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Hammond guitar amp conversion
Hammond guitar amp conversion
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Old 24th October 2007, 02:05 AM   #1
bluestube is offline bluestube  United States
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Exclamation Hammond guitar amp conversion

I finally have gotten around to my newest project a hammond AO-43 power amp (1x12bh7,2x12ax7, 2xel84, 5u4 rect) I have done a bit of reading on these forums and have not quite found the info I have been looking for. This amp has speaker outs, reverb in/out's, gain pot, I have the gibbs reverb tank for it as well. I took it from a weathered and put out to the curb L-102 Hammond. There are of course no tone controls or volume pot, nor is there an input for a 1/4 style jack.

I am interested in the conversion if anyone has any info I would be more than glad to give further information. I have done alot of reading on tube circuits, I can solder ( I made a jimmy page push pull circuit with four push pulls in my guitar) I would like some guidance on a project like this because apparently tube amps can kill you... there is a link with perfect pictures of the exact amp as mine @ http://www.retroaudiolab.com/l102.htm, my chassis is as clean as this one if not cleaner.

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Old 24th October 2007, 04:10 AM   #2
eyoung is offline eyoung  United States
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this is more than likely an amplifier for a hammond organ or a hammond leslie cabinet, sell it on ebay and buy yourself a nice vintage fender princeton or champ...

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Old 24th October 2007, 07:53 AM   #3
bluestube is offline bluestube  United States
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Default Chassis

I pulled this amp out of an organ, a hammond L-102. I appreciate the reply and suggestion my friend.
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Old 13th May 2008, 07:41 AM   #4
Patrick58 is offline Patrick58  United States
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Default L-102 Amp

Did you get any usable feedback on your project? I too have a Hammond Amp and would like some straight forward information on a conversion to a guitar amp configuration. These amps yield a really great guitar amp, so I'm told.
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Old 18th May 2008, 07:22 AM   #5
camusmuse is offline camusmuse  United States
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I have a similar "tone control" unit.....not sure of the make off hand. But it does use the el84s in pp........

my plan was to keep the iron(trannies, both power and output), the sockets, and then go from there with a "proven" design and build from scratch......

ive since had a different project fall into my lap, so this has taken back burner, but.....since the "new" project is now complete......this is next on the list......unless i can find a power tranny for a knight kn530(just for the amp section)......

good luck, and keep us posted on your progess....

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Old 29th May 2008, 04:34 PM   #6
grahamprie is offline grahamprie  United Kingdom
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Default Old Organ Amps


I used to be a service tech for Hammonds ( back in the 1960's ) so I'm familiar with the overall layout of the model L amp you've got.

Because it was intended to handle the full audio range from the organ it's at least as good as any of the hi-fi amps of the period, and probably a whole lot better than the typical guitar amps.

That said, it's still only a 15 watter, so it's not really going to be useful for anything more than a practice amp.

It's a shame you junked the rest of the organ, because the original factory-fitted speakers were made by Jensen and were very efficient - 101dB per watt or thereabouts - which made the overall volume greater and covered up the smallness of the amp.

If you really want to proceed, work out where the phase-splitter stage is and feed it with a line-level signal - remove or disconnect all the earlier stages as they have frequency compensation round them to make the volume pedal of the organ work like a 'loudness' control.

The reverb tank is driven by the 12BH7 stage and the output was controlled by some switches you no longer have, so I'd suggest you forget reverb for now...

I've probably still got schematics for the L series - I'll have a look in the loft.

The best one to play with would be the PR-40 tone cabinet amp - it had three channels - two like the one you've got and one with four output tubes for the bass. The box had a 15" bass driver and a pair of 12" - one in the front and the other in the top...

While we're on the subject of old tube amps, the ones in the Leslie speakers were also very good - they used a pair of 6550's and produced a solid 50 Watts. Leslie made a pre-amp so you could plug a guitar straight in...

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Old 29th May 2008, 05:56 PM   #7
croat47 is offline croat47  United States
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I have an AO-35 in it's own stand alone cabinet. I was told by Capt. Foldback (http://www.captain-foldback.com/) that this is a prime candidate for conversion to guitar amp. Again, low power, but could be great for an acoustic/blues amp in the LR.

Any input there? Schematics for a number of Hammond equipment can be found on Capt. Foldback's site.
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Old 29th May 2008, 07:37 PM   #8
grahamprie is offline grahamprie  United Kingdom
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Default Old Organ Amps

Hi again,

I've got a boxful of old organ manuals in the loft, but without knowing which model the chassis you refer to came from, I've got no idea where to start.

The units sold in the UK often had different reference codes to the US stuff - mostly because of the different mains voltage, but also because the franchisees here were independant and often specified variations of the standard product in the belief that they would sell better. Certainly all the cabinets were made over here ( to save on shipping ), so the styles are different.

I'll do anything I can to help, but it may take a little time to locate the information.

All the best, Graham.
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Old 6th June 2008, 06:42 PM   #9
hammondnut is offline hammondnut  United States
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Default Hammond Conversion AMP - Goodsell Amps

Goodsell Amps in Atlanta started as Hammond conversion from A100 and others. Richards amps today are very demanded and unbelievable based on his experience as a Hammond tech and his reverse engineering of the older Hammond tube amps.

If you heard them you would understand the desire to build one. At 17 watts the conversion are so unbelievable it can be described. Look around at the comments on the web.

Link to Atlanta Discount Music for his amps

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Old 25th July 2009, 07:30 PM   #10
doctorfixit is offline doctorfixit  United States
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Default AO-35 conversion

Here is a schematic for the AO-35 with modifications I have made so far. It has plenty of volume but it's crunchy distorted. I am investigating how to clean up the sound a little . Note at this point there is no vol control.
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File Type: gif ao35_2a.gif (12.2 KB, 618 views)
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