Hammond PR20 amp, 3-in-1 amp?

[Brad1]

Hi all,

A friend hauled one of these home the other day after a church stuck it out back to throw away. Very, very good condition. (Go to link, and open PR20.zip, which contains the schem, and the instruction sheet.)

http://www.captain-foldback.com/Hammond_sub/schematics/

It appears to be a 3-in-1 amp, with one amp section powering the 2 15" Rola AlNiCos (Jensens?), one powering the upward-facing 12" Rola, and one powering the front-firing 12" Rola (I think that's the Reverb channel one).

I was wondering about sticking some kind of pre-amp in front of it, but I'll be darned if I am sure about a proper way to do it. The nearest I can see to an "input" is right in the top center, where the "Test Input" is, which is calling for a 24VAC signal. (My friend said he thought the input would be the black and red wires that go to the amp from the power supply that were labeled "HV". I said..."Uhhh...I doubt that")

Any ideas if this can be made into a funky guitar amp, or shall we just connect the speakers to a jack and see what happens with a head plugged in? Or possibly even just use the speakers in something else?

Would be cool to try to use the amp, with a (somewhat) adjustable bass and reverb channels.

Thanks,

Brad

[Brad1]

Bump...anyone?

Just a bump up in case it was missed.

Thanks,

Brad1

[Brad1]

Found what I need....

http://www.tonewheelgeneral.com/buil...t=Transformers

(Down to "Hammond Suzuki Leslie 122XB Input Transformer")

One of those across G1 and G2 with a 1/4" jack at the other end, a fuse, switch and power cord...and it should be good to fire up with my homebrew tube preamp, or POD...whatever.

Just in case anyone else ever stumbled across one and got curious. MAy be kind of an interesting critter the way it's set up. Then again...it may sound like crud. Worth $30 to find out, I guess....

And, "Thanks" to 'Captain Foldback', Lars Mikael.

Brad1

[Brad1]

Working...but have questions...

Added grounded cord, switch, fuse and the transformer to supply the 1/4" input to the G1 and g2 points.

Fired it up, nothing blew up. So far, so good. Plugged a Yamaha DG Stomp in. Cranked way up, it worked, but the signal was weak. Plugged the DG Stomp into an ART Tube MP, and pushed the +24db switch.

With the DG Stomp turned up to just cause the Tube MP overload LED to go red (and by adjusting the input of the Tube MP), and cranking the Tube MP output, it was much stronger, and actually sounded darn good! That three-spring hanging reverb in the Hammond is nice and smooth! Actually pretty self-noise quiet, also. With the DG Stomp on clean settings, the thing bloomed nicely.

For some reason, they connected the top upward-facing 12" to the non-reverbed speaker. The 2-15's and the front 12" could all be reverb adjustable from off to high with a 4 position switch. Naturally, it sounded best with the bass reverb off, and only a bit on the 12", which I swapped the wiring so the front 12" was always unaffected, and the top had the reverb channel. With that setup, and the open back, it just kind of threw sound all over the room. Kinda cool!

Anyway, I think it still wants to see more input. I'm positive it can get much louder, and still sound good. A Hammond organ supplies the G1 and G2 amp inputs with a 24VAC signal, and I just measured the Tube MP, and it's putting out 16VAC before clipping.

I've searched the Internet for solutions. Not sure what I can do. How can I get that up to 24VAC that this thing can use? I ran a sine wave through the transformer and measured both the outputs. It seems to be a 1:1 ratio, with no real change in the signal strength.

Is there something I could add before the 1/4" input to boost it? Do I need to change something in the Hammond circuity? Does anyone have any ideas to get this thing pumping like I think it can?

Like I said, it sounds pretty good now. Definitely loud enough for a club gig, and may even make an interesting recording amp. Mic up a 15", the reverb 12", and the non-reverb 12", and see what happens.

Does anyone know of, perhaps, a preamp circuit that I could build that would put out that kind of level? My homebrew guitar preamp put's out about the same as that DG Stomp. I may see about modifying that thing, if I can figure it out, and have a low and a high output.

It doesn't even actually have to be tube-based. A clean SS boost might work just as well, with a tube preamp or POD or DG Stomp going in.

Or, possibly, I could tap off a small amp speaker, like one of my Vibrochamps, to do something?

Anyway, if anyone has any ideas, thanks in advance.

Brad1

[6267]

You'll notice that there is a 12au7 in front of the 6bq5's and after the 24v test point. Below that on the schematic, the reverb return, the 6bq5's are being driven by a 12ax7 after a transistor. I would try a 12ax7 or 12bz7 (not good for hifi, or even gain stages, but a good phase inverter for gain in certain cases like this one) in the place of the 12au7. You could tweak around with the cathode resistor, and perhaps bypass it with a 100uf 25v capacitor. This will increase the circuit gain.

Beyond that, you need more gain stages. Sometimes those connecter sockets for the organ are mounted on the same plane as the tubes, and just so happen to be the same size as an octal socket. You could drill out the rivets (Hammond loves rivets) and install a socket there. You'd want to put say a 6sl7 there, and set it up similarly to the 12au7, where each triode is amplifying one 'side' of the transformer's output. An RCA handbook will give you appropriate circuit values for whatever B+ voltages you have around. Run the output of each triode off the anode through an appropriately sized cap (check the RCA manual, or just use .1 unless it motorboats, then go smaller). This tube (depending on how it's set up) will have the effect of turning a 1V guitar signal into upto a 70 volt signal. I'd aim at probably trying to get more than 24v into that input. It'll give you a nice overdrive.

Finally, this poweramp has what's called a balanced topology. This means it does not have a phase inverter, but that the signal is split at the very beginning by that transformer. This is a very hi fidelity way to play guitar! I'd install the transformer inside the PR20, and put an XLR jack on the outside--that way you can use it with a guitar and get less hum with an adaptor cable, and you can use it for microphones etc.

[Brad1]

Interesting..I printed this out, and will now start doing some research.

Looks like I may have some experimenting to do.

The umbilical connectors are actually on the power supply chassis which is at the bottom of the cab, with the amp section chassis on a dividing shelf about mid-cab, so can't put a tube there. But, there is room enough on the top (and underneath) of that amp chassis to create another tube hole.

I was wondering about pumping even more than 24V into it. That should make it bark pretty good! I suppose I could even consider putting a pot in there somehow to adjust the level for different input devices?

Not concerned about ever plugging a mic into this, really. I'd like to leave the initial input a 1/4" TS jack as all it will ever see is guitar in some form.

I've also been trying to see about some kind of MOSFET booster, or something that may just amp it up to the input. But, I'm not entirely sure how much that little transformer that I installed can take. And, to do it at the outputs of that transformer would likely mean more work since I would now be working with that balanced signal, instead of just +/-. Be much more fun, and probably better sounding, doing it with tubes.

With what is basically three ~15W amp sections = ~45W, this thing should be able to pump some good volume through two 15" and two 12" AlNiCo speakers, I would think.

Let the research continue

And, thanks for your input. If I make any progress, I'll post.

Brad1

[korey]

Brad1, go to www.diyaudio.com, click on tubes and then find the thread for the PR20. I have posted a thread about how to make the PR20 sing! It takes a hammond AO-28 pre-amp (or simular) with a 12BH7 tube to drive it to 24 volts output. Hammond organs back then used this wiring scheme to drive their tone cabinets and leslie tone cabinets. Mine is connected to my home stereo using a hammond AO-28 pre-amp (out of my hammond C3) to drive the PR20 tone cabinet and it just shakes the house.

[Brad1]

Korey,

Actually that WAS me posing as Duke. I like to keep myself as an "enigma wrapped in a mystery" to the online world. Nobody is completely anonymous on the web, but why make it easy, I say, so all my screen names are different, and no two passwords are the same.

And, thanks.

Brad1