50's vaccum tube synth/organ mods/circuit bending?

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not-tachyons
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50's vaccum tube synth/organ mods/circuit bending?

Post by not-tachyons » Thu Jul 13, 2017 9:56 pm

I came across this old hammond chord organ for free on craigslist. It's really interesting, in that its not a tone wheel organ, It's a synthesizer based on vacuum tube oscillators. They're amazingly still in tune over 50 years later. They were basically meant for the home market in the 50's and that's where the fully electronic sound generation came in, to make it lightweight. It really doesn't sound like an organ at all. It was engineered in the same line as the novachord polysynth and solovox . Anyways, these things aren't actually organs, but were popular, dirt cheap and being thrown away everywhere because they don't fit into the organ community very well. But i'd say its certainly an early synthesizer, not really a novachord, but i really haven't heard anything sound this rich in vintage analog warmness. Everything has this tube driven sound coming from chorused/detuned tube oscillators.

Since its very easily serviced, I'm curious about what kind of modifications can be done to these. There's the basics, like a line out, or somewhere there's already a line in for the tube amp it has. But in the synthesizer part of it, there's a primitive envelope for the solo voice, some vibrato controls, a few switches for wave-forms, and a few more switches that go through some basic passive filtering, and just a unique sound generation with multiple sections. It seems like there's a lot of room to circuit bend this. An extra pot here and there to push around the envelope circuit maybe, maybe putting in an LFO somewhere for a unique effect, the different oscillator sections use some interesting methods to create polyphony, stay in tune, with a few divide down registers, a weird filter bar for the chord buttons. I'm not too familiar with this stuff, but i have someone to help me work on it when i get to that point.

These things are free or under $100 all over craigslist, which is great, but they also end up in the trash and i feel a lot of people here would get a kick out of tinkering with them like this. At least for the built in tube amp and 50's overdrive sounds, these are easily worth more than being thrown away. But i'm wondering what are some more ways i can push the sound it makes. I'm sure I can throw in some modern circuits to manipulate its sound even more, it was made right when the first transistor radios came out and is pretty simple. Or even just a pot or switch here and there. I found one mod, that's just soldering in a switch that makes it go from stock to full on tube overdrive. So yeah, i wanted to share this, remind everyone to check craigslist, and see what ideas you might have for it.

The residents used a custom tuned S-6 chord organ here
some different schematics for everything
general info, introduction, pics
service manual, details the chorused-detuned tuning process for the oscillators
here is a regular tone wheel organ that was circuit bent with an adsr envelope and a few other things.
Last edited by not-tachyons on Thu Jul 13, 2017 10:15 pm, edited 1 time in total.

AIDS
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Re: 50's vaccum tube synth/organ mods/circuit bending?

Post by AIDS » Thu Jul 13, 2017 10:15 pm

You've picked a very dangerous bit of kit to be circuit bending there.

The DC high tension voltage on the valve/tube heaters is the last thing you want to tangle with.

not-tachyons
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Re: 50's vaccum tube synth/organ mods/circuit bending?

Post by not-tachyons » Thu Jul 13, 2017 10:20 pm

AIDS wrote:You've picked a very dangerous bit of kit to be circuit bending there.

The DC high tension voltage on the valve/tube heaters is the last thing you want to tangle with.
Oh no I'm not touching these tube circuits. This was way before UL standards were on everything, definitely not on this organ at least, and I still need to get it serviced and modified for safety reasons. There's a shop behind my house that specializes in vacuum tube audio equipment and synthesizers. Kinda lucky, but before i went in or dropped money i wanted to have some clear ideas on what fun modifications I can have done to this thing

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Re: 50's vaccum tube synth/organ mods/circuit bending?

Post by DavidH » Wed Jul 19, 2017 8:17 am

If you like the Hammond Chord organ, google the Hammond Novachord - one of the most fascinating non-tonewheel Hammonds out there!

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Re: 50's vaccum tube synth/organ mods/circuit bending?

Post by AdamAnt316 » Thu Jul 20, 2017 10:41 pm

not-tachyons wrote:
AIDS wrote:You've picked a very dangerous bit of kit to be circuit bending there.

The DC high tension voltage on the valve/tube heaters is the last thing you want to tangle with.
Oh no I'm not touching these tube circuits. This was way before UL standards were on everything, definitely not on this organ at least, and I still need to get it serviced and modified for safety reasons. There's a shop behind my house that specializes in vacuum tube audio equipment and synthesizers. Kinda lucky, but before i went in or dropped money i wanted to have some clear ideas on what fun modifications I can have done to this thing
Well, there were UL regulations back then, just somewhat different than what we have now. Tubes were a fact of life, as was dealing with the possibility of high voltage being where it shouldn't be, and UL helped to cut down on them to some extent. Fortunately, those chord organs use a power transformer, so it shouldn't be as dangerous as a 'hot chassis' device which had the power line connected to one side of the chassis in order to save money.

My personal opinion is, have it restored to original condition so that you have a chance to listen to it in its natural form before deciding to make any modifications (exactly what would be possible with something like that, I dunno). I'm planning to replace the filter capacitors in my Hammond Solovox (actually, I have two of them now) before I make any decisions on what to do next. I'll likely be replacing the existing capacitors (and other components which may require replacement) with ones of the same values as the old ones, to preserve the circuit.
-Adam

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