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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Nov 2018, 10:41 pm   #21
Miguel Lopez
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Default Re: Playing with valve oscillators

This is an oscillogram of the LC oscillator output at 25 kHz. Sweep is set at 20uS/div and amplitude is set at 20V/div. What seems to be a crossover distortion is in fact a flaw of the oscilloscope that I've been too lazy to solve.
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Old 9th Nov 2018, 6:43 pm   #22
TrevorG3VLF
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Default Re: Playing with valve oscillators

Passive circuit with gain greater than one.
I assume that the phase shift is zero.

There is an old book by Dean (p197) which I cannot find. This used a pair of pnp transistors in a Darlington pair to get high input resistance and low output resistance.

There were three capacitors in series, two were driven by 47k resistors, the third went to the transistor base.
With C=80microF, 1cycle/minute
With C=100nF, 11Hz
I believe this low frequency system was used to generate vibrato in electronic organs.
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Old 10th Nov 2018, 6:32 pm   #23
kalee20
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Default Re: Playing with valve oscillators

Quote:
Originally Posted by Miguel Lopez View Post
Quote:
Originally Posted by kalee20
You need an RC network with a gain >1 at a particular frequency
With passive components only? Doesn't that opposes the law of conservation of energy, or even more isn't that a "perpetum mobile"
No, you can have a voltage gain >1 with just RC components (though power gain is of course less than 1).

See photo below, which'll make things clearer. So with a cathode follower having a gain > 29/30, you can make it oscillate!
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Old 19th Nov 2018, 6:36 pm   #24
Miguel Lopez
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Default Re: Playing with valve oscillators

An update for today.

Both oscillators are already mounted and running. I will need to apply some decoupling as signal are appearing on each other.
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