• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hammond PR20

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Hi all.
New here. My name is Duke.

I'm looking for answers, and hope someone has some ideas.

I am an electronics tech, just not an engineer.

The secenario: A friend obtained an old Hammond PR20 Tone Cabinet. Since none of us have an organ to connect it to, we decided to see about modding it for guitar.

I downloaded the schematics from Capatin Foldback's site (Lars Mikael...a nice, helpful person), and proceeded to experiment. Here's his site, BTW: http://www.captain-foldback.com/

I emailed him, and he said I needed a transformer to interface a 1/4" jack to the G1 and G2 inputs (and ground) of the three amplifier sections. So, I ordered one from Tonewheel General and installed it. (It's the HS122XB Input Transformer here:

I also connected a grounded cord, switch and fuse for power. Checked all the tubes...all very good.

Lit up the first time. I knew I was going to have some level problems even before I plugged anything in. I took a POD 2.0, and a Yamaha DG Stomp over to see what they would do. Well, they only put out a few volts, cranked. They played, but volume was low. I knew that would happen, so I plugged each into an ART Tube MP preamp, and pushed the +20dB button. NOW, it was getting closer. But the highest clean signal (before clipping) I could get out of the combinations was about 16V. (I had an O'Scope measuring, and I was also testing with a sine wave from a signal generator).

The PR20 G1 and G2 input lists 24VAC at the test points, and Lars said that's approximately what a Hammond will feed it.

The thing actually sounded pretty good, but I know if I can get it to put out more and make those tubes work a bit harder, it might even sound better.

So, there's my dilemma. I'd like to be able to take a small signal from a preamp unit, whether it's my homebrew guitar pre, a POD, a DG Stomp..etc., and bump it to ~24V...without having to add a Tube MP, etc. :confused:

Do I do it before that tiny Hammond transformer, which would be easier because it's only +/-? Or do I do it after, with both legs run to something before entering the amp?

HOW do I even think about it? Can a different transformer work at the input, and get rid of that Hammond one, which seems to be a 1:1...no boost? Do I build an active circuit boost? Would it have to be tubes, or would something S.S. work?

Like I said, it works now, and it actually sounds pretty good, but I think I can get a lot more air to move from those 4 speakers.

Any ideas will be appreciated.


Duke :)
Hi again. I hope this isn't against rules, but I thought I'd bump this because it was falling fast, and was hoping it might be an easy answer for folks who know more about design than I.

I was guided to this site from another forum who said you guys may have ideas, since it is practically a do-it-yourself modification.

I was hoping for something like..."you just need to get a "Brumbley 64Y5 transformer"...or "here's a link to a circuit that you can build to run into the 1/4" jack"...or something like that.
If I have the schematics, I can build it. I just don't have the extensive knowledge to feel comfortable engineering anything.

If I get no responses hence, I'll graciously allow this post to sink slowly into oblivion, and die a natural death. I certainly don't want to make a pest of myself.

If there are any other sites that anyone could recommend that may be more suitable, I would appreciate any links.

Thanks, and have a good day.


Sorry that I can't help with your problem, I am one of those that tend to get more advice on these forums than I can offer...

What I can say is that there are a really good bunch of helpful and knowledgeable people on this forum. Most of them are in to hi-fi, but there are quite a few in to guitar amps as well. I have seen and experienced that those members who at least try to help themselves or understand the problem and take the time to describe their problem in a considered and courteous manner invariably get advice and you most certainly have done that.

Your bump was done in a polite and undemanding way, I am sure nobody will take offence ;)

This forum is a shining example of helpfulness without the unproductive flame wars some sites see. I hope you like it here and get the advice you need!


Hi Duke, I have the same hammond tone cabinet. I also had the same problem. I actually got lucky and bought a hammond C-3 tonewheel organ, hammond PR20 tone cabinet and leslie 22R tone cabinet for $30 (yes $30 for all three) at a local barn auction about two years ago.

I'm a tech hobbyist not a keyboard player so I wanted these units for the electronics alone. I was wanting to use the PR20 as an extended bass unit for my existing sound system but was encountering your same problem, not enough volume.

These units should shake your house, and they do. Here's how:
I assume you want full performance so if you have the time, patients and money, replace all out of spec resistors, all coupling capacitors and electrolytics. The oil capacitors on mine were OK. They are the big rectangular silver boxes on the top of the chassis next to the output transformer. Those things last forever!

Sinse I already had the C3 organ, it had the pre-amp inside which uses the G-G outputs. The only way I could get any volume out of mine, was to take the pre-amp out of the C3 and plug it into the G-G inputs on the PR20. The pre-amp from the C3 is: Hammond model# AO-28. This pre-amp has an RCA PHONO JACK located above the 'swell' volume control for use with a seperate auxilliary input such as (in those days) a turntable.

I carefully marked all the wire connections on my AO-28 just in case I wanted to put it back in the organ someday. I have an old 'H H Scott LK-48 which has a center channel output jack on the back which I run into the AO-28. The AO-28 then connects to the PR-20 via the two G-G inputs. I use two altec 12inch 30watt speakers with my LK-48 and the PR-20 in the corner of my living room has incredible eyeball rattling bass.

I have seen people selling just the AO-28 pre-amp off of e-bay within the recent year, so keep your eyes open.

I also have the schematic for the AO-28 if you would like to see how it's used to drive those G-G outputs. For some reason, I haven't been able to upload pictures from my computer to this website, so if you want, I can send them to you via e-mail.

If you can get ahold of an AO-28 pre-amp, you're on your way!
Thanks, guys.

I've gotten several ideas from here, and a couple other sites...including something I'll have to research to maybe add another tube.

Funny thing is I used to have a Hammond M3 until I moved from Denver in December, when I sold it to a guy who had a non-working one. He wanted to put the best parts of each into the best cab. Maybe he has some extra parts still lying about.

I actually thought about getting a Hammond pre-amp to see what happened.

Anyway, I'll study all this stuff. Just thought I'd check back to say "Thanks". If I figure anything out, I'll post back if I can get it screaming like I think it could.

I stumbled onto this thread while searching for information about the PR-20 tone cabinet (info on the cabinet itself). My current project is exactly what this topic is about and I have successfully built a unique guitar amplifier form the chassis carcass. I figured I'd take a few minutes and share it with you.

The 3 individual output stages are loosely based on the Marshall 18 watter. The overall structure is loosely based on the PR-20. I built a preamp on my protoboard that splits the signal 3 ways. 1 pre out goes to the 8 ohm amp, 1 goes to the 4 ohm out, and the 3rd goes to the reverb circuit before hitting the other 8 ohm out. I hacked on my cruddy Marshall 4 x 12" cabinet for the 3 individual loads making the bottom pair the 4 ohm, the top right the 8 ohm, and the top left the reverb out. Yup, the reverb has it's own amp and speaker. It's very 3 dimensional. I won't bore ya'll with the gory details. I'll let the pictures do the rest of the story.

Here is a copy of the eBay picture as purchased:
Here is a picture of how it currently sits on my bench:
Here is the current schematic:

I hope this info is useful or at least mildly amusing.

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