[R-390] Tube cutoff characteristics and AGC

[David Wise]

Note that AGC characteristics are greatly influenced by
the screen grid supply.  The datasheet curves were taken with a constant
screen voltage.  If you add series resistance (so the screen voltage
increases with decreasing current and vice versa), the effective
cutoff becomes more remote.  Hammarlund fussed around with
this in the SP600. 

Dave Wise

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of Drew P.
>Sent: Monday, June 06, 2011 9:51 PM
>To: r-390 at mailman.qth.net
>Subject: Re: [R-390] Tube cutoff characteristics and AGC
>
>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
>
>______________________________________________________________
>R-390 mailing list
>Home: http://mailman.qth.net/mailman/listinfo/r-390
>Help: http://mailman.qth.net/mmfaq.htm
>Post: mailto:R-390 at mailman.qth.net
>
>This list hosted by: http://www.qsl.net
>Please help support this email list: http://www.qsl.net/donate.html
>