Hello guys.
I'm building an amplifer with 2 GU81M russian pentodes,
and I'm planning the tank circuit. The GU81M's has very high output capacitance, thus, I would like to use a roller coil in conjuction with a vacuum cap with low min capacitance to be able to tune the higher bands. Can get hold of very heavy duty russian 3.7uH roller coils, and also 5-250pf vacuum caps.
On my last amp (2x813) I had some issues with the plate choke, so this time, I want to try another approach, that being to feed DC at the 50ohm reference point in the tank circuit instead of at the anodes.
I have seen schemes of it being done succesfully, by inserting a teflon insulated magnet wire, through the tank coil, keeping the dc blocking cap at the normal place, and getting by using a small choke at the feedpoint.
If I had used other tubes, I would definately have done just that, and wound the tank coils using brake line for the low band coil, and something larger for the hiband coil, with a wire inserted before winding.
Now I'm thinking about still using the coaxial feed for the lowband coil using brake line, and putting the 3.7 uh roller coil and vacuum cap at DC potential, with a blocking cap between the roller coil and the low band coil, see attached picture.
I will be housing this in a large compartment, capacitors and rollers will be motor driven.
Will the dc component on the vvc be of any trouble?
Here it is described by john fielding, scroll down to 'A better anode feed method'
https://ogrik2.ru/b/john-fielding/valve-amplifiers-explained/30339/8-designing-an-anode-tank-circuit/9Also included a sketch of how SM6APQ have done on multiple amps,using copper pipe.