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PJL · 12th Jul 2006, 6:49 pm

Can someone help me out here as I could be wrong but it strikes me that alot of TRF's work OK without the AF stage so I have a theory!

The 6J5 seems to be incorrectly biased. The characteritics says grid should typically be -8V which would imply 2mA anode current from the cathode bias but 2mA across 220K = 440V so it is operating in that not too happy saturated region below the characteristic curve which could explain why the gain is low.

You can check the theory by looking at the anode volts which should be 100-150V to get a reasonable output. If it is low then you would need to increase the 3.9K bias resistor (and decoupler) to get it into a sensible working region.

I am no expert but I am not sure the choice of valve was right for a high gain AF stage.

12th Jul 2006, 6:49 pm	#12
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Regarding home made TRF Can someone help me out here as I could be wrong but it strikes me that alot of TRF's work OK without the AF stage so I have a theory! The 6J5 seems to be incorrectly biased. The characteritics says grid should typically be -8V which would imply 2mA anode current from the cathode bias but 2mA across 220K = 440V so it is operating in that not too happy saturated region below the characteristic curve which could explain why the gain is low. You can check the theory by looking at the anode volts which should be 100-150V to get a reasonable output. If it is low then you would need to increase the 3.9K bias resistor (and decoupler) to get it into a sensible working region. I am no expert but I am not sure the choice of valve was right for a high gain AF stage.