Washed Out Reverb Issue

[dorrisant]

What would cause the reverb circuit to sound washed out? I have had problems before with the reverb being too strong and feeding back too much... this is not it.
This amp is an AB763 BF Super Reverb. I can turn it up all the way and really not too much feedback but the signal is all washed out. There really is no definition to individual notes... Everything sounds like one big morphing chord that changes depending on the notes you pick out. New JJ tubes for the driver and recovery, different tubes produce same result... original tank and TX. I ordered a new TX just to satisfy my curiosity. Also, my tank that I use for testing sounds exactly the same as the original.

Kinda stumped here... still looking for clues at the scene of the crime.

[xtian]

Just guessing, but a common failure is the cathode bypass cap on the driver stage.

[dorrisant]

I will check that out.

[Stevem]

Have you scoped the input to the pan and the output to the mixer to look for signs of oscillations ?

[dorrisant]

No Steve, I have not scoped it but I will. I had surgery a little over a week ago and I'm a bit slower to get back in the groove than expected. I will do this and report back.

Thanks for reading this guys. Much appreciated!

[dorrisant]

Cathode bypass cap for the driver stage is new/good... I checked it before I put it in. I scoped it, looking for oscillations... none. I replaced the tank, cables and rev TX... no dice. Put all of that back. Then... I isolated the return jack from the chassis and ran the ground to the cathode ground for the recovery stage. This cut the hum down but still not completely gone. I don't think that it is magnetic coupling for the PT... It still hums even when the tank is disconnected. I'm out of ideas.

[tubeswell]

Just fixed one where there was no reverb. Turns out it had a broken pan spring (which was easy to spot), blown Reverb transformer (which I had to troubleshoot with a VaC source) and a 12AT7 driver tube only working on one triode, which I spotted by measuring the voltage drop across the cathode resistor.

So there's some ideas for you.

[dorrisant]

I tried all of the above. Changing the ground point for the recover stage helped the most.

[tubeswell]

Maybe the one of filter cap grounds has become unconnected from the ground return? This causes 'sponginess' of notes under signal conditions because inter-stage feedback through the power supply resistors occurs if the supply nodes are not properly decoupled. Taking some HT voltage readings along the supply resistor dropper chain will give you a clue. If you measure two supply nodes at the same DC voltage, you will know. Also check the DC connectivity between each filter cap lead and the chassis. (I had this once on a DR reissue - the PCB trace for the Pre-amp filter cap ground had lost ground connection, and jumping this to a known good ground return point at the V1 cathode ground cured this)

[dorrisant]

Thanks man, I will check that.

[Stevem]

the amp now has the output stage screen filter tied in with the 2 preamp filters and that's wrong, it should be landed with the ground for the main stack of filters.

Also your Reverb stages tube sockets may be highly tarnished or have some corrosion making for very high resistance .
Wetting the tube pins up with contact cleaner or WD40 from a Q tip and installing and removing them and rewetting them 3 or 4 times will bypass that potencial issue.

[dorrisant]

the amp now has the output stage screen filter tied in with the 2 preamp filters and that's wrong, it should be landed with the ground for the main stack of filters.

I don't see how. I did not touch the ground for the driver stage... which is tied to the same filter as the screen supply. I did isolate the reverb return RCA jack ground from the chassis with Switchcraft fiber washers. Then I ran a ground wire from the isolated ground tab to the front of the chassis where the recovery stage filter cap is grounded. Maybe I didn't explain well enough.

All of the sockets are brand new... no tarnish at all. Checked all of the filter caps... even changed the one supplying the recovery stage for grins. Voltages dropping along the power rail as expected... so I am kinda lost here.

[Stevem]

Sorry I did not mean the filter thing to be related to your issue , it just a common issue that makes for more general hum in Fenders then there should be.

You might check the pan itself, in peticular the two coils mounted on the iron cores at each end,
If they are loose they can not drive the springs or gather vibrations from the spring on the other end.

In this case I run medium super glue down the core hole to lock them on, or shave down a Tooth pick and jam two of them in there.

[dorrisant]

No offense taken Steve. You are always very helpful. I tend to leave out details in my descriptions. I appreciate anyone who reads this and posts a reply.

I have tried three different tanks by the way. Two were brand new Mod brand, the other was an original Accutronics from another Twin that works perfectly... has the same number as well.

[tubeswell]

Have you checked all the coupling caps to and from the reverb circuit for DC leakage? (i.e. by properly lifting the AC-side of each cap from the circuit and powering the amp up to measure for DC on the AC-side lead of each cap). Even 1V will throw off the bias on the grid off following pre-amp stage. Maybe focus on the coupling cap immediately after the recovery stage to begin with.