[R-390] Tube cutoff characteristics and AGC

[David Wise]

If I may append a comment to Don's wonderfully clear description,
a visible side effect of the 6AU6 is that the carrier meter
will read low.  This is because for all normal signals the AGC
line will hover around the narrow range of -4V to -6V.  Conversely,
if you make your system _more_ remote-cutoff than before, say by
replacing the 6DC6 with a 6BZ6, the meter will read abnormally
high, because to reduce the gain to the appropriate value, more
AGC voltage is needed.

Dave Wise

>-----Original Message-----
>From: r-390-bounces at mailman.qth.net 
>[mailto:r-390-bounces at mailman.qth.net] On Behalf Of 2002tii
>Sent: Sunday, June 05, 2011 3:15 PM
>To: R-390 at mailman.qth.net
>Subject: [R-390] Tube cutoff characteristics and AGC
>
>I wrote:
>
>>The 6BA6 is a "remote cutoff" pentode, meaning that it takes a
>>relatively large negative grid bias to cut off plate current.  The
>>6AU6 is a "sharp cutoff" pentode, meaning that its plate current cuts
>>off with a much smaller negative grid bias.  Remote cutoff tubes are
>>generally used in stages with AGC control, to provide a proper linear
>>AGC action.
>>
>>In any case, all of the tubes on a common AGC bus should have the
>>same cutoff characteristic, so the IF gain is distributed properly
>>between the stages.
>
>To elaborate a bit:

[great explanation snipped]