[R-390] Tube cutoff characteristics and AGC

[2002tii]

Drew wrote:

>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.

I've never been particularly impressed with Heath's engineering apart 
from the old VTVMs and a very small handful of other products, and I 
always thought the performance of the old transceivers left a lot to 
be desired -- so I hesitate to speculate why they might have chosen 
as they did in this case.  6AU6s were always the least expensive of 
the 7-pin pentodes because they were the most common, so that could 
be the answer.

Dave pointed out that resistance in the screen circuit tends to make 
the cutoff more remote.  I'm not sure I'd phrase it quite that way, 
because unless you start with a screen voltage much higher than 
normal (which would be hard to do, because the screens of 
small-signal pentodes are generally run from the plate supply), it 
wouldn't actually move the cutoff point more negative -- but it does 
tend to reduce the change in transconductance with grid voltage by 
making it harder to turn the tube on fully.  More like compression at 
the top than extension at the bottom.  Anyway, the Heath circuit uses 
significant resistance in the screen circuits, so that may reduce the 
bad behavior of the sharp cutoff tubes to some extent.

Best regards,

Don


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