Tube expo convertor.

[René Schmitz]

At 01:00 15.12.00 -0800, Eric wrote:

>You clever lad! 
;-)

>This is caused by high-energy electrons,
>by secondary emission electrons and a little by ionized gas.

Tubes with bad vacuum would not be very accurate, the result would 
be a kink in the response. I doubt that second emission is playing 
a major role here since only electrons of more than 10eV can do that, 
which are very rare here. 

>I built a tester for large transmitting tubes last year,
>to check for gas by doing something similar.

This circuit started as a test circuit either. 

>Wonder how stable this is. It would vary a lot with
>different brands and samples of a given kind of tube.

Right, it won't beat records in stability. 
The maximum plate current (@ Uak =0) varied between different 
types, and even different samples of the same type. Also it varies 
with the heater current. 
Whats interesting to see is that the situation here is essentially 
the same as in the bipolar expo circuit, the operation depends on 
temperature in the same way. As if physics doesn't allow an exponential 
response without temperature influence.

>Think I'll keep using V/Hz on my synths. There's no problem getting
>MIDI-CVs that will do it. And it's very stable.

I'll stick with my method of interfacing a regular bipolar expo 
circuit to a tube oscillator. I've build a quad VCO using that method 
and so far it works quite well, though I used a DIAC as the discharge 
switch so purists might say I'm cheating. ;-)

Bye,
 René


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159