CGS beam modulator

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The CGS beam modulator module is based on Eric Barbour's second beam tube modulator design, featuring a 6BN6 FM detector tube. In fact, I used a 4BN6, and as my filament supply was only 5.5 volts, I put three 1N4004 diodes in series with it to drop it down to the appropriate level.

Pretty much everything else was as per the circuit diagram, except for the inclusion power supply decoupling using a 680R resistor and a 100mfd capacitor.

The most fun part of building this module was with the physical construction of it. I tried to capture some of the feel of old equipment, even using an antique knob for the screen voltage. All the aluminum pieces were junk that was to hand. It is yet to receive a coat of wrinkle finish paint - I have to find some. Photos of this module are below.

I have also build a second one of these units that uses pentode pre-amplifiers for both the signal input, and to steer the screen voltage away from the value set by the screen pot.


Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.

See also


Beam Modulator by Ken Stone, 2001, with permission of the author - archived

External links