistors have drifted.

I would like to fix this problem and blaceface the reverb channel. Which schematic is the best one to follow? After that I'm going to experiement with the normal channel. First things first, what would be the first thing to check?

[Ray Ivers]

How do I check the pan to make shure thats not the problem.

Quick and dirty? Swap the cables and try the 'crash test' again; if you get no spring noise at all, either the drive transducer is bad or a connection has come loose.

I would like to fix this problem and blackface the reverb channel. Which schematic is the best one to follow?

As per your suspicions, I'd also check the B+ dropping resistors too (an easy check to make)... and I personally would use the AB763 TR schematic with 47K PI plate resistors, but YMMV.

Ray

[Tom Phillips]

Why 47k Plate Resistors?

Ray,
This is the first I remember hearing about using 47k PI plate resistors in the AB763 circuit. I never thought of trying that and always just change them out to 100k & 82k when I do a BF conversion. Can you explain what you like about the 47k set up?
Thanks,
Tom

[Ray Ivers]

Tom,

I've always liked 12AT7 PI/drivers in Fender amps, and I feel the 47K Rp's are a better match to this tube than the 82K/100K '12AX7 PI' values if you're looking for clean headroom - and if you're not, IMO you're better off using a 12AX7 PI with the TR AB763 values; just my $.02.

Ray