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Old 22nd Jul 2017, 10:42 am   #21
Oliver35
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

There's some good stuff on those sites, I'd not come across those! I have two small crts lurking that I was thinking about building a waveform scanner with, but that's a job for the future. At the moment, I'm cobbling my keyboard together- I should post some pictures once I get organised..

Oliver
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Old 19th Aug 2017, 11:36 pm   #22
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Well, it's been a while, and various domestic matters have kept me away from the soldering iron, but I'm still doing bits and pieces on this project and pressing unlikely odds and ends into service. I've currently got a single Transitron-style oscillator built around an EF39, working with a keyboard controller giving me a tunable octave range with a square-wave (ish) output. Oscillator range can be varied by switching through capacitors.
Unsurprisingly perhaps, my optimism always slightly exceeding reality, outside the tunable octave I'm running into problems with the non-linear response of the valve. At the bottom end of the two-octave keyboard I get far too little difference between the steps in the potential divider (which is built with 24 2.2k trimpots in line with a 2.2k resistor) and at the top end I start to get an inverse response to the increasing control voltage, presumably due to the valve doing one of those things that valves do that I have so far failed to grasp. Saturation? I don't know, I have a cavalier attitude to theory at the best of times..
I now need to work out a clever solution to this problem, although a tempting (and far lazier) solution would be to use a separate valve for each octave and thus use the sweet spot in the middle of the response. That does run the risk of giving a different tone characteristic to each octave, although that probably doesn't matter. This will never be a serious instrument, just a toy and an enjoyable learning curve!

Having uploaded some photos this evening, I've now discovered that the focus is dreadful on many of them. A couple of photos of the keyboard unit though, which I'm quite pleased with. It has its own inbuilt 20v DC supply for the control voltage, and an array of unlikely hardware to boot. Dymo tape is, of a course, a crucial element in all of this..

Oliver
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Old 6th Oct 2017, 11:52 am   #23
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

what a beauty love the case!
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Old 6th Oct 2017, 4:37 pm   #24
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Youtube "Space Patrol" TV puppet series to hear some of Fred Judd's music made with his circuits.

David
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Old 7th Oct 2017, 12:33 am   #25
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Oliver,

Your project is just marvelous.

I remembered a circuit I once looked at that used a 6SK7 as a very effective voltage controlled resistance (in the audio frequency range). The simple arrangement was created by Seeburg, for one of their Juke box amplifiers. They did it to create one of the most impressive volume leveling (compressor or AGC circuits) that I had seen/heard. In any case I copied it from the schematic (attached). I think it varies its effective resistance from a few hundred k to a fairly low value.

This idea could possibly have an application to make a voltage controlled oscillator too, since you to like to experiment.

The trick with their circuit is they couple the signal into both the plate and control grid, turning the valve into a voltage controlled variable resistor which in their case created a voltage divider at audio frequencies with the series 100k resistor.

Hugo
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Old 7th Oct 2017, 12:47 pm   #26
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Some therimin pages I had bookmarked - http://www.thereminworld.com/schematics and - http://www.theremin.us might give you some ideas. From what I remember from experimenting years ago I find it easier to use a valve as the oscillator and "tune" it using solid state.

Andy.
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Old 10th Oct 2017, 7:15 pm   #27
Oliver35
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Sorry to respond so slowly, I hadn't spotted that there had been further activity on this thread. Thanks for the positive comments, though- Snufkin, the case is actually pretty crude, but I like things to look the part! It's only scraps of MDF, but painted with good old Hammerite to get the correct flavour!
Hugo- I'm glad you like it! That circuit is what I had originally assumed I could do, and later abandoned, as I couldn't make it work. I can't remember where I cribbed the idea from, but the execution was different to the circuit you've posted- i don't believe that coupling between control grid and anode was there, or starters- so I shall experiment as you say and see what happens. I was hoping a VC variable resistance will give me a wider frequency range to play with. David, Andy, thanks for the links- I shall look them up. I'm afraid I have an odd 'kink' in my character though, which makes me try and do everything with valves!

Oliver
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Old 11th Oct 2017, 4:04 am   #28
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

"Kink"
thats not a kink, its nostalgic purism.
I have it too

DONT have it repaired !!!

best wishes

Joe
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Old 11th Oct 2017, 8:41 pm   #29
Oliver35
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Default Re: Valve voltage-controlled oscillator- what am I doing wrong?

Joe, I wasn't going to- although I think 'nostalgic purism' is perhaps a more elegant way of phrasing it.. The trouble is, I wasn't born until the 'valves had gone out all over Europe', so I'm not sure I can claim nostalgia in my defence! Stubborn contrarianism, more likely.

Oliver
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