AIUI the 1GHz & 3GHz circuitry was entirely passive. Perhaps they could have used much the same structure from that CRT in an amplifying valve, maybe with a tiny anode far away.
NT
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9 years ago
Interesting question/rant? from EEVblog Dave
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9 years ago
Nothing like a bit of nostalgia to brighten your day. Thanks!
I still can't see why they need a 4CX250 transmitting tube in V331. The voltages and currents don't seem to be that high. Maybe it's the low capacitances and wideband operation into UHF region. But it's the first time I have ever seen that done.
Another amusing thing is the 47 ohm or 100 ohm parasitic resistors in virtually every grid of the 6DJ8's, except V223 and maybe one or two others.
The idea is the piece of wire connected to the grid makes a Colpitts or Clapp oscillator when combined with the input capacity and the capacitance from cathode to ground. The Colpitts and Clapp are perhaps the world's most vigorous oscillators - if given half a chance, they will merrily sing at vhf or uhf frequencies.
The oscillation may cease when a scope probe is placed on a node, or it may only start at certain portions of a waveform when the gain and currents are right. Usually the frequency is high enough so the circuit is sensitive to hand effects when it is waved nearby.
The resistor in the grid is in series with the tank and kills the Q so the oscillations can't start. The same thing happens with transistors, and placing the resistor at the base has the same effect. I noticed that some people put the resistor in the emitter. This will certainly kill the oscillations if the resistor is large enough, but I wonder if a smaller resistor in the base would also work and have much less effect on the gain.