High voltage on patchcords?! was: RE: More tubes

Rene Schmitz uzs159 at uni-bonn.de
Thu Nov 25 11:09:30 CET 1999

At 11:12 24.11.99 +0100, Haible Juergen wrote:
>	>Patch cords are an issue I had not considered yet. 
>	>Converting all of the inputs and outputs to lower voltages 
>	>seems impractical to me, 
>Wouldn't dividing outputs down by 1:5 be safe enough, and
>still hot enough to drive the next tube stage ? (Thinking of 
>all these passive tone controls between the gain stages
>of guitar amps ...)

Safety first !!! Never run a hot signal thru a connector which you can
touch !! 
Some might say: "I'm carefull", but on that rare occasion, where you were
unattentive, it zaps you and possibly you get a heart attack. Its a flirt
with desaster!

In many multistage VT-amps the plate voltage *has* to be divided down. A
single gain stage might give you a gain of 50-80. But on the same time,
your peak to peak voltage level must be less than the cathode voltage.
Otherwise you will "forward bias" the grid ,makeing it act as a VT-diode.
The grid must usually be negative against the cathode. Running 60Vpp
levels, (on a assumed 1V input, g=60.) you would have to put the cathode on
at least 60V, when you don't want grid current. Usually the cathodes are
held only 1-2V above ground, the maximum input voltage swing is 1-2Vpp! In
many guitar (pre)amps, there is gainwise no necessity for two or three
stages, the extra tubes are used for the "loss" of the frequency control. A
single stage usually suffices to drive the poweramp. The unsymetrical
loading of the previous stage which occurs when the grid current sets in
(and the load impedance varies), may be a pleasant effect.

With just one triode we can amplify the signal back to a hot level. For
control voltages I propose to use a 0..10V range. Amplifing control
voltages to high level with semiconductors seems OK for me, where a linear
gain is needed. Or where the voltage must go near zero (ADSR...). There is
a nice circuit in Nationals AN-72 which can do that with 1/4 LM3900 and one
HV-transistor. The advantage is that using lower levels but slightly
increased complexity, makes it safe for you or anyone who plays with it.
And you can easily interface it to your existing synthesizers. 

Why draw artificial borders thru the circuit technology? Use whatever suits
your needs and tastes. Use of semiconductors within a tube circuit is not a

The whole point of my VTVCO is that the interface is safe, while providing
compatibility with existing circuits.


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