Reverb Tank reqd foruse as test equipment

[blindboybenton]

Hi

Im finding myself in need of a test Reverb tank for Reverb tank faults on valve amps. Ive until now used another customers tanks from their amps. I need to buy one
But im unsure of the best type to get. Is there anything i need to look for

When i look at them online. i see so many, makes my brain hurt

John

[Enzo]

The output end on all of them is the same. But there are a couple types at the input. All the old Fenders with a tube drive reverb use a AB type pan, such as 4AB2C1B or 8AB2C1B The second character is A meaning a low impedance drive transducer. There will be a small transformer on the amp to drive this.

The other end of the scale is the EB pan. 4EB2C1B or 9EB2C1B. Second character is E, next to the highest impedance for pans. This is the one used by most solid state drives. Peavey almost universally uses a 4558 IC to drive the reverb, and uses that pan.

An FB pan has even higher impedance and some use it, but the EB pan will work well enough in those circuits, at least for test purposes. There are also a few DB pans, and again, usually the EB will work there. I have never seen a CC pan, but they may exist somewhere.

SOme old Ampeg amps used a tube drive, but not a transformer, the signal came right off a tube plate through a cap. Those want a high impedance drive too, like 4FB. An ED should work well enough there.

The output will almost always be grounded to the metal pan for shielding. I don't think I have ever seen one where the output was not grounded one way or another. But a lot of the solid state drives do not reference the input side to ground, and so need a input jack not grounded. For test purposes, just use insulated input jack styles. If you NEED it to be grounded for some amp, just use a clip wire. I have never seen a tube/transformer drive where the input cared about grounding.

SO you'd need two types of pan for this.

BUT. If you understand how they work, you really don't need a pan.

The output from the pan is just a low level signal, and so the return to the amp is nothing more than an input. SO to test that part of the amp, just plug a signal into the reverb return jack. My test? I usually just touch the return with something. If I hear a loud hum, it is working. In fact a favorite old Fender trick was to take the output of the pan and plug it into the often unused "Normal" channel on the front. Now that channel controls act as reverb controls, and the actual reverb return is left unused.

On the drive end, that tube driving a small transformer is nothing more than a small power amp. Instead of a speaker, we connect the pan input transducer. But for test, we could connect a small speaker to hear what was there. Or connect the drive to some other small amplifier to see what is there.

For higher impedance drives, like those expecting the EB pan, we can usually plug that into an amp input to hear what is there. And certainly in either of those drive cases, we can scope the output.

I VERY rarely need to get out a pan to test the reverb circuits.

[Mick Bailey]

The problem with valve amps is the reverb drivers are often op-amps and the tank impedance varies. Many op-amp circuits also can't use grounded inputs.

The number of springs, orientation and tank length don't matter much, but the coil impedance does. Especially with capacitor-coupled reverb like Ampegs. Transformer driven tanks are most often 8 Ohm input. With those circuits you can connect a small speaker to the input lead of the tank and hear it working - the little driver amp will produce almost 1 watt. More often than not, touching the output connection lead will give a buzz and that's all I need most times to test if it's working.

Tanks have different grounding schemes as well. You may end up troubleshooting a hum that's created by using the wrong tank. Personally I've never had to use a test tank.

The first test is the tank itself. If the coils are ok then the tray is probably ok, but check for mechanical faults. I once had a reverb from the tropics that measured OK, but when I pulled it out of the bag I thought it was stuck in with orange cotton wool. The springs were all bound together with insect pupae.

If the tray is OK then inject an audio signal into the recovery stage input. Or just touch the lead. If you hear a signal the stage is OK.

To test the driver stage you need to listen to it under load. An audio probe and load resistor of the correct nominal value for the tray is all that's required.

You could build a reverb test box with selectable resistances, switchable grounding, RCA in/out and a simple signal generator. It will cost less than a selection of trays and be usable with a whole range of amps and driver configurations.

[blindboybenton]

Many thanks

Ive learnt some stuff. Ive just done a Fender Princeton reverb amp with a reverb fault.. The tank is fault. confirmed with a swap out. One of the transducers was o/s

John

[Mick Bailey]

Ah - Enzo beat me to it while I was typing my reply.............

[Mick Bailey]

I've had a couple of amps in lately that are very high impedance input and output, and use Acos piezo cartridges. Not an easy fix at all.

[Enzo]

Oh dear lord. The old SIlvertones had piezo reverbs. They had all the reverb tone of a screen door spring on the porch door. I hate those things.

[Mick Bailey]

For anyone not familiar with this type of reverb, here's on I just pulled off the shelf. It's awaiting new cartridges.

The recovery stage is an EF86 with the wick turned up to make the most out of the pitiful signal.

[g1]

The old SIlvertones had piezo reverbs. They had all the reverb tone of a screen door spring on the porch door. I hate those things.

Bird amps also used the peizo tanks (maybe that's what Mick's tank shown is out of?).
When it came to reverb, Bird was definately not the word .

[Mick Bailey]

Yes, it's out of a Bird. Quite possibly other small British manufacturers used them as well. Surprisingly, Vox used them too.

You should get a prize for spotting it's from a Bird!