EF86 to cathode follower question on new build...

[redtoploader]

Hello everyone! first post here, and I have a question on a build i'm thinking of doing!

I have a nice, almost new british military ef86 lying around, and i'd like to try it in a new build. I'd like to use it as the input tube, and then buffer it with a cathode follower with 1/2 a 12a*7(probably an ay7)...then send it to a tone stack, then to the other half of the 12a*7 for recovery, then to a volume pot. All this goes to a LT pail PI.

I've never used a cathode follower as a buffer stage before, and i've never put the volume pot in this spot. Would you use a coupling cap from the ef86 plate to the input grid of the CF? Also, would you use a 1M resistor from the CF grid to the ground?

[oldhousescott]

First of all, welcome!

I think I would go:

EF86 --> CF --> tone stack --> volume pot --> recovery stage

This will give you a chance to keep from overloading the recovery stage and also save you a coupling cap.

As long as the EF86 is biased at a reasonable point (plate at about 1/3 to 1/2 of B+ for that node), you should be able to direct couple the plate of the EF86 to the grid of the CF and let it bias the CF. Use 56k to 100k in the cathode. No need for a grid to ground resistor on the CF.

[redtoploader]

Thanks Oldhouse!!

I just read some more about cathode followers on valve wizard...and 56k is what I came up with for Rk for the formula provided....b+ will be between 300-320.

I'm also going to try to get the screens on the ef86 about 30% or so lower than Va.

[redtoploader]

EDIT: make that 290v for B+