Vacuum tube VCO

[Bitrex]

Hi all!

I am experimenting with a vacuum tube VCO similar to the one in the link below, but with a few modifications to accommodate the parts I had on hand; I'm using a cascode arrangement instead of the 6AU6 and a 2D21 thyratron in lieu of the 5696.

http://www.cgs.synth.net/modules/pic/tubevco.jpg

The circuit really does produce a nice sawtooth wave that tracks well, however I am a bit disappointed by the tuning range, particularly on the low end. The lowest frequency I can coax out of the circuit is about 250 Hz, and increasing the size of the capacitor across the thyratron doesn't seem to make any difference. The ECC88 grounded-grid amp seems to drop completely out of conduction and oscillation stops when the tube current tries to go below about 25 microamps.

Does anyone have any advice on how I might improve the frequency performance? Thanks.

[neazoi]

I do not know what could cause the problem.
I have the first circuit on this page K.Giannopoulos official site and I was able to extend the range to the very low ends as well to the highs by adjusting the 100k pot as well as the CV in.
If you build just one vco there may be a problem, as you cannot re-set the 100k as you play the notes. In my case I am building the second schematic with 60 such vcos for a 60-poly so each vco is dedicated to a key, thus independently tuned on both cv in and 100k pot.