Ultimate Hammond Conversions thread

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donzoid
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Ultimate Hammond Conversions thread

Post by donzoid »

I've seen a lot of info scattered around the forum on converting Hammond organ (and other brands of course) iron and chassis into guitar amp builds and wanted to create a thread for it. I had a few side conversations with other members on the forum and by coincidence, two of us are sitting on Hammond AO-43 chassis and we both are starting them shortly.

For those of us who like to feel "environ-mentally responsible" there are several chassis from different Hammond models that seem to be popular candidates for conversion:
AO29 - M-1 series
AO14 M-2 series
AO35 M3?
AO43 - L series
AO71 - H series
I also have a very old S-series I am disassembling. The chassis is much too large, but the iron should be useful as it has a 6v6 pair for the output.

Specs on the AO-43 original configurations:
Tube complement -
2 -el84/6BQ5 output section
5U4 rectifier
2 - 12ax7 preamp
1 12BH7 (I believe this was the reverb tube, but the reverb unit itself was offboard in a separate chassis). Nonetheless, its an extra useful noval socket.

Transformer info (from the service manual schematics)
T302 PT, pn AO-24157
5v 3A
315v secondary (Listed on schematic as 160mA AFTER the 64R wirewound resistor (so B+1 is approx 1.4 x 315 at or around 440v?)
6.3v approx 1.8A
315v at plate on the schematics, but that's after more dropping resistors.
14H choke


I'll start with some photo's of the prep work. My approach here is a total shoestring. I'm going to at least attempt to re-use every component possible save for the PS caps, and tone stack caps /resistors. I'm even likely to re-use this funky turret-eyelet hybrid board.

I'd like opinions from "the asylum residents" as to what you think is the best choice for conversion. This one is a bit too hot for a 2xel84 Rocket, but I am considering a big ol' Zener to drop the B+. Other builds considered were an 18 watt stout sort of thing, or an ef86 side and a 12ax7 side. I'm a high gain guy, and I use pedals. It needs to be simple 'cause I'm new to this, and I normally use single channel amps. I'd like to try a Rocket....just not sure how best to drop the secondary volts coming off the PT.
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lord preset
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Re: Ultimate Hammond Conversions thread

Post by lord preset »

I am just getting ready to start an AO-39 conversion - also 2-el84 and similar voltages but it's a smaller chassis. I'm using this as a guide to do an 18 watt Lite build:

http://home.comcast.net/~seluckey/amps/ ... ammond.htm

The author was able to drop B+ by using a 5Y3 rectifier tube and a beefy power resistor.
eniam rognab
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Re: Ultimate Hammond Conversions thread

Post by eniam rognab »

hey don! cool thread :wink:

i am starting a half power rocket with AO-43 irons! new chassis though, got the PT mounted and i am playing around with OT/choke locations

im looking forward to sharing this with all of you, so you can laugh at my noobness!!

to hammond!
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donzoid
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Re: Ultimate Hammond Conversions thread

Post by donzoid »

Cool, preset! I also looked at a 5Y3, because I have a couple old stock ones but it would also have to have some other extra drop to bring my B+ down. I thought about using resistors. I'm going to take a closer look at those pix. Since this link is using the ef86, that is the "non top-boost" Vox channel correct?
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donzoid
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Re: Ultimate Hammond Conversions thread

Post by donzoid »

Nice chassis eniam...you're going to make mine look bad, aren'tcha?

I'm going to "reverse" my pots and use the unscathed side of mine as the front. To keep my lead dress short, that means the knobs will be more to the right side (facing the front) than a standard Rocket. But I'm OK with that. Alternately I thought of placing a new plate on the side that's punched with odd holes. Either way, I'm going to use a mahogany face plate overlay so it will match my Express build.
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Re: Ultimate Hammond Conversions thread

Post by eniam rognab »

imma do like a swirley finish on my chassis with sand paper or a wire brush or something, and label my knobs crazy talk,

more-uppers-downers-talent-engage-liftoff-spunk-exacerbate

tee-hee! :D :oops:
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donzoid
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Re: Ultimate Hammond Conversions thread

Post by donzoid »

I'm curious what tonal differences (if any) would be introduced by using the resistor (as in the AO-39 conversion they use a 250R with the 5Y3 to drop B+) vs. a Zener "dump" in the same place. My preference would be whatever way has the least effect on the tone/response. I know the 5Y3 has the most sag of the rectifier choices.
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Phil_S
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Re: Ultimate Hammond Conversions thread

Post by Phil_S »

donzoid wrote: 315v secondary (Listed on schematic as 160mA AFTER the 64R wirewound resistor (so B+1 is approx 1.4 x 315 at or around 440v?)
If you use a 5U4GB, B+ will be ~375. If your choice of amps requires less than 125mA, you can use a 5Y3GT which will give you B+ ~345. I dislike the Zener approach, etc. Maybe that's just me. Solid state, you will won't get quite as high as 440 -- I think maybe ~425 after ordinary losses if you are "lucky." I usually figure the loss at around 4%.

If you think the choke is up to the job of taking the current load of the whole amp, you might go for choke input (between rectifier and B+1) which should shave 10% off the B+. With a 5U4GB, that should get to ~310, which a pair of EL84 should be able to manage. 6AQ5 will also survive at 310 OK. Certainly 6V6, no problem!
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xtian
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Re: Ultimate Hammond Conversions thread

Post by xtian »

FYI, here's what I did with the last AO43 I got:

http://ampgarage.com/forum/viewtopic.php?t=22152
I build and repair tube amps. http://amps.monkeymatic.com
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Phil_S
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Re: Ultimate Hammond Conversions thread

Post by Phil_S »

donzoid wrote: 315v secondary (Listed on schematic as 160mA AFTER the 64R wirewound resistor (so B+1 is approx 1.4 x 315 at or around 440v?)
If you use a 5U4GB, B+ will be ~375. If your choice of amps requires less than 125mA, you can use a 5Y3GT which will give you B+ ~345. I dislike the Zener approach, etc. Maybe that's just me. Solid state, you will won't get quite as high as 440 -- I think maybe ~425 after ordinary losses if you are "lucky." I usually figure the loss at around 4%.

If you think the choke is up to the job of taking the current load of the whole amp, you might go for choke input (between rectifier and B+1) which should shave 10% off the B+. With a 5U4GB, that should get to ~310, which a pair of EL84 should be able to manage. 6AQ5 will also survive at 310 OK. Certainly 6V6, no problem!

OTOH, line voltage might be higher these days, contributing extra voltage on the secondary.
passfan
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Re: Ultimate Hammond Conversions thread

Post by passfan »

I use organ iron and other parts all the time. You have a great start to an 18 watt with that 0-43 chassis. Here's what I did with an older Conn chassis. Hammond AO-43 PT and wurlitzer OT

http://ampgarage.com/forum/viewtopic.php?t=24307
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eniam rognab
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Re: Ultimate Hammond Conversions thread

Post by eniam rognab »

yay my first question, hope its a good one!

so, im doing the headphone (actually meter) trick to place my OT. i found a spot next to the PT that reads 5mV and i think i am happy with that. however i still need to locate the choke and i am wondering what steps i need to take to ensure it doesnt couple with my output. OR should i not worry about it and mount it wherever convenient.

is there a way to include a choke in my headphone trick test?

fyi, the OT measures 0mV when placed on the left side far back, 90 degrees to where it is in the pic. should i just put it there instead? i would like the real-estate for my tubes and such but if that is better i will go with that. it does seem like kind of a fuster-cluck with all that iron in one place!

any and all input appreciated!
neal

hi passfan, recognize anything? :wink:
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passfan
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Re: Ultimate Hammond Conversions thread

Post by passfan »

So much for an 18 watter , eh ? 8)
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Re: Ultimate Hammond Conversions thread

Post by eniam rognab »

haha yup bigger more better :lol: :lol:

so i decided to mount the OT on the far left corner so disregard that ugly picture

but i am still wondering about choke coupling, is this an issue?
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lord preset
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Re: Ultimate Hammond Conversions thread

Post by lord preset »

donzoid wrote:Cool, preset! I also looked at a 5Y3, because I have a couple old stock ones but it would also have to have some other extra drop to bring my B+ down. I thought about using resistors. I'm going to take a closer look at those pix. Since this link is using the ef86, that is the "non top-boost" Vox channel correct?
The link mostly addresses an EF86 AC-15 Lite circuit, so yes, non-top boost, but the author also did a Marshall Lite with the same chassis. Both are documented in the pdf at the end of the article. I am going for the Marshall circuit because I'm doing this on the ultra-cheap with existing parts/tubes and I used my spare Ef86 in a previous build. 8)
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