Fender DDRI Reverb problem

[vintasonic]

48 minutes ago
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#1
I have an interesting problem with Fender Deluxe Reverb Reissue reverb.

The amp came to me with no tank so I put the recommended 4AB3C1B tank in and it sounded awful, the reverb was overdriven and distorted. I put a 400Hz sine into the amp, looked at the input to the reverb transformer and it had a weirdly distorted signal, there was an additional peak and a flat spot. I checked all voltage, ac and dc and they all match the schemo.

Here is everything I swapped out - tried another thank, different cables, 12AT7 tube and the reverb driver transfomer - all new stock. Checked for wrong components in reverb driver circuit, all match schemo.

In trying to find the problem source (circuit, tank, something else) I tried a 4EB3C1B -  used in the Fender Hot Rod series, which use an opamp to drive the tank. With this tank it works fine, no distortion. I then tested with a 4FB3C1B which also worked but the reverb gain was low (which I expected). As I'm sure many of you know, the only difference between the tanks is the input impedance.
A = 8 ohm
E = 600 ohm
F = 1475 ohm

Just for the record, I have fixed dozens of reverb circuits, replaced thanks, troubleshot many a weird problem - this one has me guessing. I have a suspicion it may be the filter caps - they are the IC caps which often fail in Fender amps - but they look clean, no sign of leakage. I alligator clipped new caps one by one across the current filter caps, no difference at all. My solution will be to use the 4EB3C1B if I can't find another solution.

Thanks!

https://www.ampwares.com/schematics/65_Deluxe_Reverb_RI.pdf

[sluckey]

I would try another 4AB3C1B tank.

[PRR]

He "tried another thank". Apparently before going to the E and F tanks.

Me, I'd be reaching for the bright light and magnifier, be sure these parts were what I thought they were, or just dyslexic distortions.

[sluckey]

He "tried another thank".

No. He tried "a" tank. The amp came to him thankless. 

My thinking was a dead tank.

[vampwizzard]

if the signal looks like crap before hitting the transformer, could it be a vibrato channel preamp issue? Maybe a bad ground around R31 or R39?

[vintasonic]

Hi

Just to be clear - I tried 2 - 4AB3C1B tanks.  After the first one acted weird I tried another, I have a stock of several tanks of each.

The signal to the input of the tube for the reverb driver is clean, the ouput only gets distorted when I connect the "A" tank, so it's got something to do with the tank loading the circuit.  Again, the 4EB3C1B shows no similar loading problem.  I measured the cathode resistor and it is the right value. If it were a problem with the cathode bypass cap then the gain of the circuit would be incorrect. It's a very simple circuit, I have run out of things to troubleshoot or swap out.

In addition, I was really suspicious of the caps so I lifted the grounds and tacked in all new filter caps - no change.  I don't believe it is two bad reverb tanks from stock because there was at least one other one in this order that was used in another amp.  All new, all MOD tanks from CE

Left the 4EB3C1B because it works fine and sounds good, I cannot get the driver stage to distort with this tank no matter how hard I drive it.  The 4AB3C1B starts to distort when the channel volume hits around 4-5.   

Absolutely stumped at this point.

[sluckey]

Do you have another AB763 amp that you could try those MOD tanks in? The driver circuit is such a simple circuit. There's no logical reason that a correct tank won't work. There has to be something wrong in the circuit or the MOD tanks (I agree, not likely). I know it's aggravating working on that PCB but I don't accept that using a 4EB3C1B is the fix. The real problem is still there.

Some things I would try... measure actual resistance between chassis and pin3/8. Also measure actual resistance between chassis and pin 2/7. Change the 500pF cap.

Don't give up. I believe you will find the logical answer. I'd love to hear what you find.

[vintasonic]

Sometimes there's fixing the problem as opposed to finding the problem 

I left the 4EB3C1B and will give it back to the owner, if he has problems with it I'll go back and try again.  It sounds fine so I don't think it will be an issue.  I agree that it's a simple circuit but after I replaced virtually everything in the circuit I can't invest anymore time if it's working.

It comes down to a:) somehow the reverb cables, which is a possibility.  I used an 3' RCA pair but I have had a lot of problems with various RCA cables.  The first one I grabbed caused oscillations, which I have seen before.  So maybe, or b:) both the MOD tanks are bad, again not likely but not impossible.  I've been fixing things for a years and impossible things happen all the time

Also, this is a Deluxe Reverb re-issue, if it were a vintage amp I would have continued to investigate.  Nothing against the re-issues, it's really more a matter of the value of the amp.

If it comes back, I'll fix it and let you know what I find

[mresistor]

most likely if it were a vintage amp you wouldn't be having these problems

[tubeswell]

If its a DRRI, its (quite) possibly a bad trace/pad/joint somewhere  - could even be in the PS ground return rail. Try measuring for DC continuity between each filter cap ground lead, and the chassis. (I've found open traces in several RI amps)