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Subject: Re: understanding tube amplifier schematics

Date: 02/05/01 at 9:03 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Re: understanding tube amplifier schematics posted by Lee Vander Boegh on 02/02/01 at 1:34 AM:

Hi, Lee.

It was only one vacuum tube instrument, the Bertan HVL-25 Electronic Load
which is shown at http://www.bertan.com/hvl-25.html

I don't have its schematic, but the situation was as follows. Two 6EN4 color
TV shunt regulator triodes were used for service at voltages up to 25 kV and
full-scale currents in three ranges of zero to 3 mA, zero to 300 ľA and zero
to 30 ľA.

The problem I had was that the 3 mA and 300 ľA ranges worked fine, but the
tubes couldn't be given enough negative grid bias to bring their total
plate current down below 20 ľA on the 30 ľA range. I solved the probem by cooling
the filaments for that range by adding a series resistor to the paralleled
filament pair. I think it was 500 ohms at 20 watts, but I'm not quite
sure of that.

In any event, the tubes were run at their nominal 6.3 volts of filament
voltage on two of the ranges and at less on the third.

As far as amplifier schematics, I don't know what to suggest. I'm not an
audio enthusiast by any means, but try looking at http://headwize.com/

Those folks seem to have a lot of information about vacuum tube amplifiers.

Good luck.

John Dunn - President
Ambertec, Inc.

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