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Old 24th Feb 2021, 9:57 pm   #19
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: MK.123 spyset BFO fault.

One other thing to note is is that if C48 has failed o/c then I think the circuit would have lost some benefits of being a pentode. I think the screen grid g2 needs to be decoupled by a capacitor at RF because otherwise the valve will begin to take on some of the less attractive features of a triode. This includes an increase in the effective feedback capacitance to the grid if the (non-decoupled) screen grid can also swing at RF in a similar way to the anode. This increase in feedback would make Miller Effect more significant and this could degrade the starting gain as well. This might be partly why it starts up better with your new 1nF padding cap at g2.
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