Convert JR-20 into outboard reverb + guitar amp?

[Seibertmeister]

Hey all- a while back, I ended up with a somewhat beat-up but mostly functional (except for a slight hum that was undiagnosed) Hammond JR-20 Tone Cabinet. A friend was buying a Leslie from a studio we've both worked with and they practically begged him to take the Tone Cab, which I ended up with somehow. I'm assuming based on what the studio told me that the reverb was functional when I picked it up, though I know they're not terribly reliable in their old age.

Anyways, the engineer at the studio mentioned at the time that my best bet was to convert it into a standalone spring reverb unit, and it's languished for a few years while I've been off at school. I don't really see myself getting much for it in sale, and the space it's occupying will need to be cleared, so I'm curious if anybody has experience pulling these oil spring reverbs and converting them to outboard units. I'd be interested in using it as a studio piece, primarily. I don't have much electronics experience but I know a little and could get help from pro friends in the area if I knew where I was going with it.

Separately, I'm curious if the amp in the JR-20 would sound good for electric guitar- I don't see myself using the cab, as the 4x10+1x12 thing is weird enough even if it wasn't so big, but to build a head or a combo out of the amp is an enticing idea- if it would sound good. Bonus points if you've got thoughts on whether the speakers have any practical use.

I don't want to totally scrap the cab if I can get some functional pieces out of it. I might try listing it for sale but I can't see anybody in my area (CT) wanting/needing this given the popularity of Leslies vs Tone Cabinets, and I don't really want to ship something so heavy and fragile.

[Papus]

You can obtain a more usable and probably better sounding spring reverb in any cheap nasty solid state organ.... just needs a preamp and recovery amp to drive the piezo transducers in the spring tank.

[David Anderson]

The amps in most Hammond Tone Cabinets are power amps only + reverb, and they are low-gain, so you need a much hotter signal input to drive them than standard line level. You need a preamp for use with guitar to boost a guitar signal enough to drive these power amps, plus you need volume and tone controls, and the JR-20 needs a balanced input signal. Add to that that the oil-tube reverb units may or may not work anymore.

In other words, if you wanted to use the JR-20 for guitar, I think the best approach would be to build an outboard preamp with all necessary controls (volume, tone, etc...) and balanced-line output and then hook it to the repaired/unmodified JR-20.

The studio engineer was probably just telling you something to send you on your way. They do that. Most of them are only experts on the recording side and have only general knowledge on the sound-producing side.

Would it sound good for guitar? I don't know. I'm sure there's someone with a different opinion, but most people I've talked to don't think JR-20s sound very good for organ. I've known a few people who rebuilt them for Hammond use and were disappointed with the results for organ even when they were working properly.

I have heard a Hammond DR-20 adapted as I proposed above for guitar, and it sounded pretty good. But then, it's a very different design with 2 x 12" speakers, a standard guitar setup.

[muckelroy]

+1 to what David said.

It's a weird tone cabinet, but when weird is what you're after, then it will fit the bill.

I've refurbished one of these amps, and I wasn't happy with how it sounded. But to each their own.

The salt crystal pickups often fail on these reverb units, as do the reverb drive transformers. Everything else is easily repairable with common parts, to which anyone who is skilled at working on tube amps ought to be able to refurbish the amplifier.

If you are curious whether the spring unit works, you don't even need the amp to do it, just need an oscilloscope. You could take an oscilloscope across one of the outputs of the reverb pickup to ground, and gently thwack the spring. There should be approximately 300 to 400 mV peak to peak (or more maybe) in a functioning pickup.
-- Sometimes it does produce signal but much less - maybe 30 mV. Those are essentially dead pickups.

I am currently working on a stand alone reverb unit build for somebody, but it is on hold for a custom built-amp to drive the thing. I have a working spring unit here that the build will be based on. These builds have been done before, but it is not easy. Theories abound as to how to replace a pickup if it dies, and I won't entertain those theories because it is much, much easier to adapt your amp to a modern day reverb tank than it is to try to revive a dead oil spring reverb unit pickup.

[Baldwin51]

The pick ups don,t last long in any humid place, i have replaced a ton of them over the years until they were all gone, they use a low top end freq, i built a wooden box and made a stand alone, takes up a small space, used a Hammond pre amp which i added a stage using a 6sn7 tube and a one to one output trans former from a cv hammond preamp and staggered the out put voltage sending the reverb from cabinet to cabinet including two, the stagerring voltage created a slight delay ,

[Seibertmeister]

Thanks for the replies. As for using the amp portion for guitar, my understanding has been that it's the speakers that really don't sound great (too much bass in these particular 10's and the upfiring 12), not necessarily the amp itself- hence why I was considering rehousing it into a head or combo. I'm happy to put together a preamp, or contract an amp-builder friend of mine to do it, but wasn't sure it was worthwhile. If they really don't sound great, I likely won't bother.

As for the reverb portion- the guy who sold it to me likely didn't know how unique/fragile these oil verbs with the crystal pickups were (I didn't until doing this research). I know it wouldn't be a particularly easy build, but I'd be really curious to hear what it sounds like as outboard in my own studio... I love spring verb in lots of places, and my understanding was that these particular verbs were unique and really beautiful sounding, but I've never heard one in person myself. Kind of figured that if it sucked, I could repurpose part of the build with an accutronics tank or something and salvage most or all of the expense on parts. That said, I don't even know how I'd house it at this juncture. Maybe I won't bother.

[muckelroy]

It's worth finding out if the reverb works or not. Get a scope and find out. If it's dead, don't mess with it or give the reverb unit away to someone who may want it.

I have posted a number of SoundCloud clips of the reverb on my FR-40 restoration thread, and I also have some video of a JR-20 working somewhere, haven't posted it that I recall. I'll try to dig it up tomorrow if you want to hear it.

[David Anderson]

It wouldn't be too difficult on the JR-20's amp to boost the high frequencies in the bass channel. They simply use a few capacitors to roll off the high end. You could also lower the Treble Channel's range to what's appropriate for guitar.

I don't know of anyone who's ever raved about the oil-tube reverb system. I thought the consensus was that the necklace reverb was arguably the best.