[Boatanchors] Tube reciever design banter

[Drew P.]

On characteristics of the pentode section of the 6MU8 pertaining to AGC control, Carl wrote:

"The full RCA spec sheet has it as Sharp along with the curves. Its dead in
the water at -9V which doesnt give a good AGC window without getting fancy."

Interesting...using a sharp cutoff pentode as a gain-controlled stage.  I've wondered about Heath's choice of the sharp cutoff 6AU6 as RF and IF for the SB/HW transceivers.  At first glance, seems like this shouldn't work very well, but in actuality it does.  You mention an AGC window - this would occur just as the tube is reaching cutoff, where the typical sharp cutoff pentode really isn't so sharp.  There could be a quite substantial AGC voltage which, up until that near-cutoff region of the transconductance curve, would not vary the gain of the controlled stage at all.  Then, within that very small region where the curve bends over, there would be a large change in stage gain for a small change in AGC voltage. Would I be correct in surmising that such application of a sharp cutoff pentode is to obtain AGC delay?

Drew