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Old 18th March 2006, 02:27 AM   #1
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Default Hammond Amp Question

Hi, I recently acquired an old (1958) Hammond amp and power supply out of a pr40 tone cabinet. My original reason for buying it (cheap at a local auction) was for the 6 el84's and 12ax7's. I hooked it up just to try it and it sounded ok. It's a little different animal in the fact that it has one channel for 2oohz and below, one channel for 200 and up, and another channel for 200 and up that fed a reverb unit. I'm thinking about recapping it and using the 2, 200 and up channels as stereo and the lowhz channel as a center or sub. The high channels drove a 12 each while the low drove 2 15's. The power supply uses 2 5u4's. Does anyone have any expeience with these and would I maybe want to change coupling capacitor values (is that how they are controlling the weird frequency response)? I've been chomping at the bit to try a somewhat simple project and thought this may be a good one to get my feet wet, as I don't have much doe (50 bucks for amp reverb and speakers) in it. Any help or advice would be greatly appreciated and I promise to read Rozenblitz again when I'm done. Thanks Jay
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Old 18th March 2006, 05:42 AM   #2
lndm is offline lndm  Australia
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You are probably right about the coupling caps but better to post the schematic to make sure. A centre channel would want higher than 200Hz.

You should change out the electrolytic capacitors before you get serious.
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Old 18th March 2006, 06:03 AM   #3
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Looking at the schematic

http://hammondb3organ.net/schematics.../pr40/pr40.php

I would say the coupling caps are how they are doing it.
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Old 18th March 2006, 07:42 AM   #4
lndm is offline lndm  Australia
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Good one Tim. It pleases me that schematics for these good instrument amps are still available.

It is a little hard to read but in the bass amp section (of the original model) you will see C13, a 0u0022 shunting cap that cuts the bass at around 200Hz. Also look at the three 0u0015 caps soon after.

For the tweeter amp look at C1,2,4,5.
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Old 18th March 2006, 03:14 PM   #5
poobah is offline poobah  United States
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One note here guys... having a B3 that I worked on a bit.

Hammond was well aware of the fact that their organs could not produce a frequency over 6 kHz. They weren't dummies either... so they wouldn't have spent alot of energy ($$) enhancing the high frequency behaviour of the OPT's.

Just something to keep in mind... You can get sweet sound out these amps... but I don't know about the highs.

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Old 19th March 2006, 12:05 AM   #6
lndm is offline lndm  Australia
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That might explain the small caps shunting some of the grids to their cathodes.
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Old 19th March 2006, 01:06 AM   #7
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Good point Poobah, I would have never thought about the organ (I bought it too-a C3) not being able to produce uhf's. I think I'll still play with it, its not like scrapping a leslie. Maybe some new hammond iron for an old Hammond amp. Besides I,ve got a bunch of coupling caps and el84's laying around and the irons hot so to speak. Thanks everyone for the good input, if I don't let all the magic smoke out I'll let you know how it turns out. Jay

Thanks for the schematic too!
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Old 19th March 2006, 05:42 AM   #8
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By the way, Where in Kansas are you?
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Old 19th March 2006, 02:03 PM   #9
poobah is offline poobah  United States
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Hey Jay,

Have fun with PR40... butcher at will. Be kind to the C3 though. I know way too much A's, B's , & C's; lemme know if you need some help.

You need to recap the C ($100) before you play too much though...

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Old 19th March 2006, 02:07 PM   #10
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To extend what Poo said, Hammond also knew that the 200Hz+ amps were not going to produce low frequecies, so the output transformers were designed accordingly. You'll never get 20Hz out of them.

Although I get red in the face everytime I read something like "those junky output transformers would be good in a guitar amp" I think it's true in this case. No, the transformers are not junk, actually they sound pretty nice, they just won't go low, but a guitar amp doesn't need to go down to 20Hz...
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