Andrew Holme wrote:
I'm wondering if they're actually common-grid oscillators relying on
anode-to-cathode parasitic capacitance. This circuit oscillates:

http://www.holmea.demon.co.uk/temp/osc2.gif

A very perplexing circuit. I'm not sure the original author of this
circuit completely understood how it actually worked either! There are
at least two plausible mechanisms for the RF feedback, the plate-cathode
feedback from inter-element capacitances, or plate-grid feedback from
the tank itself through the unmarked capacitor to the grid.

I think the feedback mainly comes from the capacitor to the grid from
other side of the tank. (ie a Hartley oscillator) That cap is just a DC
blocker I believe. You could probably replace V1 with a JFET and the
circuit would work fine without a drain-source feedback capacitor.

The RFC in the cathode circuit is to prevent C1 from shunting the RF on
the cathode. This makes the plate-cathode feedback more likely,
especially as the plate circuit is floated by its own RFC. I have no
idea of the properties of a CV6 tube so I can only guess what is really
going on.

The cathode circuit looks very much like it is a self-quenching circuit,
but the diagram below suggests it is separately quenched? This is an
IFF transponder? The feedback loop is interesting, looks like reflexing
to me. I'd guess the entire circuit oscillates at the quench frequency
and the modulation is recovered by filtering that output?

I've always wanted to build a toy walkie talkie from a similar circuit.
Super-regen on RX and AM on TX by changing the source and drain
circuits on T/R.

--
Alan Yates
http://www.vk2zay.net/
The Moon is Waning Crescent (17% of Full)