Organ Mate Tube Reverb -- Microphonics?

[neddyboy]

I have a new reverb issue for you guys to ponder! I'm working on a tiny little Organ Mate freestanding reverb unit from the '60s designed to add onto an organ. I've done all the usual stuff -- cleaning tube sockets, replacing the old electrolytics. I have not replaced the filter cap can yet though. Waiting for parts.

When I first tried it, it worked, passed wet signal, but above about 1 on my amp volume it begins to squeal. Microphonic tube, right? I even confirmed it by holding the tubes and reducing the feedback. BUT...when I swapped in other old tubes I still got the squeal. I even bought two brand new JJ tubes (12AX7 & 12AU7) and although the squeal was reduced, it still happens. The only difference is that the tubes don't ring when tapped.

I'm willing to believe all my extra 12AX7s and 12AU7s could be microphonic, but the new tubes aren't. If I set the reverb level low, the squealing doesn't start until the amp volume is at 2, but if I turn up the reverb intensity it will squeal sooner. When I moved the reverb unit further away from the amp the squealing lessened but still happened at higher volumes, ie 3 or 4. I put the chassis in a cardboard box -- it helped, but still squealed. Swapping reverb tanks made no difference. Mine is a later circuit board version, so there's very little chance of bad lead dress, but I pushed the leads around. Also no difference.

I'm tearing my hair out on this! It sounds like a microphonic tube, it seems related to the volume of the amp, when I tap the chassis it pings like a microphonic tube, but if I tap the new tubes I get no ping. I've attached a schematic. I'm not aware of any other components in this circuit that can be microphonic. Maybe the reverb transformer? It doesn't react when tapped either. Argh.

Any suggestions?

[sluckey]

Show us a pic of your unit. I'm thinking that a shielded enclosure is going to take care of most of the squeal.

[neddyboy]

Hi Sluckey! Really? You're right, I don't have the cage on. I'll be trying that shortly. Thanks a second time.

[sluckey]

Also shield the bottom of that reverb tank and place the entire unit about 3 feet away from your amp. Does that help?

[neddyboy]

OK, I put the whole thing back together. I had taken the chassis off the the tank and installed Fender-style shielded reverb leads for ease of working on it. I put the cage on, mounted the chassis on the tank, and lay the whole thing on a sheet of aluminum foil about 5 feet from the amp. The result was the same as when I moved it away from the amp earlier, without the cage, shielding, etc. I can, of course, install it in a plywood box with sound insulation around it, but hell, it was designed to mount in the bottom of an organ naked to the world! It seems like it shouldn't be this sensitive.

One more symptom detail: the squeal feeds on itself, like feedback (or a microphonic tube). It rises in intensity as the volume is raised. I'm going to wait 'til my filter caps arrive and get those installed, then go through the schematic and make sure everything's right. Maybe an old dry solder?

[sluckey]

How are you connecting into this unit? Guitar to input, output to amplifier instrument input? The organ would have been feeding quite different signal levels into that reverb unit.

[neddyboy]

That is very true. I'm using it as you described. I don't have an amp with an effects loop. Would a buffer pedal help?

[neddyboy]

I've built a box for the chassis/tank, and that's helped with the howling. It still happens but at higher volume. I finished replacing the filter caps and a few ceramic caps, and it sounds pretty good! The stock pot seems to adjust intensity of reverb, so I need to install a mix pot. My thought was to wire the left tab of a pot straight to the output jack, the right tab to the input of the reverb, and the center tab to the input jack. It can't be that simple, can it? What value pot should I use?

[neddyboy]

The pot that came with the original Reverb Mate is an intensity control, and I'm trying to add a wet/dry mix control. Can anyone give me an idea of how to do this? I have no idea what I'm doing! The attached schematic shows what I've tried but it acted more like a volume control. Thank you!

[PRR]

Not understanding why you want a "mix" pot, if you are going guitar->reverb->amp.

Why would you ever want Zero dry signal?

The OrganMate is wired to pass dry signal all the time. Intensity increases signal from tank, from none to max, so adds wet signal to output. The dry signal passes 100% until the Inten pot is nearly full-up, then the dry signal drops a bit.

This seems to me how "all" in-channel reverbs are controlled? Dry all the time, and you mix wet on top.

Remember this WAS used in performance, so should be versatile enough to be musical yet simple enough for a musician to operate without distraction.

[PRR]

If you must have 0%-100% dry/wet, two pots avoids major cross-fade complication.

[neddyboy]

Hi PRR, thanks for responding. I want the mix pot because as it stands now, the original pot adjusts intensity & volume at once. I don't know if this is as it was designed, but it goes from no signal passed at 0 to super cavernous unusable reverb at 10. All the way through the rotation it seems like nearly 100% wet signal.

I've never owned one of these and know little about how they worked in an organ installation. I'm feeding it with guitar signal, not a preamplified signal like the organ would have. I assume this must affect how the unit sounds and how the adjustment pot behaves. What would you suggest? Is there anything short of adding a gain stage ahead of the Organ Mate that would make it work better for guitar? 

[PRR]

> original pot adjusts intensity & volume at once

I wonder if it is mis-wired.

The plug and 1-2-3 notations make it hard to see at a glance. This is not supposed to be wired like a "normal" volume control, output from wiper. Input should be at wiper, output at top of pot.

[neddyboy]

It's currently wired like the schematic shows. I mis-wired it earlier and I've triple checked it since!

[scoutjones]

@neddyboy: How is this working out for you?  I am working on one of these right now.  My plan is to make a cabinet and use it as a stand alone reverb unit.  Any advice on what you've replaced and how would be appreciated.  Mine was also pretty noisy but does better when it's in an effects loop.  Also, the whole thing seems microphonic.  So far, I've replaced the filter caps but since then the signal level is very low and the noise is worse. 

Also, regarding the input stage, I had thought about putting a 12at7 for more gain when the unit is between the guitar and the amp. And adding a volume pot at that stage to lower the level when it's being used in an effects loop.  But my level of expertise is pretty low.  I don't know if that would actually work. I know enough about tube amps to not die.  I've completed a project or two but it's always a steep learning curve.

thanks

[neddyboy]

Hi scoutjones, sorry for the delay. Honestly, I ended up setting mine aside for a while because I got frustrated. Another poster pointed out the unit was intended to be used with preamplified signal, so I'm not surprised yours sounds better in the effects loop. Mine is also very microphonic. I'm waiting until I have a tube amp I can modify the way the Organ Mate manual recommends. Until then it's on hold. Wish I could be of more help!