1 V per octave Tube VCO

[René Schmitz]

At 16:17 27.12.00 +0100, Magnus Danielson wrote:
>Good work!
>
>Now, have you checked out Rene's tube expo curcuit? It would be
>interesting to see this curcuit in a all-tube setup. It should not be
>that hard to replace the op-amps with tubes. Naturally one has to
>calibrate the offset.
>
>Now, tapping out on thin ice here (yeah, the ice is thin in Stockholm,
>so I am not considering doing in real life just yeat), if the cathode
>on Rene's ECC83 does not have to see a low impedance, one could run a
>ECC83 as the input op-amp with a fairly high output swing, have that
>balance trimmed and then just use a resistor divider to divide down
>the swing and also the offset error. Maybe someone could enlight me
>what is seriously wrong with this plan.
>
>Anyway, that way you would get a 3/2 ECC83 input setup.
>
>Any takers? Eric? Rene?

Hi Magnus, and list!

Not that an all tube setup is impossible, but I think the sonic improvement
is rather low, plus even the simplest two transistor bipolar expo would
perhaps easily outperform this circuit in stability and tuning accuracy.

The tube expo is drifting along already, and using 1-2 tubes for a feedback
amp won't help on the stability either, considering the (comparatively)
small open loop gain that they will permit. Even if you trim your circuits
offsets, it will still drift with varying heater current (think of line
voltage variations for example). 

My personal opinion is that the control section can make good use of the
precision of semiconductor opamps, while the audio section takes advantage
of the sonic characteristics of tubes. 

But, I don't wan't to discourage you or anyone, to lay his hands on these
circuits and to squeeze more performance out of them. 

Bye,
 René 



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