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retailer · 26th Nov 2019, 1:31 pm

I did not really give it much thought but as Argus pointed out the preamp valves are indirectly heated so it shouldn't make any difference which way around the filament wires are connected and yes H/K leakage does cause hum - I have a few shoe boxes of ECC83 preamp valves and simply grabbed a hand full and plugged them in without testing any - on powering up the amp for a sound trial I noticed a slight hum that got louder with each minute - replacing the first preamp valve cured the hum - on the valve tester the suspect valve came up with around 1/2 meg H/K resistance slowly decreasing as the valve warmed up.

If I wind a mains transformer for an amp build I always (if possible) use a 3.4V bifilar wind for the filament supply - that way I'm sure of an exact centre tap.

26th Nov 2019, 1:31 pm	#6
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: Filament wiring 'Phasing' I did not really give it much thought but as Argus pointed out the preamp valves are indirectly heated so it shouldn't make any difference which way around the filament wires are connected and yes H/K leakage does cause hum - I have a few shoe boxes of ECC83 preamp valves and simply grabbed a hand full and plugged them in without testing any - on powering up the amp for a sound trial I noticed a slight hum that got louder with each minute - replacing the first preamp valve cured the hum - on the valve tester the suspect valve came up with around 1/2 meg H/K resistance slowly decreasing as the valve warmed up. If I wind a mains transformer for an amp build I always (if possible) use a 3.4V bifilar wind for the filament supply - that way I'm sure of an exact centre tap.