[Heathkit] Suppressor Grid Bias

[Bob Macklin]

Just use a 6L6 in the AT-1 the way it was deigned of put it back in the
closet.

Even a Chinese or Russian 6L6 will work OK in the AT-1.

Bob Macklin
K5MYJ
Kent (Seattle), Wa,
"Real Radios Glow in the Dark"

----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <heathkit at mailman.qth.net>
Sent: Thursday, January 29, 2009 4:57 PM
Subject: [Heathkit] Suppressor Grid Bias


> 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
>
>
>
> _______________________________________________
> Heathkit Mailing List provided by QSL.net
> List Administrator: Duane Fischer W8DBF
> ** For Assistance: dfischer at usol.com **
> ----
> List Info: http://mailman.qth.net/mailman/listinfo/heathkit
> Mailman FAQ: http://mailman.qth.net/mmfaq.html
>