Reverb tank feedback: could this pig stop squealing???

[Victor Denance]

Hello folks!

My customized Super Reverb build is finished and works great (I took great care of the lead dress and it's as clean as any pre CBS amp).
However, the reverb tank squeals so bad I can't turn the reverb past 4 (which is already quite wet, but I'd enjoy pushing further from time to time).

I taped a heavy piece of cardboard at the bottom of the tank, wrapped the whole thing into a big ole winter quilt, and left the "package" unscrewed floating at the bottom of the cab.
But still, the squealing is there. I have to move the tank out of the cab to make it stop.
I changed the two verb tubes and it's still there.

Also, while checking voltage, I noticed the verb recovery triode plate voltage was at 200V, while other V1 ab V2 and V4b plate voltages were around 270V.
Could this be the culprit?
I also did not have a 0.003uF cap for the reverb recovery decoupling so I used a slightly different one 0.005 or 0.001 IIRC
Could this again have an effect to the feedback?

Where else should I look for?
Could I have a bad tank? (it's not a real Accutronics, it's a cheaper Belton)

Thanks so much for your help!

Victor

[6267]

Those voltages sound suspect, because either v2 etc aren't conducting, or the reverb recovery is conducting too much...I'm assuming same valve type & plate resistor, as well as B+ node. Check for solder flakes etc. that might be shorting stuff out on the recovery stage, although if the anode resistor is significantly larger than the other stages, the voltage will be lower.

I've gone through the squealing reverb blues more than once. The first step is to realize that these things transmit high frequencies faster than slow ones. In some amps, this means positive feedback. Acoustically or driven by the transducer, the HF bandwidth must be limited enough to prevent oscillation, but not so much to affect tone.

Mechanical: see if the transducers in the tank wobble. If so, shim them with toothpicks (between coil and metal frame thing) to prevent microphony or other vibration related coupling. Tape over the little corners & holes in the tank. I taped my cardboard backing down too. Check the cables for microphony as well, you may be surprised! Also see if it squeals without the tank attached, since that probably means the recovery stage is just getting oscillations triggered by the tank. Move the tank around.

Electronic: try shunting the highs from the recovery anode to the cathode, or back to the grid, or from the grid to ground (or whatever you prefer). A .0047 from grid to ground may work, or from anode to grid. I used a ceramic high voltage type for reliability. Sounds fine. You can also try removing the bypass cap, this will help prevent oscillations in the recovery stage, but you may need to make up more gain somewhere else, maybe not. Not sure if that schematic uses one, but try a 100k grid resistor in the recovery.

Good luck, that's the process I used. Ended up being the tape on the corners of all damn things for me, and I ended up using a pretty large shunt (.015) off the return input. Still has plenty of highs, somehow. I think sealing the tank had a bigger effect, however.

[Victor Denance]

Thanks a million sir!
I'll go through the mechanical part first, then I'll warm the soldering iron up, then I'll let you know.

Enjoy the weekend and best regards,

Victor

[Victor Denance]

Update: this is one really squealy pig!

I isolated the whole thing w/ heavy tape and it still feedbacks a lot.
The squeal continues when I unplug the reverb in jack, and stops when I unplug the reverb out jack. Does it mean a thing?

Guess I'll have to go through the electronic process too.

Thanks!

[Steve A.]

Are you sure that the Belton tank is a compatible replacement for the Accutronics one that Fender used?

You might try measuring the DC resistance of the input and output of the tank, and posting it here.

One last question: are you sure that you are plugging the tank in correctly? I ask because I always thought the labels on the old BF/SF amps were downright confusing. To make sure that I plugged the tank back in correctly I would color code one of the cables and its correct jack...

Steve Ahola

P.S. If you have a friend (or even an enemy ) with a Fender BF/SF reverb amp you might try plugging your tank into the Fender amp to confirm that it works properly.

[6267]

Ok, so every tank has an input which is driven by the amp, and an output which goes back to the amp. You're saying that the squealing continues when the output (back to the amp) is pulled? If that is the case, something weird may be going on with your recovery stage. If you meant that removing the tank input wire doesn't stop the oscillation, then it's still likely the tank. It means that the acoustic feedback loop includes the tank, the driver, and everything after it until the speaker. You can check this by isolating the speaker or tank (say with a pillow or heavy blanket).

If removing the wire that goes from the tank *back* to the amp to be reamplified (output) doesn't stop the oscillation, the problem is probably with the recovery stage. You may want to examine its operating conditions and bandwidth...check the cathode bypass cap, the cathode resistor, try putting in a grid stopper before you start soldering in anti-oscillation caps.

If removing the wire that goes from the tank *back* to the amp to be reamplified does stop the oscillation (or starting amp with tank disconnected and leads floating), you definitely have an acoustic, and probably not electronic problem. It could be that the Beldon tank couples some frequencies acoustically or mechanically much differently than an accutronics tank. If isolating the tank as best as possible doesn't cure this, you may have to limit the high frequency bandwidth available to the recovery stage of the tank. .0047u ceramic 500v, anode to grid is a good starting point, or .0047u grid to ground or cathode (less fancy). Increase as necessary. Sometimes reducing the gain of the recovery stage and making it up elsewhere is a good solution as well.

Essentially, the answer is if you can't stop the high frequency acoustically, even if its an acoustic problem (due to say a small combo or other layout issue), you will have to compromise and try to filter that resonant frequency in the circuit somewhere. I hope it's a high-pitched squeal, easier to get those and keep good tone!

I haven't drank all my caffeine yet, so sorry if that's a bit muddy
Good luck, and is this beldon tank about the same size as a Hammond/Gibbs/Accutronics tank?