Tube DIY Asylum

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RE: This is similar to the old Berning TF-10

Posted by Ralph on August 8, 2017 at 13:42:54:
I don't, but FWIW your circuit is nearly identical.

The second order filter is great, and IME you don't need such a large grid bias resistor to make it work. Bypassing the output of the IC is effective though, if you are worried about 'solid state contamination' though :)

You are also better off with really slow timing constants in the servo. You really are only compensating for drift in the tube once the whole thing warms up. So there is no need for the servo to have any speed to it.

I recommend an additional circuit that lights an LED or the like, in case the tube is unresponsive to the servo. Very handy for detecting a bad tube and you don't want DC at the output in any event.