reverb feeding back in new Princeton build

[norburybrook]

Had a strange thing today.


The reverb in the new Princeton build started to slowly over time feeding back until it was howling . I didn't realise it was me and thought it was the singers mic, I was surprised when it was coming from the combo, i hit the footswitch and it stopped. I was using the trem at time as well. It started again slowly when I engaged the reverb, seemed to only happen when the amp was hot, listening carefully when cold there's a slight 'overtone' on the note it feeds back on present but over time it starts to just 'feedback' .

i have removed the 1M resistor and added a dwell 1M pot on the back of the amp, could this have anything to do with it? do i need to shield the cable run to and from this pot? never heard anything like it before from an electric guitar amp. Sounds like a microphone feeding back.


M

[didit]

Hello Marcus ..

Photos of the current as-built guts might help us help you.

Best .. Ian

[norburybrook]

Hello Marcus ..

Photos of the current as-built guts might help us help you.

Best .. Ian

Thanks Ian, I'll do it tomorrow been rehearsing with it all day.


M

[10thTx]

IF you have a 500p silver mica going into the dwell pot, you might experiment with a ceramic in there and see if it makes a difference? I've a number of silver mica caps be bad from the start or seem to go out. The other thing I can think of is do you have your reverb tank isolated in a reverb tank bag or something similar?

With respect, 10thtx

[norburybrook]

IF you have a 500p silver mica going into the dwell pot, you might experiment with a ceramic in there and see if it makes a difference? I've a number of silver mica caps be bad from the start or seem to go out. The other thing I can think of is do you have your reverb tank isolated in a reverb tank bag or something similar?

With respect, 10thtx

It's not in a bag no, it's isolated on neoprene washers from the base of the combo. does it need a bag? I didn't think modern spring verb's were in bags anymore


M

[Stevem]

I have had issues like this take place and it was due to high resistance being made at the RCA connectors either at the amp or the pan, most times at the pan.

[norburybrook]

I have had issues like this take place and it was due to high resistance being made at the RCA connectors either at the amp or the pan, most times at the pan.

Steve,

what was the 'high resistance' caused by?


M

[didit]

Best bet 10thtx. The tank output transducer essentially is a microphone. Should a heavy towel or other damping material make the feedback disappear that’s your likely answer. All my reverb tanks are in padded bags.

Best.. Ian

[norburybrook]

Best bet 10thtx. The tank output transducer essentially is a microphone. Should a heavy towel or other damping material make the feedback disappear that’s your likely answer. All my reverb tanks are in padded bags.

Best.. Ian

Ah, OK, of course they're a microphone I'll look into that. I didn't know you could buy bags for tanks as I've never seen any advertised. I'm surprised Mike at modulus amps where I get my parts didn't mention a bag as he also sells a full princeton reverb kit and that has NO bag.

edit: I was doing a bit of googling and someone mentioned a strong speaker can have a magnetic pull on the reverb springs, and if close enough will begin to induce a squeeling feedback as oscillation occurs.

I'll do some investigating tomorrow.



M

[telentubes]

I make reverb pan bags out of Tolex. Single or double layer (double is better isolation from vibrations from the cab), simply folded together and secured with spray adhesive. I've used staples as well. Then fasten to the cab with a couple of screws. I've found the reverb tank stuck to the speaker magnet after a ride in the car, so securing it is a good idea. Pretty sure "store bought" pan bags are made from Tolex.

[Stevem]

The feedback was due to tarnish on plating of the the Female rca connectors.
They need to look shinny , not dull .

Another thing I have had that can make for this issue is the fit of the coil windings on there iron core at each end of the pan.
All too many times they rock around a good amount on the core .

What I do to all reverb pans is to flow some medium thickness super glue on the top and bottom of the winding coil to lock it onto the core.
Besides helping to suppress feedback it also helps to make the reverb sound better since the coil is coupled better to the core.

[norburybrook]

The feedback was due to tarnish on plating of the the Female rca connectors.
They need to look shinny , not dull .

Another thing I have had that can make for this issue is the fit of the coil windings on there iron core at each end of the pan.
All too many times they rock around a good amount on the core .

What I do to all reverb pans is to flow some medium thickness super glue on the top and bottom of the winding coil to lock it onto the core.
Besides helping to suppress feedback it also helps to make the reverb sound better since the coil is coupled better to the core.

Steve, thanks for this. I've taken the back off the amp today and checked the contacts, they're all good. I unscrewed the MOD reverb tank, it's on isolated bushes but i rested it on some neoprene to isolate it without screws. I got it to feedback again easily once i'd found the right note ( C#) on the D string. once oscillating you could feel the the vibrations in the tank walls. I removed the tank from the amp and that stopped it feeding back but tapping the tank I can hear the resonant frequency, the C#.

Does this sound like your coil winding/core issue? I'm not sure i understand exactly what you mean by the coils/core, do you mean the little transformers on each end? they're covered with plastic, sould I remove the plastic and drop some super glue on the coils? or is this a 'faulty' tank?

M

[norburybrook]

I've just swapped the tanks from the one in my Bandmaster and that's cured it. Much better sounding reverb, and no rogue frequencies or feedback.

I'm going to return the tank.

M

[ericlc]

Good to hear that the problem is solved. Did I read it well that this was a MOD reverb tank? I have MOD reverb tank in my Dumble clone. It took me a while to get the reverb to work. I checked the component or wiring many times but could not find the cause of having no reverb signal . In the end I checked the reverb tank. It turned out that one of the internal wires was not connected. Straight out if the box. I put the reverb tank in a padded bag. Made it myself from some scrap tolex.

Best,
Eric

[norburybrook]

Good to hear that the problem is solved. Did I read it well that this was a MOD reverb tank? I have MOD reverb tank in my Dumble clone. It took me a while to get the reverb to work. I checked the component or wiring many times but could not find the cause of having no reverb signal . In the end I checked the reverb tank. It turned out that one of the internal wires was not connected. Straight out if the box. I put the reverb tank in a padded bag. Made it myself from some scrap tolex.

Best,
Eric

yes a MOD tank, i thought they were supposed to be the best/most authentic. It's why i always like to use OLD reverb tanks if i can, they just sound 'better' unfortunately my donor one i had lying around was the wrong impedance.I think it had come from a solid state reverb in a tube amp.


M