rebuilding JR20 tone cabinet

[kevinrontel]

I was recently gifted a beat-up JR20 tone cab, with the AO-15 power amp, which I'm gonna rebuild as a guitar amp. It's my first experience with a Hammond amp; my previous conversions have been with Bogen PA amps. Anyway, I have a million questions, but I'll restrain myself to two for now:

1) Since this two channel amp has two separate inputs (Pins 1 and 6), I assume it doesn't have any crossover circuitry (I sure didn't see any in the schematic), and instead that any hi/low splitting should happen before it gets to the amp. Is this the case?

2) Does anyone have an easily share-able list of the amp's electrolytic caps? That'd save me some time when I'm ordering replacements.

Thanks!

[MihevicB3]

There is only a single channel at the JR-20 6 pin input plug. It is a balanced signal at pins 1 & 6 referenced to pin 2 that is signal common. The signal at pins 1 & 6 are 180 degrees out of phase with each other. Hammond did this to reduce signal noise in the cable from the console to the tone cabinet. I am not familiar with the signal output from a guitar amplifier, but I believe it is single ended. One way to get a balanced signal to the JR-20 would be to put an isolation transformer between the guitar amp output (transformer input) and the JR-20 input (transformer output). The transformer output winding should be center tapped and would connect to pin 2. Pins 1 & 6 would be across the transformer output winding. You would have to check the signal level at the output of the guitar amp to get an idea of what the transformer turns ratio should be. The JR-20 requires a fairly high level to get the full power output of the amplifier.

The signal for the treble amplifier and the bass amplifier are RC filters built into the JR-20 power amplifier. The frequencies > 200 Hz are sent to the treble amp and the frequencies < 200 Hz are sent to the bass amp.

Below is a parts list for the JR-20 I created several years ago. Some of the Mouser part #’s may not be correct. The 4uF and 12uF power supply filter capacitors should not need to be replaced unless they are leaking. They are oil block capacitors and are very stable.

John M.

[kevinrontel]

John,

Thanks, boss. This should get me started. I've got a transformer that'll do the line-to-balanced signal conversion. And the parts list is gonna save me a good chunk of time.

Gratefully,

KR

[Papus]

There is only a single channel at the JR-20 6 pin input plug. It is a balanced signal at pins 1 & 6 referenced to pin 2 that is signal common. The signal at pins 1 & 6 are 180 degrees out of phase with each other. Hammond did this to reduce signal noise in the cable from the console to the tone cabinet. I am not familiar with the signal output from a guitar amplifier, but I believe it is single ended. One way to get a balanced signal to the JR-20 would be to put an isolation transformer between the guitar amp output (transformer input) and the JR-20 input (transformer output). The transformer output winding should be center tapped and would connect to pin 2. Pins 1 & 6 would be across the transformer output winding. You would have to check the signal level at the output of the guitar amp to get an idea of what the transformer turns ratio should be. The JR-20 requires a fairly high level to get the full power output of the amplifier.

The signal for the treble amplifier and the bass amplifier are RC filters built into the JR-20 power amplifier. The frequencies > 200 Hz are sent to the treble amp and the frequencies < 200 Hz are sent to the bass amp.

Below is a parts list for the JR-20 I created several years ago. Some of the Mouser part #’s may not be correct. The 4uF and 12uF power supply filter capacitors should not need to be replaced unless they are leaking. They are oil block capacitors and are very stable.

John M.
[ATTACH=CONFIG]28455[/ATTACH]

If any of those caps are made with PCB's you'll want to replace them pronto - whether they're stable or not. That stuff is seriously hazardous to health and should be properly removed and disposed of.

[David Anderson]

If any of those caps are made with PCB's you'll want to replace them pronto - whether they're stable or not. That stuff is seriously hazardous to health and should be properly removed and disposed of.

As long as they're still sealed, people have been living with these things in their homes for years. I honestly don't know what's in the oil-block capacitors. I do know other vintage capacitors we run across fairly often that have PCBs in them. I'm not going to mention which ones they are so as not to start a panic. From a certain point of view, you may cause more problems by putting the PCBs back out in the environment unless you dispose of them very carefully. PCBs cause major problems when they get loose in the environment.

IMO, the JR-20 is not a great cabinet by most measures and may not make a very good guitar amp even if you figure out an interface to drive it. The result may all depend on what's upstream. Perhaps if you drive the JR-20's input fairly hard, you'll get something useful out of it.

As John points out, but doesn't make completely explicit, the AO-15 amp contains no electrolytic capacitors whatsoever.

[kevinrontel]

I spent some quality time with the schematic yesterday. That amp looks like a pretty bare-bones affair. Ultimately, the pre-amp/tone stack that I end up using (whatever that ends up being) will play a major role in how this turns out. In any case, I'm gonna dismantle the cabinet and use various bits of it to make a pair of 2x10" cabs. I'll likely pull the reverb circuitry out of the amp and turn it (the reverb unit) into a stand-alone component. With the reverb stuff out, I should be able to fit the (as yet non-existent) pre-amp in the amp chassis.

In short, I'm pretty much gonna build a new amplifier/cabinet. Cool!

(Oh, and I was not aware of the PCB issues with the caps. Thanks for the heads up there.)

[David Anderson]

Just a piece of unsolicited advice from someone who's been working on tube amps since 1990:

If you're going to build a new guitar amp, start with a new chassis and cabinet -- and a chassis/cabinet that's designed to be a guitar amp cabinet with the controls in the right place to be useful.

There are tons of 2 x 6V6 guitar amp designs out there from the 1950s that sound great as designed, and you can buy a cabinet with a professional finish in which to install them. If well-built, these do actually have some resale value when you're done. There are kits you can buy that come with layout diagrams.

I have built a setup for someone to use Hammond DR-20s for guitar amp, and I did not modify the cabinets at all. I built an interface that connected via a standard 6-pin cable and that provided power to the cabinets and a balanced-line signal. As you noted, Hammond power amps of this era are basically just driver stages and output stages. The preamp circuits are in the organ.

I don't mean to be discouraging, but I have seen the results of build attempts like this, and they are usually not that satisfactory in the end. (Just the amps themselves in these are very heavy.) I did an autopsy on one after it started to catch on fire during its first gig. On the other hand, one less JR-20 tone cabinet in the world is no great loss.

The oil-tube reverb unit may not work at all if the crystal pickup has failed. It's about a 50/50 chance that it still works. The reverb drive transformers in these amps also fail sometimes.

[muckelroy]

David is right. 200 Hz is a fairly low cutoff point, and not a whole lot makes it to the bass channel. So for a typical guitar signal, most of the signal goes to the treble driver, which faces downward, and little of that makes it out to the room unless it reflects off a surface behind the cabinet.

It might be a nice setup for someone who plays in the bass range a lot, or perhaps for a pedal steel player. Your milage may vary. Good luck, whatever you decide to do.

[kevinrontel]

This is all excellent food for thought, and it only makes me more excited to work on this project. The more work I have to do (build a cabinet, pretty much re-do the entire amp structure, and build a pre-amp, not to mention any ambitions I have with the reverb) the more mistakes I'll make and the more I'll learn from 'em. The simplicity of the amp is a big selling point*. Its basically a blank slate waiting to be re-arranged and tweaked up to such point that I either throw it against a wall or it becomes yet another weird-looking addition to my guitar rig. I'll report back in a few months and let you know which of those two scenarios came to fruition.

*Another big selling point was the price, which was $0.

[David Anderson]

David is right. 200 Hz is a fairly low cutoff point, and not a whole lot makes it to the bass channel.

The low E on a standard electric guitar is around 82Hz, so what happens in that range does have a significant impact on how a guitar amp sounds. If a guitar amp runs full frequency range, you can get strumming noises that muddy up the signal, especially if the amp uses a smaller speaker. Quite a few guitar amps have some sort of bandwidth-limiting to block lower frequencies from the signal path.

The interesting thing about the dual Bass/Treble channel Hammond Tone Cabinet amps is that they use a pretty large output transformer for the bass channel, much larger than you'd see in a guitar amp of the same power rating. This was how Hammond achieved full power bandwidth for organ bass. But those output transformers are also wound for very low-impedance loads. I think the AO-15's Bass Channel should have a 2Ω load on it. I suppose you could try two 15" 4Ω woofers in parallel. The Treble Channel needs a 4Ω load.

From a certain point of view, the amp would lend itself to being used as a lower wattage bass amp. But then you have only around 10 Watts per channel. And the amp will always be very heavy compared to any typical guitar amp due to the multiple transformers and chokes.