Hammond JR-20 AO-15 amp.

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hamerhed2003  
Junior tube assistant
Posts: 2
(1/30/04 1:22 pm)
 Hammond JR-20 AO-15 amp.
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 Picked one of these up from the local dump and am considering making it into a guitar amp. I am needing suggestions as to the best way to convert it. It has two OT's one for bass one for treble. I would like to use it with a 1-12 cab. Also whats up with the two BIG chokes?

Schematic

Edited by: hamerhed2003   at: 1/30/04 1:23 pm
 
Tiny Daddy
I will work on all amps
Posts: 462
(1/30/04 1:56 pm)
 Re: Hammond JR-20 AO-15 amp.
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 Do you just want to modify it slightly, in which case you need a schematic? Or are you gonna strip it all out and start over? With all the rust I would sure check for leakage from the power line to chassis. Gonna paint that or what?
 
hamerhed2003  
Junior tube assistant
Posts: 2
(1/30/04 3:20 pm)
 Re: Hammond JR-20 AO-15 amp.
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 There is a link to the schematic below the pic. I'd like to modify it as little as possible. Whatever I need to do to the front end to make it work with guitar, maybe a tonestack, and whatever I need to do with the output section. The picture makes it look much worse than it is it mostly dirt. It'll polish up nicely.

Edited by: hamerhed2003   at: 1/30/04 3:41 pm
 
hamerhed2003  
Junior tube assistant
Posts: 3
(1/30/04 3:33 pm)
 Re: Hammond JR-20 AO-15 amp.
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 This is basically 2 amps powered by one PT isnt it? I'd like to make combine the two if possible. What amps use 4-6v6s?
Maybe it would be best to find a schematic for a 4-6v6 amp and totally rebuild it. I would need a new OT to do that wouldn't I?

Edited by: hamerhed2003   at: 1/30/04 3:34 pm
 
jc
I only work on Fender's
Posts: 665
(1/30/04 4:58 pm)
 Re: Hammond JR-20 AO-15 amp.
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 What is the deal with the output transformers secondaries? There is a second winding that is center tapped and then feeds back into the circuit.
This isn't ultra-liner since those taps are on the primary side. Any idea?
 
Tiny Daddy
I will work on all amps
Posts: 464
(1/30/04 7:50 pm)
 Re: Hammond JR-20 AO-15 amp.
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 Unless you just crave to run a quad of 6V6, I would delete V7, V8, V9, V10 and T2. Rewire so V4, V5 are 6L6GC driving T4. The reverb driver/recovery should be interesting with a guitar reverb, I would try it. But there is no preamp so maybe install 12AX7's at S1, S2 and add a tone stack, borrowing some circuitry from Fender.
The power supply needs some cleaning up. I would ground the high voltage center tap and install fixed bias. And maybe get rid of the oil caps, they may leak something flammable and are probably only 10uF anyway. Add a capacitor input filter so the B+ is 450 Volts. Install 500V filter caps - check out Doug's multi-section electrolytics. How's that for starters?

 
Tiny Daddy
I will work on all amps
Posts: 465
(1/30/04 7:58 pm)
 Re: Hammond JR-20 AO-15 amp.
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 There are extra cathode windings on the output transformer for negative feedback, plate-to-cathode. Don't see that too often.
 
GroundhogKen
Forum Moderator
Posts: 2236
(1/30/04 8:21 pm)
 Re: Hammond JR-20 AO-15 amp.
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 That's a neat trick--getting the negative bias voltage off the HV center tap with a resistor.

Ken

 
whittele
I only work on tube amps
Posts: 477
(1/31/04 8:06 am)
 Re: Hammond JR-20 AO-15 amp.
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 The TopHat Ambassador uses four 6v6gt. I made a few of these. Love em. two knob reverb and a midrange control control that will take you from Fender thru vox into marshall land. Check out the blue guitar for schem's.
 
6G6
Junior tube assistant
Posts: 66
(1/31/04 9:16 am)
 Re: Hammond JR-20 AO-15 amp.
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 I'd begin with a good cleaning and a close examination.
An autopsy might reveal fried transformers.
Then again it may have just been collecting dust on a shelf for 40 years before someone decided that they didn't have any use for it.
My guess is that if it had been any of us, we'd have saved the transformers before dumping it. : )
I vote for the four 6V6s.
A lot depends on what is left. If only the chassis can be saved, then it could end up being almost anything.  
 
hamerhed2003  
Junior tube assistant
Posts: 4
(2/2/04 12:28 pm)
 Re: Hammond JR-20 AO-15 amp.
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 How hard would it be to use this powersupply section on a 5e3 circuit? Would I want to dump the chokes and the 5u4?

Thanks guys for all the help past and future.
 
6G6
Junior tube assistant
Posts: 69
(2/2/04 1:05 pm)
 Re: Hammond JR-20 AO-15 amp.
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 It could be (more or less) 2 5E3s in sterio.
Or it could be a "5E3+" with double the power.
They would still share a single power supply.
I think I'd leave the 5U4 or use a GZ34.
I am curious to see what it ends up being.
 
hamerhed2003  
Junior tube assistant
Posts: 4
(2/2/04 2:05 pm)
 Re: Hammond JR-20 AO-15 amp.
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 I think a 5e3+ with a hi-lo power switch would be cool.

Edited by: hamerhed2003   at: 2/2/04 2:38 pm
 
hamerhed2003  
Junior tube assistant
Posts: 4
(2/2/04 2:38 pm)
 Re: Hammond JR-20 AO-15 amp.
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 dp

Edited by: hamerhed2003   at: 2/2/04 2:38 pm
 
 
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