exponential VCO problems

[Paul Perry]

At 11:21 AM 14/07/97 -0700, Eric Barbour wrote:
>>You have built an expo converter without linearity problems
>>with tubes ?!?
>
>No, I didn't. As someone else pointed out, it is not possible
>with tubes. (Actually it is possible but very difficult--it 
>requires a special tube made by Raytheon in the 1950s, a 
>square-law tube called a QK329. Very rare today.)
>
>At no time did I apply all the tired old paradigms of solid-state
>synth design to my circuit. The VCOs use thyratron tubes, which

.......and at no time did he succeed in making a useful analog synthesiser,
if there is no high Q resonant filter. 
And if he can make an exponential convertor with only one QK329 I would be
amazed...a diode break point function generator would be more to the point
if one is being thermionic.
And if anyone wants to really build an analog synth  using valves, they
could do worse than consult the works of Graninio Korn on analog computation
ways and means. Of course it is possible to make a resonant filter from
valves, playing around with voltage controlled transconductances......

The whole enterprise reminds me of what that great Dead White Male, Dr
Samuel Johnston, said about a dog walking on its hind legs: it is not done
well, but one is amazed to see it done at all!

BTW, a very amusing way to make an arbitrary waveform generator or function
generator - and as analog as hell - is to cut out a piece of cardboard
shaped like the function and tape it to a oscilloscope screen with a
photodetector in front and then set up a feedback loop so as to follow the
edge.....

Paul Perry (melb aust)