Tube Modular Synth from Modified Hammond Solovox

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tablebeast
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Tube Modular Synth from Modified Hammond Solovox

Post by tablebeast » Wed Jul 25, 2012 9:26 pm

Inspired by the work of Eric over at Metasonix I have decided to build an all-tube modular synth, DIY style. Eric has schematics up for his early rack modules that I will eventually be building myself. But before I get to THOSE modules, I plan on restoring and modifying one of the earliest synthesizers ever, the Hammond Solovox! I just aquired the best of the three versions, model L and it comes with the correct (and extremely complicated) keyboard. The previous owner has even built the cabinet and keyboard which were usually two separate devices together as one unit.

Here is the blog entry with more pics for this first project:
http://xfmr.blogspot.com/2012/07/hammon ... cking.html

And here are the planned mods:

1. This unit uses an interesting method to create three layers of sub-octaves from the single oscillator, effectively making 4 synchronized tones from the one tone. Normally these sub-octaves are simply switched on or off, but I will be adding individual outputs along with individual trim controls for each of the four voices.
2. Next, there is a global mute switch that mutes all four voices simultaneously. I will replace it with 4 individual switches.
3. Then of course I will add an audio input so that I can send any signal through this weird circuitry.
4. It has a vibrato circuit that I will be adding additional controls to.
5. There is a switch for a fast or slow attack using vari-mu tubes. From my experience building vari-mu compressors I will be adapting a circuit that gives fully adjustable attack and release controls.
6. I will replace a single triode tube with a dual triode tube so that I can add the extra gain stage in between the tone generator and preamp section to drive the signal into soft-clipping.
7. Finally it has an output power amplifier of about 10 watts that drives a built in speaker. I will add a dummy load for the speaker so it can be switched off, but the amp will remain properly loaded. To the amp output I will put a padded line level output with a trim control that can drive headphones or an unbalanced output to send to a recorder, PA, or another amp's input!

This will be the first synth in my modular tube rig with the next planned unit being a heavily modified Cordovox tube synth/amp originally made for accordions!
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hammondsolovox101.jpg
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Post by numan7 » Wed Jul 25, 2012 9:32 pm

:woah: interesting - i'm looking forward to hearing what you do with it! :popcorn:

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Post by Rod Serling Fan Club » Wed Jul 25, 2012 9:39 pm

Ilona watching. Is the divide-down circuitry neon based? I've been playing with neons lately.

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Post by Rod Serling Fan Club » Wed Jul 25, 2012 9:40 pm

Fucking phone. Ilona watching is right!... Shoulda read "I'll be watching"

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Post by tablebeast » Wed Jul 25, 2012 10:08 pm

Rod Serling Fan Club wrote:Ilona watching. Is the divide-down circuitry neon based? I've been playing with neons lately.
No, the divide down circuitry uses all vacuum tubes! From the Model L manual,
"Each divider includes three triodes. One acts as a driver and pulse rectifier, supplying sharp and narrow negative pulses to actuate a symmetrical feed-back tripping circuit comprised of the other two triodes. Either one (but only one) of these two triodes can be conducting at any one time, for by drawing plate current it holds the other in a cut-off condition.

Suppose, for example, that the first triode is conducting and the second is cut off. Now a negative input pulse impressed on the grids of both triodes will not affect the second one, which is already cut off, but will cut off the first. This produces a positive pulse at the plate of the first triode, which is applied to the grid of the second triode through its feed-back connections, insured that the first triode remains cut off. The situation is now exactly reversed, with the first triode cut off and the second conducting.

The next impulse will act on the second triode, cutting it off again and making the first conductive; and thus two input cycles are required to produce one output cycle. Each frequency divider circuit therefore divides its input frequency in half, producing an output signal one octave lower than the preceding divider. The triode plates of each divider stage furnish an output signal of rectangular wave shape as well as the signal to drive the next divider stage."

A very interesting circuit to be sure. I have attached a schematic so you can maybe make more sense of what this description is saying.
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solovoxmodellschem.pdf
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Post by Moog$FooL$ » Wed Jul 25, 2012 10:13 pm

keep us posted TB!!

:tu: :tu: :tu:

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Post by infradead » Thu Jul 26, 2012 12:48 am

oh wow. so cool
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Post by tablebeast » Thu Jul 26, 2012 2:03 am

Looking closer at the schematic, the circuits are really no different than modern transistor-based devices, just completely barebones! The amplifier section is a power amp that drives a speaker, but it also functions as a VCA essentially. I am not aware of another vari-mu VCA power amp. Anyway there is probably a good way for me to add some control voltage inputs to the amp, master oscillator, and vibrato oscillator sections. I want to keep this unit all-tube, no solid-state shortcuts, so I will have my work cut out for me.
Last edited by tablebeast on Thu Jul 26, 2012 2:44 am, edited 1 time in total.

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Post by Kent » Thu Jul 26, 2012 2:19 am

Fantastic! I'm really looking forward to seeing and hearing the end result.

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Post by General Electron » Thu Jul 26, 2012 7:10 am

"Symmetrical feedback tripping circuit" would be an excellent name for a band.

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Post by revmutt » Thu Jul 26, 2012 7:36 am

Nice. I own two Solovox, one is not currently working. Mine do not have the pedal stand. That is very handy looking.
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Post by Rod Serling Fan Club » Thu Jul 26, 2012 10:53 am

Thanks for the schematics. The master oscillator looks super simple like a relaxation multi-vibrator type of thing. I'm not very familiar with trimmer condensers. I'm assuming its some sort of variable capacitor thing?

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Post by oljud » Thu Jul 26, 2012 11:03 am

Wow. So, you will add some sort of patch bay? Can you do FM?

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Post by Dave Peck » Thu Jul 26, 2012 11:43 am

Rod Serling Fan Club wrote: a relaxation multi-vibrator type of thing.
I'll just let everyone fill in their own punch line for this. Enjoy!

BTW, this looks like a really cool project!

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Post by Rod Serling Fan Club » Thu Jul 26, 2012 5:51 pm

Dave Peck wrote:
Rod Serling Fan Club wrote: a relaxation multi-vibrator type of thing.
I'll just let everyone fill in their own punch line for this.
I've been trying to come up with a name for the neon-lamp based LFO/oscillator thing I am working on. It's a relaxation multi-vibrator type of thing btw. One idea was "neon syncoscillator" which is a good description of what it is from user standpoint but I'm not sure I like the name so much. Maybe I should go with "Relaxation MultiVibrator". :woah:

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Post by Moog$FooL$ » Thu Jul 26, 2012 6:05 pm

but will it do through-zero?

:razz:

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Post by tablebeast » Fri Jul 27, 2012 2:38 pm

I had to search "through zero" and see all that drama attached to it. Sheesh! Neat design, but I am pretty sure this old tube synth won't do it! However tubes have weird things that transistors and ICs don't, screen grids! As Eric Barbour's creations prove, there are a lot of unique things you can only do with tubes.

As I look more and more at this simple circuit I see more and more possibilities. I think a proper audio input may prove problematic because it may wind up acting more like a ring modulator on some settings. Could still be cool, but might need some more support circuitry. I won't know for sure until I do some more experimenting.

I have also decided to change the two power tubes in the output section with a single 6SN7 dual triode so that I will have more heater current and another tube socket available for the added CV buffer triodes. It will have a much lower wattage output, but will be much more appropriate for a line output as it won't have to be padded, I can tap a balanced output from the transformer, AND will still drive a speaker to a bedroom level volume. Since it can now be hooked up directly to a PA or external amp the built in speaker becomes more of a novelty than a necessity.

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Post by tablebeast » Mon Jul 30, 2012 2:11 pm

She arrives today! I will get some inside pics of the guts and stuff as soon as I can!

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Post by numan7 » Mon Jul 30, 2012 2:18 pm

:agonizer: ... :popcorn:

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Post by dslocum » Mon Jul 30, 2012 2:19 pm

TB,

Not sure where you are, but if you're close to NJ, I'd be pleased to help with this project. If nothing else, I'd love to see it.
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Post by tablebeast » Mon Jul 30, 2012 2:22 pm

I am in Asheville, NC. Thanks for the offer, I would for sure take you up on it if we were closer. Don't worry I will be asking for plenty of help right here, though! First step is just getting it to work as it is, then I will add the mods on at a time. If I can develop a good set of mods, I may consider doing more of them for other people. If nothing else I will document the conversion well enough that a skilled tech could copy what I've done and do the mods themselves.

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Post by tablebeast » Fri Aug 03, 2012 12:32 am

It arrived the other day, but I have been too busy working on other projects to dissect it yet. There are already a few mods from what I can see, including two 1/4" jacks that I am pretty sure are not stock. The way this thing was built into a standalone instrument is neat, but I think I will just borrow the idea and redo the whole thing myself as the woodworking is utilitarian at best.

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Post by metasonix » Fri Aug 03, 2012 1:14 am

Yes, they did not come with output jacks. Solovoxes were intended to be used as add-ons for organs and pianos, which is why the keyboards usually look like this, they were designed to mount under a larger keyboard. And they were "closed". In those days, an external amplifier or a line-level recording output wasn't even a dream. Organ keyboards always had to be "self-contained".

Calling the Solovox a "synthesizer" is really pushing it. The main oscillator is a very simple LC type, the keyboard switches in custom-made inductors to produce the correct pitch. If an inductor goes bad, you have to cannibalize another Solovox to fix it. The "vibrato" is more like a bad tremolo and the attack control is extremely crude--you get two choices, fast or slow, it messes with the bias current of the 6SK7s to do it. The "voicing controls" are a joke. It was a very different era, and the "weird sounds" the Solovox did were more than unusual enough to sell it.

At least Solovoxes are easy to find, having been very popular in the late 1940s and 1950s with professional organists. I would seriously suggest buying at least one more keyboard section for parts.

Beware of the different models: I seem to recall the J had loktal tubes and lots of little interstage transformers, the K and L had octals, and there was a later L model with miniature tubes. J and K models used a mechanical vibrating reed to make the tremolo effect instead of a tube oscillator. All quite different.


Also, the power transformer is barely adequate to run the thing and gets very hot. Major failure mode. You might consider adding a big switching-type DC power supply just to run the 6.3v tube heaters. A 5V supply will work okay.

You can use 6V6s for the outputs instead of 6K6s. 6V6s are still being made. Might have to change the two 1000-ohm resistors on the cathodes so the tubes bias properly.

Don't even joke about adding CV inputs.

If you don't believe me, here's Lorin Parker's take.
Last edited by metasonix on Fri Aug 03, 2012 1:29 am, edited 1 time in total.

tablebeast
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Post by tablebeast » Fri Aug 03, 2012 2:06 am

Wow, Eric, thanks for your input and that link. I've always liked Lorin's work and I'm surprised I didn't come across this link earlier. I am still new to the concepts in this Solovox and I am trying to figure out how or even IF it could fit in with a modular setup. CV inputs aside, what do you think about changing the 6K6 outputs for a single 6SN7? I know it will knock the output down considerably, but I am wondering if it will still drive a speaker? In any case, the octave divider and amp circuitry are itneresting enough to me to warrant the purchase on its own, so anything else is gravy. Thanks for the heads up about the power transformer, maybe I'll swap it out.

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Post by johnnly » Fri Aug 03, 2012 4:39 am

This looks great - will definitely be keeping an eye on your progress with this project.
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