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Old 20th Dec 2018, 11:46 am   #1
Neil Purling
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Default Valve Multivibrator

I have loads of EF39's and 6K7's but only one ECC83. I figure i'd get it to oscillate if I showed it enough HT voltage. I want it to oscillate at 1Kz, or therabouts to troubleshoot radios.
I have several HT transformers in my scrap pile. The lowest voltages are 160-170V, wheras the rest are typically 250V.
I know you can make one with transistors, but I have loads of valve-related junk and high-voltage capacitors.

It will give me something to do and be useful as well.
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Old 20th Dec 2018, 12:18 pm   #2
paulsherwin
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Default Re: Valve Multivibrator

https://mysite.du.edu/~etuttle/electron/elect36.htm

Just about any pair of triodes will work. If you want to use pentodes, triode wire them. The HT voltage isn't critical so long as it doesn't exceed the valve ratings.
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Old 20th Dec 2018, 12:42 pm   #3
G6Tanuki
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Default Re: Valve Multivibrator

I've used an ECC83 multivibrator in the past: mine was designed to oscillate around 10KHz when running by itself, but was then 'locked' to a 100KHz crystal-oscillator (by injecting a whiff of the 100KHz into one of the ECC83's grids) to provide me with accurate 10KHz calibration 'pips' when aligning receivers.
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Old 20th Dec 2018, 1:45 pm   #4
kalee20
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Default Re: Valve Multivibrator

It should work well. The conventional Eccles-Jordan multivibrator is a pretty forgiving circuit.

In fact, using valves it should be better than transistors! With transistors, it can get into a stable state (both transistors 'on') which persists indefinitely. With valves, even with both valves 'bottomed' there is usually enough voltage gain around the loop for oscillation to start.
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Old 20th Dec 2018, 1:49 pm   #5
turretslug
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Default Re: Valve Multivibrator

You may be pleasantly surprised at just how low a voltage a valve multivibrator will oscillate, too- just as long as you take care not to load it too heavily.
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Old 20th Dec 2018, 5:33 pm   #6
ionburn
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Default Re: Valve Multivibrator

Quote:
Originally Posted by turretslug View Post
You may be pleasantly surprised at just how low a voltage a valve multivibrator will oscillate, too- just as long as you take care not to load it too heavily.
I would agree with that. I have never made a multivibrator with valves, but do have a 6C4 crystal oscillator running reliably at 17V with a transistor buffer stage.
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Old 20th Dec 2018, 5:46 pm   #7
Neil Purling
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Default Re: Valve Multivibrator

I followed the link in post #2.
The Square-Wave Multivibrator:
I know that I don't have a pair of 1000pf capacitors for C. What would it do to radically increase the value to 0.01? I have dozens of those.
I have a ganged pair of 470K for R2. I will have to see what I have got a pair of for R1, but I don't think I have a ganged pair.
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Old 20th Dec 2018, 5:56 pm   #8
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Default Re: Valve Multivibrator

Some time ago, I built a small test rig to draw Ia-Vg curves for valves which could then be used as standards for a valve tester. The first time I put in an ECC81 (with just one side hooked up), it started oscillating with no help at all from me. I had to go to some trouble to stop it; lots of ferrite beads on the leads etc.

I think it's Tanuki who occasionally points out that every amplifier aspires to be an oscillator. I think you can add to that that once a valve finds out it can oscillate, it prefers to do that rather than just amplify .

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Old 20th Dec 2018, 6:02 pm   #9
paulsherwin
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Default Re: Valve Multivibrator

Quote:
Originally Posted by Neil Purling View Post
I know that I don't have a pair of 1000pf capacitors for C. What would it do to radically increase the value to 0.01? I have dozens of those.
That will just reduce the operating frequency. It should still work.
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Old 21st Dec 2018, 11:47 am   #10
Herald1360
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Default Re: Valve Multivibrator

Usually summed up as "amplifiers oscillate, oscillators won't"
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Old 21st Dec 2018, 1:36 pm   #11
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Default Re: Valve Multivibrator

Oscillators have a plan-B they can also hoot on the wrong mode.

David
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