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Old 3rd Jul 2017, 9:52 pm   #1
Oliver35
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Default Valve voltage-controlled oscillator- what am I doing wrong?

Popping my head up again, hopefully for guidance out of the extensive thickets of my own stupidity.
I've lately been reading a lot about early electronic music pioneers, not hitherto an area of technology I've paid much attention to. Characteristically, once the transistors start creeping in I tend to tune out, but I've read much interesting stuff about the likes of Tristram Cary, Delia Derbyshire and Daphne Oram, the first and last especially being great experimenters electronically as well as musically. Also characteristically, I quickly decided that reading about them was insufficient, and nothing would do but I should have a go myself. As a result, I've rummaged around in the valve box and come up with a plan to build what could best be described as a 'noisy thing' using RF pentodes (all Octals- I only really have Octals) that I can expand with filters and so on as I fiddle. I read something to the effect that a voltage controlled oscillator (VCO) was best implemented with thyratrons, but as I don't have many Octal based thyratrons, I thought I'd try building a VCO using a pentode in a Transitron arrangement, giving me a choice of square, sine or sawtooth waves dependent on configuration, the voltage controlled frequency achieved by using a second, triode-strapped pentode as a dynamic resistance. So much for the theory. I built it, and it didn't work. The oscillator oscillated with gusto, but the addition of the second valve killed it stone dead. In the end I got it to work, and respond to varying control voltages, by partially bypassing the control valve with a resistor (shown dotted in the attached schematic). It still doesn't work well though, the range being severely limited. I discovered that without the bypass resistor, it was oscillating- just extremely slowly, with a series of faint ticks from the high-impedance transducer I'm using to test it. I assume the valve offers far too high a resistance, but I was surprised at how insensitive to the control voltage it appeared to be. What have I missed? I'm certainly no expert at designing circuits, optimism and lucky chance are my two main assistants, my theoretical knowledge being virtually nil (see schematic.) Can this work viably under any circumstances, or am I barking completely up the wrong tree?
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Old 3rd Jul 2017, 10:58 pm   #2
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

I don't think varying the grid volts on a triode has the effect of varying its resistance (slope of Va/Ia)- only how high the Va has to be to get any current at all.

A crude VCO could use a pentode to vary the voltage at its anode to feed a neon relaxation oscillator.

Better than the neon would be a thyratron, though. 6K25, perhaps?

A valve Schmitt trigger circuit could also be used instead of the neon to provide the "switch with hysteresis" needed.
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Old 3rd Jul 2017, 11:04 pm   #3
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

I have been threatening to do this stuff as well!!
Have a look here:
https://www.cgs.synth.net/tube.html

There is a whole bunch of interesting stuff, with the theory as well
Joe
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Old 3rd Jul 2017, 11:22 pm   #4
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

I've heard of valves being used as dynamic resistances, but I know nothing of the theory. Thyratrons can be used, and the grid offers a means of incorporating voltage control, but as I mentioned above, I'm unwilling to use them if I can avoid it, partly because they generate sawtooth waves (which sound nasty) and partly because I've only got one 6k25, one EN31 and one EN32, and I'd like to ultimately build three identical square / sine oscillators. I have a decent stock of EF37a and EF39 pentodes, which mostly drove my choices. As I say, it does work, up to a point- just not over a wide enough range to be truly useful.
I have to admit I have no idea what a Schmitt trigger is, even less how it would be implemented with valves. Google may be required..

Oliver
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Old 3rd Jul 2017, 11:28 pm   #5
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Hi Joe, I've been reading that site too! It does indeed have some great stuff, the VCF and VCA circuits I'm going to use unmodified, apart from using 'big' valves, as I have them. However, his VCO is a single-thyratron circuit.
One thing that is missing from that site is any kind of valve-based envelope generator. That's another project, but not one I feel man enough to tackle just yet!

Oliver
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Old 3rd Jul 2017, 11:38 pm   #6
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

A Schmitt trigger is circuit that switches "on" rapidly at a threshold rising input voltage but doesn't switch "off" again until the input voltage falls to a lower level than the on threshold.

It was originally cooked up by a Mr Schmitt using triodes sometime in the 1930s.

In multilegged bug form a simple square wave oscillator uses 1 R, 1 C and a Schmitt inverter. Frequency depends on R, C and supply volts.

Dr Google will indeed enlighten
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Old 3rd Jul 2017, 11:58 pm   #7
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Oh, I see- as indeed a thyratron does! I shall read further. Could be useful in designing an envelope generator, too..

Oliver
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Old 4th Jul 2017, 12:25 am   #8
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Arrow Re: Valve voltage-controlled oscillator- what am I doing wrong?

Early oscilloscopes used thyratrons. Later designs used the Puckle time-base. That latter oscillator might be worth investigating for your purpose.

Al.
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Old 4th Jul 2017, 11:46 pm   #9
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Maybe I over-complicate things..
One thing I hadn't tried (and should have done) was just shoving a CV voltage straight into the control grid of the oscillator, which would otherwise be returned to earth or to HT via a resistor, or to a variable point in a potential divider. I suppose the clue's right there.
Anyhow, it works- or at least, works better than anything else I've yet come up with using hard valves. A well-regulated 0-6v DC gives me a pretty reasonable range, although ultimately I'd like it to be better. I need to experiment a bit with biasing- perilously little of my Quixotic approach to design relies on doing any kind of maths..
Tomorrow I'm hoping to progress the crude keyboard I've nailed together to give me some form of control..

Edit: I notice now I've omitted the output on the drawing, obviously from the anode. I really should pay more attention to what I'm doing..
Oliver
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Old 5th Jul 2017, 1:57 am   #10
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Quote:
Originally Posted by Oliver35 View Post
Tomorrow I'm hoping to progress the crude keyboard I've nailed together to give me some form of control..
Wonderful - an analog synthesiser in the making! This Ondoline uses thyratrons, in case you're interested in getting some ideas from an established (vintage) source?
http://forum.vintagesynth.com/viewtopic.php?f=1&t=79506
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Old 5th Jul 2017, 7:53 am   #11
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Perhaps a two-valve multivibrator would be a more predictable current to frequency converter?

The opamp VCO designs use a schmitt and an integrator and a switch... all do-able with valves.

For a simple 'stylophone' arrangement you can temper the resistor values in the keyboard, but if you want to come on like Keith Emerson with modulated oscillator sounds, you'll need an anti-log converter.

Keeping things in tune was bad enough with opamps.

David
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Old 5th Jul 2017, 9:32 am   #12
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

One approach that I would investigate, although not sure whether some aspects are too modern with the approach, would be to use semiconductor diodes as varicap diodes in a higher frequency oscillator (valve), then use double triode digital divider circuits to get back to audio.

Thoughts about what the mechanism of working in a Theremin might also provide ideas.
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Old 5th Jul 2017, 11:57 am   #13
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

It depends what frequency range you want the VCO to cover! Small range... reactance valve, Varicap, DC-biased iron-cored inductor. Large range, some sort of relaxation oscillator.
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Old 5th Jul 2017, 12:48 pm   #14
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Maybe simpler than valve dividers, make your reactance valve controlled HF oscillator part of a Beat Frequency Oscillator.


If your VCO covered just 100-120kHz (should that be kc/s in this context?) mix it with another 100kHz and feed out through a 20kHz LPF. 0-20kHz in one sweep. You can square it, integrate it or whatever downstream.
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Old 6th Jul 2017, 10:39 pm   #15
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

These are all great suggestions- I like the idea of a BFO, although in truth I don't need that wide a range in between the stepped capacitors- the keyboard I'm building only has a compass of two octaves, and having just drilled the seventy-two holes needed in a paxolin sheet for the PCB mount trim pots I'm using, that's sufficient range for me! I'm now making up a strip of 24 microswitches, although I now won't be able to do much more until the weekend. I am a bit obstinate when it comes to embracing solid state technology, so I doubt I'll be using varicap diodes- I did originally think of using the tempered resistor approach, and may yet do so. I'm rather letting the idea grow like topsy, although I would like to include some form of vibrato, which might be difficult in that instance. I've got a nice spring reverb unit I can throw in the mix, too..

Oliver
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Old 7th Jul 2017, 1:34 am   #16
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

I have some Hammond keyboards that I will use ( if I ever get around to building something) I have played with microswitches myself and found that its very difficult to achieve any sort of "attack" time. The hammond keys have multiple contacts that the time can be measured on keystrokes to obtain "loud and soft" notes. ( piano and forte )
I like this thread so I might even start playing with a few "experiments".

regards
Joe
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Old 7th Jul 2017, 2:34 pm   #17
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

That kind of dynamic response is a bit beyond my capabilities at the moment! I'm hoping to design a rudimentary envelope generator that will give me a level of control over attack and release at the very least, but only from a binary input. Microswitches are far from ideal- I have a large number of the roller arm type- as they have a definite 'click' as they throw over. I did buy a lot of small leaf switches as well, but they would have been much harder to mount, the microswitches clamping quite nicely between a couple of lengths of aluminium angle. They also have normally-open and normally-closed contacts, which may prove useful. The keyboard itself is crudely manufactured from MDF and a handful of old piano keys that I managed to get my hands on, so it's all rather rough-and-ready and experimental (mad?). Dymo may have to play a part in all this.

Oliver
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Old 18th Jul 2017, 10:59 am   #18
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

http://www.electronicpeasant.com/pro...nth/tubes.html here are some nice circuits

Three tube based synths here too http://www.sdiy.org/pinky/

Also check out the phantastron circuit adapted for audio
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Old 18th Jul 2017, 11:13 am   #19
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

http://www.experimentalistsanonymous.com/orchard/

http://nicksworldofsynthesizers.com/scope.php this is basically how the Oramics oscillators worked.

F.C Judd also published many early valve based designs Ill see if I can scan one of the early books I have from him if you like (or at least the diagrams)
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Old 19th Jul 2017, 1:37 am   #20
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Superb resource snufkin !!!
Absolutely ten out of ten!
Thankyou

Joe
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