[patrick-turner]

Last night I said I had a plan in mind for dc current control of the lamp resistance.............

"""Then I built a high input Z rectifier to produce a max +Vdc of +3Vdc. This is applied to a base of a common emitter stage which inverts the at the collector output so that a Vdc can be derived to power a BC640 bjt with Re 180r so that as it turns on it feeds collector current up to 22mA through 4 series lams each rated for 12V x 50mA.
Suitable R + electro Cs slow down any change in lamp DC current and its the DC though the lamps that causes the most change in their resistance, which varies between 27r cold to up 115r with about 22mA.

I have not tried my 3 bjt Vo regulator yet, but it does seem to behave far better than the j-fet, and up to a slightly higher Vo.

The ß just goes over 0.33 at 3.5Vrms input to my circuit.
But at turn on, ß = 0.1, so oscillations will be max and distorted until lamps warm up SLOWLY.

This NFB is a much lower resistance circuit so I expect no troubles getting all F between 100kHz and 1MHz

I should get far lower THD, because the VNFB produced has no visible THD on my CRO, unlike the j-fet which produced several % 2H.""

This is all very well, and a fabulous plan and while there wasn't any oscilator amp hooked up, the experiment showed it might work, but unfortunately, time constants got me yet again when trying to implement a circuit with long time constants into another amp circuit ( the oscillator amp ) with very high open loop gain, even at near DC.
The result was that the Vo slowly waved up and down like very slow amplitude modulation because the DC servo circuit was acting as a LF oscilator to vary the lamp resistance.
Back to the drawing board. I doubt if any simulation program would tell me it won't work before I built it because I doubt anyobe would know how to enter in so many interactive variables.

But at least it works with a j-fet, except for the highish THD. I will reserach a little more, before returing to the j-fet idea.

Sorry to bore you all to tears. Its OK, but wenya tryta build sumping good, with high aims in mind, then a special Law of Life applies :- COWPAT = 1 / N squared, where COWPAT = Chance Of Working Perfectly Any Time and N is the number of things you didn't know might stop it working, or what you didn't know, or you dropped a spanner, or the dog ate your data notebooks :-)
Patrick Turner