[R-390] Tube cutoff characteristics and AGC

[Drew P.]

Hi Don,

Thank you for the description of remote vs sharp cutoff characteristics and the effect upon AGC operation.

It is interesting to note that the Heathkit SB-100/101/102/HW-101 series uses sharp cutoff pentodes as AGC controlled RF and IF amplifiers (6AU6 and 6HS6). I believe (but am not certain) that this was done to provide a bit of AGC delay, and a very aggressive AGC response once the AGC voltage reaches the threshold.  Also, the 6AU6 seems to be run at about half the anode current of the 6BA6 - may have been selected by Heath for power supply considerations.

Care to comment?

Drew