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Old 30th Jul 2018, 9:52 am   #18
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: Bestone TRF - power up and audio stage testing

There are 2 types of early detectors:
Leaky grid - The cathode is wired to chassis and the grid wired to chassis via a grid leak resistor with capacitor coupling for the AC signal. This uses the grid to rectify the incoming AC signal. Larger RF signals will push the average grid voltage lower causing a change in average anode volts. Their main disadvantage is their low input impedance which affects the Q of the tuned circuit feeding it.

Anode bend - A correctly biased valve amplifies the input on the grid linearly, however, if the anode current is reduced to near cut-off, the transfer characteristics become non-linear. By biasing the valve in the non-linear area, the +ve half cycles of the input will be amplified more than the -ve and will contribute more to the average anode current. If the amplitude of the RF signal changes there will be therefore be a shift in the average anode current. If this were a triode, the bias is set to the non-linear area solely using a very high cathode resistor but as a pentode the anode current can also be controlled from the screen grid hence the 2M screen feed.

Both create quite a bit of distortion and I assume the bias and screen feed resistor values for a pentode would have been chosen by trial and error.

Last edited by PJL; 30th Jul 2018 at 9:57 am.
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