Hammond Solovox ('40's synth)

[maxkracht]

Curious If anyone has played around with these. I found the "tone cabinet" in a thrift store outside of Chicago about a decade ago, not quite knowing what it was, and got the keyboard from ebay several years later. This spring, I got around to repairing/restoring it after a few more years collecting dust. Several burnt out resistors, recap, and lots of cleaning. I bypassed the cathode on the preamp for more gain, and have been playing around with plate resistor values to try and get it to distort a bit more. I am toying with the idea of adding a passive FX loop, before the preamp, but feel bad about drilling holes. I temporarily rigged this up with jumpers and kinda like the way it sounds going through a fuzz pedal. Can also use this as a guitar input, but you need to hold down a key for the guitar to make any noise. Haven't thought through a way around that, though I'm not sure if I find it that interesting as a guitar amp anyway... I'm not too familiar with vintage organs, and most of the electronics on this thing are way over my head. I am pretty blown away by using a large inductor for every key. Can't imagine how expensive that would be if this were manufactured today.

Edit: Seems obvious in hindsight, connecting a wire from the "control contacts" to ground to ground turns on the control tubes. Since that node is in the keyboard enclosure, a 1.5k resistor from the orange wire on the bottom right of the schematic to ground does the same thing. Normally this would play a C note, but if I put in an effects loop and plugged a guitar into the preamp input, it would break the normal from the keyboard output so there isn't a constant C playing in the background.

solovox guts.jpg

solovox back.jpg

solovox keyboard.jpg

solovox schematic.jpg

[maxkracht]

Doesn't seem like anyone is into this but though I would post an updated schematic in case someone wants to try something similar in the future. I neglected to take pictures of the inside before putting it back together, but effects loop works. A DPDT switch shorts the main oscillator grid to ground and a 1.5k resistor from the orange/black wire to ground turns on the control tubes. The switch makes it possible to use the cabinet as a normal guitar amp. In the other position, you can still plug in a guitar, but you will only be able to hear it when someone is pressing a key. This is kind of an interesting effect in itself, though i'm not sure how useful. With most keyboard settings, a guitar seems to bog down the keyboard signal and it is fairly hard to strike a balance between the two.

solovox schematic mod.jpg

[xtian]

Glad you're having fun!

[thetragichero]

interesting, so am i correct in the idea that it's using the transformer for a phase inverter with an amplification stage before the power tubes?

[maxkracht]

Yes, the transformer is the PI, though I am unfamiliar with this arrangement with a plate resistor across the primary. Or, does the resistor before it count as the plate resistor? I played with both but put it back to stock after deciding a FX loop would get all the tonal changes I wanted. Not sure if the control tubes are amplifying much, but I didn't take any measurements and this thing is pretty confusing to me. The oscillator is always on and putting audio through the preamp and PI. The control tubes main function is to mute the output until a key is pressed. Why that requires both tubes is over my head...

[thetragichero]

yeah a lot of that is over my head lol. you're a better man than i for getting it back up and running instead of gutting to make a guitar amp like i would do

[echuta13]

Neat project! I've seen these mentioned every blue moon and they they are fascinating (don't tell anyone but I like synths too..)