Spam (madbodger) wrote,

ONOZ geek attack!

Someone asked in a vacuum tube group:

I am looking for an oscillator, like the Blocking Oscillator that can be externally sync'ed up. However, I need the extra function of having the oscillator sync up to a reference whether the oscillator is going slightly too fast as well as slightly too slow. Does such a beast exist in the tube world?

I pointed out that the colour oscillator in tube televisions had been doing that trick for many moons.

But I couldn't stop there:

Here's a quick description of how the usual color burst oscillator works. If you have a schematic of a color oscillator handy, this is a lot easier to follow.

The phase detector is the reference frequency (gated by a keying pulse in the case of color burst) driving a pair of matched diodes (6JU8, 6AL5, etc. or even crystal diodes) connected in series (anode to cathode). The output of the oscillator goes to the junction of the two diodes. The ends of the diodes, which are connected to the reference signal, are also connected to a pair of matched resistors. The junction of the resistors provides a control voltage, zero for in- phase, and plus or minus a few volts for out of phase. This is filtered and goes to the grid of the reactance tube, which has something like an LC tank to make its plate voltage and current out of phase (so it looks like a reactance). By changing the grid voltage on the tube, the conductance (and hence the effect of the reactance) varies. This variable reactance is in parallel with the main frequency determining component (a crystal in the case of a color oscillator, LC in other cases), serving to pull the resonant frequency up or down. A popular tube is a 6GH8, with the triode serving as the reactance tube, and the pentode as the oscillator.
Tags: electronics
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