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Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Detailed valve data sheets often give either a working filament voltage range for normal operation, or % tolerance, which is often specified as 5%.
12V is within the 5% tolerance range for a 12.6V filament, so there should be no important change to the valve characteristic curves given in the datasheet when operating the valve with 5% reduction in heater voltage.
From a practical point of view, the filament of your military power pentode was designed to operate across the voltage range of lead-acid 12V accumulators with an open-circuit voltage of 2.10V x 6 = 12.6V volts, and a loaded full discharge voltage of 1.75V x 6 = 10.5V.
Registered Member #4603
Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159
Powering a GU81 filament directly from a variac is a stupid idea... No galvanic isolation from the net and nearly a short circuit when the filament is cold - your variac will not like it. You need a filament transformer.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
RateReducer: In my GU-81M VTTC the voltage on the filament is around 12.65 - 12.7V with nominal 230V in the line. The problem is the tolerance of the line voltage, it is very common to get 240V here. If I was to stabilise the filament voltage, I would dare to go higher... but I almost never do this :D
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Can I run the gu81m filament coil on 12 volts
