Help with reverb feedback in a Selmer

[marlinster]

Hi again,

The tank installed in a bag and foam strips really helped a lot. No oscillation/feedback now from the tank and it sounds good now. Still some hum but I guess that's just the way it is. It helped a lot to reduce noise to remove the grounding of the pots in the metal mounting plate and use cable back to the ground trace of the board and it's all reversible.
I want to change the 68 Ohm anode resistors of the power tubes that are only 1/2 watt carbon film to 2-3watts metal film as well, after that I'm done. It's a great sounding amp...
Thanks all for your tips.

[Enzo]

The fact you can grab it and it stops tells me we have acoustic feedback. SO our job is to keep the acoustic energy our of the pan. Thus covers and bags, and indirectly my stick on foam strip.

[marlinster]

Yes I would say it's a howl. The original magnet of the old Goodmans speaker is pretty small. It makes sense.

Hope the bag makes it better.

[g1]

I can't think of a reason why the static field of a bigger speaker magnet should give rise to feedback.

I can see different speaker doing it, but the magnet would not be why.

It's the proximity of the magnet to the tank. Not necessarily the bigger magnetic field, but the fact that the bigger magnet is now closer to the springs.
I'm not sure the mechanism, but the magnet causes an oscillation of the springs. Usually a lower frequency howl than typical tank feedback.
I can't tell you the physics of it. Only that I've had it happen and varied the effect with my own two hands.

[marlinster]

Yes I tired to grab the pan and then it stops. I will cover it with cardboard also and test that.

[Enzo]

When it feeds back, reach around the pan and just grasp it firmly. Does that stop it? I am looking to see if it is acoustic in nature. I can't imagine what else.

Have you put a cover on the open side? Takes a few seconds to try.

[marlinster]

I have the tank as far away as possible now from the speaker. Further away and it's out of the cab. The different speaker definetly made it worse yes, gonna see if I can fix the original one but it would be nice to be able to swap speaker freely in the future. It's a very nice sounding amp and without reverb it's quiet.

I have tried turning it 180 degrees, tilting it etc. The most quiet position is vertical with the closed side towards the speaker. I will try a bag as well when it arrives on Monday though. Although the mounting looks good now with the grommets and a distance so it can move freely within the grommets and it's resting 100% on those.

I have tried with a 0.1uF capacitor in series with the output from the tank, and I cannot hear the oscillation now on max volume, and the 2.2k cathode resistor. I guess the oscillation is still there only I cannot hear it?

Will try more on Monday/Tuesday with the bag and the damping strips. Thanks for your tips.

I have also tried a different grounding and it's no big change. According to the original both jacks go to the ground, including the tank chassis...

[Enzo]

I can see different speaker doing it, but the magnet would not be why.

[Helmholtz]

The bigger magnet is what made it start doing this. If you reposition the tank to be further away from the new bigger magnet it should stop.

I can't think of a reason why the static field of a bigger speaker magnet should give rise to feedback.

[g1]

The bigger magnet is what made it start doing this. If you reposition the tank to be further away from the new bigger magnet it should stop.
The other issues need to be solved as well, (wrong resistor, bad tank) but I've had the same problem arise from installing replacement speakers that put the magnet closer to the tank.
I'm sure if you pull the tank away from the magnet, the problem stops, doesn't it?

[Enzo]

Here is the standard procedure. The pan is never to be tightly screwed to the cab wall. Screws should only be tight enough to keep the pan from moving around the cab. The pan should be put in a vinyl reverb bag. The open side of the pan should be covered, a piece of cardboard works fine. MAke sure the innards float freely. The wide flat top surface of the pan can resonate, so run a strip of self-adhesive foam weatherstripping down the center longways.

[marlinster]

Enzo, the MOD does not feedback with the 220 Ohm cathode resistor. When I change it to 2.2kOhm (should be according to schematic) it starts to feedback as well. The original pan was horrible, I suspect it is broken in some way. I tried to glue the transducers, dampen the springs, etc etc and still feedback.

[marlinster]

Hi and thanks,

1. Yes I'm gonna stick to this value and solve it with something else. Why does it start oscillating more when biasing hotter?

2. I installed a 1Mohm pot there instead and it has to maxed out to get rid of the oscillation, but it works. Yes it helps when having outside the cab on soft material, but that is not a solution I have ordered a bag as well to try with.

3. I tried to reverse the speaker but no change. I have not tried to ground the speaker fram but I will try that as well.

I saw just now a Selmer Zenith schematics which is almost identical with the Vanguard except that it has higher value filter caps for B+ and also they have put a 1000pF capacitor in series with the reverb tank output and 100k grid leak for the reverb driver tube. I tried a cap like that and it got better but removes too much reverb. I will try with a higher value cap and see what it does.
By the way the grounding of the control potentiometers in this are connected to the chassis, I changed it for the reverb volume(to reverb common ground point) and it made it quieter. But not enough.

Many thanks

[Enzo]

You said a MOD reverb did not feed back? But the original does? That suggests to me the problem is the pan, not the amp.

[Helmholtz]

1) 2k2 is the proper cathode resistor value. 220R cause very hot biasing, low plate voltage and poor tube operation.

2) Assuming the new Celestion speaker gave rise to the feedback issue, you might have:

- Acoustical FB caused by higher efficiency of the new speaker. To compensate for this you may reduce the reverb recovery level by adjusting the 500k trim pot in series with the rev depth control to a higher resistance.
If the results are not satisfying, you would need better acoustical insulation of the reverb tank. Does it help to unmount the tank from the cab and place it on a pillow ot the like?

- Or electrical feedback (less likely). For a test you may try to reverse speaker phase and/or ground the speaker frame.