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G6Tanuki · 19th Mar 2018, 7:08 pm

Disconnect one end of C49 then check for voltage on the grid of V6 (put the meter across R28).

If V6 has internal leakage [not unknown] then even with a good C49 you can get an unwanted voltage appearing on the grid, which will cause excessive current to flow through the valve (and so an excessive voltage across R33).

Check that R28 hasn't drifted high in value: the "Erie" ceramic-bodied resistors with the ends plugged by a brownish putty/cement are known to drift - your V6 grid-to-earth resistor could well have wandered up to a couple of Megohms!

UL84 valves are known to sometimes have this leakage issue, though they're nowhere as bad as their earlier EL41/UL41 relatives.

It's possible - in various ways - to 'zap' a leaky valve to try and destroy the conductive deposits that cause this leakage.

19th Mar 2018, 7:08 pm	#109
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	Re: Bush VHF 81 Restoration Disconnect one end of C49 then check for voltage on the grid of V6 (put the meter across R28). If V6 has internal leakage [not unknown] then even with a good C49 you can get an unwanted voltage appearing on the grid, which will cause excessive current to flow through the valve (and so an excessive voltage across R33). Check that R28 hasn't drifted high in value: the "Erie" ceramic-bodied resistors with the ends plugged by a brownish putty/cement are known to drift - your V6 grid-to-earth resistor could well have wandered up to a couple of Megohms! UL84 valves are known to sometimes have this leakage issue, though they're nowhere as bad as their earlier EL41/UL41 relatives. It's possible - in various ways - to 'zap' a leaky valve to try and destroy the conductive deposits that cause this leakage.