From Metasonix Re: [sdiy] Tetrode VCA?

[Cynthia Webster]

Hi Gang!

Eric is off list at the moment and asked me to forward this response to
Scott re Tetrode thoughts...  Cynthia

Please see below:
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on 2/5/03 10:43 AM, Scott Evans, Gen Mgr at esresource at earthlink.net wrote:

> Years ago, when studying electronics in college, we still learned vacuum
> tube theory (yeah, it has been a while). Unfortunately, I did not keep
> my textbooks on tube technology :(
> 
> I recall that the tetrode tube exhibited interesting non-linearity when
> used as an amplifier stage. A tetrode has four electrodes, a cathode,
> control grid, screen grid, and anode. It also had, as I recall, a gain
> curve that actually inverted to produce negative gain and then inverted
> again to normal gain sort of like a backwards "S" on it's side. This
> led, of course, to the pentode type tube to correct these non-linear
> gain problems.
> 
> I thought this could make an interesting VCA for producing distorted
> waveforms. Has anyone in the group tried building something of this
> sort? Are there even any of these devices still built? Just a thought.
> 
> Scott
> 
> From: "Scott Evans, Gen Mgr" <esresource at earthlink.net>
> Date: Wed, 05 Feb 2003 10:43:31 -0800
> To: DIY <synth-diy at dropmix.xs4all.nl>
> Subject: [sdiy] Tetrode VCA?
> I thought this could make an interesting VCA for producing distorted
> waveforms. Has anyone in the group tried building something of this
> sort? Are there even any of these devices still built? Just a thought.

~~~~~~~~~From Eric at Metasonix~~~~~~~~
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(Cynthia, I am off the SynthDIY list due to time shortage.
If you could respond to Scott Evans on DIY, I would
appreciate it. )

Tell them (a) nobody makes tetrodes anymore and never will again,
(b) yes, the tetrode has unique distortion because of the
secondary plate emission producing an S-shaped plate curve
(which the pentode eliminates with the suppressor grid),
(c) Metasonix uses 6EA5/EV5 tetrodes in the TM-1, TM-2 and TM-5,
(d) they are ridiculously cheap in NOS form, I buy hundreds of
them every year from tube dealers who are happy to see them go.

Sometimes Antique Electronic Supply sells them in their annual tube
clearance sales for 50 cents apiece. This is a great way to
get all kinds of "junk" TV tubes. Even though they are out of
production, they are so damn abundant that antique radio people
give me piles of them routinely, rather than toss them out.
I once figured that given the old inventory of such tubes at major
distributors, I can manufacture Metasonix modules at the present rate
for the next 1000 years or more, without exhausting the tube supply....

Eric Barbour
Owner, METASONIX (www.metasonix.com)
Senior Editor, Vacuum Tube Valley magazine