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KeithsTV · 14th May 2021, 8:59 am

According to the manufacturers service manual early versions had a single 33k resistor feeding the screen grids of both the DK92 and DF91 giving a voltage of 62V on both valves. Later versions separated the screen grid feeds with 180k for the DK92 and 56k for the DF91. The fact that you have different voltages on the screen grids indicates that your radio has the two separate resistors. Using the screen grid currents in the manual and your anode voltage measurements gives screen grid voltages of approximately 42V for the DK92 and 58V for the DF91 which are not far from your measurements. So nothing to worry about.

I suspect the manual was not updated following the change to the separate resistors.

Keith

14th May 2021, 8:59 am	#6
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,573	Re: Bush BAC31 mains unit According to the manufacturers service manual early versions had a single 33k resistor feeding the screen grids of both the DK92 and DF91 giving a voltage of 62V on both valves. Later versions separated the screen grid feeds with 180k for the DK92 and 56k for the DF91. The fact that you have different voltages on the screen grids indicates that your radio has the two separate resistors. Using the screen grid currents in the manual and your anode voltage measurements gives screen grid voltages of approximately 42V for the DK92 and 58V for the DF91 which are not far from your measurements. So nothing to worry about. I suspect the manual was not updated following the change to the separate resistors. Keith