Tube DIY Asylum: RE: Michael, I missed the 100R individual cathode R's... by mjk Do It Yourself (DIY) paradise for tube and SET project builders.

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Posted by mjk (M ) on June 1, 2008 at 14:45:34

In Reply to: Michael, I missed the 100R individual cathode R's... posted by Allen Wright on June 1, 2008 at 10:39:52:

Yes, 100R is too big as it effectively adds 100 * mu to the Rp, in this case about 400 ohms! I can only tolerate that because my effective primary impedance is over 12K a-a so I still get a damping factor of 5. Anyway, it's only temporary so I can get good measurements of the AC current to help me understand the operation of the servo controller.

I would definitely like to compare notes. I presented this on a poster at VSAC and showed the prototype controller board installed in these amps. I made some measurements and observations, and am now getting more baseline data whilst working on a revision of the controller.

The basic idea is the small common cathode resistor R5 senses the cathode current for both tubes, which reaches a minima on each zero crossing of the push-pull output stage. This is equal to the zero signal current. the negative peak detector IC1A, D1, and C1 follows this zero signal current continuously. IC1B compares the zero signal current with a set point from R9 and outputs a control voltage to the summing amplifiers IC2A and IC2B. The cathode voltage is returned to virtual-earth connections at the inputs of the summing amplifiers and is thus kept constant at a level servo-controlled by the zero-signal current. IC3 sences unbalanced current between the cathodes and applies a corrective offset to the voltage regulator.

This circuit is a very naive implementation of the idea for testing and still needs a lot of refinement. It basically works but becomes unstable at about 1/4 full power. I am trying to learn more about the nature of the cathode load using the current resistor bias network.

Cheers,

Michael

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