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1949 electronic sackbut 6as6 subcircuit

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Satellite Icarus

Satellite Icarus

Member
Joined 2010
#1
Hi diyaudio!

it's been a while since I've posted but I was wondering if any of you might be able to chime in on my latest research project.

Last time I was writing here I was looking for help on a hi fi amp i was building for my undergrad senior thesis. A few moves and thirteen years later I'm now a researcher at the Canadian museum of science and technology working to reverse engineer Hugh Le Caine's electronic sackbut instrument, which is part of their collection.

https://en.wikipedia.org/wiki/Electronic_sackbut#

My circuit analysis friend and I are a little stumped by the suggestion of a colleague that the 6as6 tubes in the oscillator section of the device are frequency counters: no feedback, no RC circuit with a time constant to flip flop a square wave at a defined rate. Just resistors:

1740962493703.png

I'm including two of the four stages. the caps on the left of the 6as6 stages go to a transformer coupled blocking oscillator. The output exiting left of the plate goes to a mixer-so the stages don't cascade. I really think this is just a buffer / gain stage to couple the aforementioned blocking oscillator to the strange resistive mixer that follows these pentodes.

I realize this is more of an amp forum than a synth forum, but since these pentodes really do look like a coupling stage to me (rather than a counter stage) I think we are squarely in the amplifier section of the tube circuit topology realm.

In short though - is there any conceivable way a pentode, wired like this or something like this (the circuit tracer may have missed something!) could act as a clock divider (e.g. take a square wave and output other square waves at ratios of the original frequency)?

Thanks in advance for your perspective, and let me know if there's additional info you might like to answer this more accurately!
 
#2
Satellite Icarus said:
In short though - is there any conceivable way a pentode, wired like this or something like this (the circuit tracer may have missed something!) could act as a clock divider (e.g. take a square wave and output other square waves at ratios of the original frequency)?
Click to expand...
I don't really see how. To do that you need essentially a flip flop which is really just a bi-stable circuit which would be triggered by the input. Since you haven't shown either the input or output connections, it's impossible to tell.

Regardless, a single pentode cannot form a bi-stable circuit. You'd need two active devices with opposing grids and plates coupled. So there really isn't a way to divide the incoming signal with a single tube.
 
#5
Hearinspace said:
Just curious , What's the rail G3 is connected to?

Is there copyright that doesn't allow posting more of the schematic?
Click to expand...
No, it's more that if no one recognizes this topology easily this gives me a reason to get approval to post more / confirmation it's not a common setup. I'm working on cleaning up the cut up multiple .sch we have into a much more legible vector schematic for the whole thing / still chatting with my supervisor about where I can post the whole thing. More on that soon
 
#6
Suncalc said:
I don't really see how. To do that you need essentially a flip flop which is really just a bi-stable circuit which would be triggered by the input. Since you haven't shown either the input or output connections, it's impossible to tell.

Regardless, a single pentode cannot form a bi-stable circuit. You'd need two active devices with opposing grids and plates coupled. So there really isn't a way to divide the incoming signal with a single tube.
Click to expand...
That's also been my guess. I will be looking through some historical tube delay / comparator / one shot circuits just to make sure but I appreciate the feedback
 
#7
Hearinspace said:
Just curious , What's the rail G3 is connected to?
Click to expand...
also the suppressor grid (pin 7 in the schem) is connected to all of the other 3 suppressor grids and then to part of a discriminator circuit in a fairly complex keyboard control circuit (the sackbut had multiple concurrent pitch control mechanisms: a glide strip, a keyboard, and a pitch bend control dependent on bending keys left or right). As mentioned I hope to post the full schematic eventually so these interesting arrangements can be discussed more easily
 
#8
Tom Bavis said:
The 6AS6 has a suppressor grid that has more effect than most pentodes (It's a "dual control" pentode) and the voltage on the third grid can control gain.
Click to expand...
could you implement some kind of comparator or frequency dependent delay or other forms of clock dividers with this with just a few extra resistors as above? I very well may simply not have the imagination! if it is possible it is not a common use of a control pentode, from what I can tell (e.g. https://www.radiomuseum.org/forum/dual_control_pentodes.html)
 
#9
It is possible to make a frequency divider with a single tube section. I had an old organ (Baldwin maybe) as a kid that had half as many tubes in the generator rack than keys on the keyboard. It was highly sensitive to temperature and humidity probably due to its old carbon comp resistors and paper caps. Each stsge is basically a sawtooth oscilator tuned just below the desired frequency fed sync pulses from a higher octave. Every other pulse triggers a reset.
Look here for some ideas.

http://www.r-type.org/articles/art-309.htm
 
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