[Heathkit] Suppressor Grid Bias

[Drew P.]

Hello all,

I've been communicating with Chris about his AT-1 off list and I think 
that the material might be useful to others, and so Chris has given me
permission to repeat it here.

> I wonder if anyone's tried an EL34 in place of the
> 6L6 in an AT-1 ?
> 
> Here's specs & connections for each:
> http://www.mclink.it/com/audiomatica/tubes/6l6.htm
> http://www.mclink.it/com/audiomatica/tubes/el34.htm
> 
> I'd be inclined to not connect pin 1 of the EL34
> directly to the cathode, rather, put a little resistance to
> it to make the supressor less negative than
> the cathode.    Maybe use a pot to control it ...  ??"

You'd have to be careful with the dissipation rating of the pot...
you might overload it and smoke it...and we all know that (snicker) 
"smoking pot" is illegal! 

Seriously, adding resistance in series with the suppressor grid 
could cause the final to become unstable and go into oscillation.
I have a Hallicrafters S-120 that uses this technique in the IF
amplifier stage with a front panel mounted pot labelled "BFO".
Increasing the resistance first increases the gain and selectivity
via regeneration, presumably from reduced shielding by the suppressor
grid. Further advancing the "BFO" pot causes the IF amplifier to 
oscillate, giving a BFO function of sorts.

The S-120 is an absolute minimalist type of radio, it is an "All
American 4" (12BE6/12BA6/12AV6/50C5, selenium and a power resistor
in place of the 35W4) and so certain design compromises were made.

Maybe if you do elect to insert resistance in series with the EL34's
suppressor you might also connect a bypass cap to ground.

I think that the EL-34 would work without changes to the AT-1's 
suppressor grid circuitry.  If you feel inclined to tinker, you might
try different values of screen voltage (dropping resistor?) and 
control grid bias (grid leak resistor and/or fixed bias in series with
the grid leak resistor).

Drew