AW: exponential VCO problems

[Paul Perry]

At 09:15 AM 16/07/97 +0200, HJ wrote:

>How do you build a tube opamp?

The level shifting is normally done by resistor networks - there is always a
high voltage negative rail in a valve analog computer system - sometimes
aided by neon constant voltage tubes (from memory 0B2 - the 0 probably means
zero heater votage)

it is possible to make a basic differential input op amp from a twin triode
long tail pair input stage and a pentode output, with carefully adjusted
positive feedback to get the gain tweaked up to something like 20,000 - but
it is nothing as good as the first 709, let alone the 741!

I have seen a 100,000 gain dc valve op amp that worked by modulating a valve
chopper and then consisted of a string of rf amplifiers.....this was the
size of a normal valve cro and had a bandwidth of 100KHz

>	>more if tubes are used as voltage-variable transconductances.
>	>This
>	>is workable

a lucky guess on my part!

>I have no idea how to do this. 
neither do I!, but I think  pairs of variable mu pentodes as in a ladder
filter, or 
for a non-resonant filter, I would modulate an rf signal and change the
carrier frequency to lose more or less of the sideband to a rectangular
tuned circuit....there are various reactance modulation schemes as were used
in the old rf sweep test generators

for anyone wanting to look at this seriously, a good book is Analogue
computing at Ultra-high Speed D.M.MacKay and M.E.Fisher

some general analog tips but no valves in Korn & Korn electronic Analog and
Hybrid Computers

BTW, sometimes the valve heaters are run on rectified dc with a 6v car
battery floating across it.....apparently this gives a more stable heater
supply (valve synth tip of the day)

Paul Perry Melb Aust