14-04-2021, 12:43 PM
Mike Watterson Wrote:They are though sharp cut off (apart from odd circuits for 1jxxa dual grid types), serious AGC needs an amp and drive to g2.
I'm looking at AGC with rod pentodes!
I'd discounted varying Vg2 for AVC purposes. True, it does control gain. But, reducing Vg2 reduces gain, and also reduces cut-off potential on g1. So signal handling is reduced as a result of the reduced grid base.
For AVC purposes, the output is relatively constant. It's the input which varies. When the input is large-amplitude, reducing Vg2 would reduce the input-handling capability, just when we really need it. With a strong signal from a modulated signal generator, I'm seeing the AM disappearing as I reduce gain with Vg2 - I have made a super limiter...
I have experimented with varying Vg3, finding (with 24V on g2) that gain reduces to zero as Vg3 is taken to -25V. But the down side is that anode resistance ra seems to drop too, meaning that damping on a tuned circuit load increases as gain is reduced - selectivity reduces.
Trials will continue over next few weeks, but I thought I would tap into any already-existing knowledge on here in the meantime!
