[AMRadio] 100 V filament transformer primary

John Lyles jtml at losalamos.com
Tue Feb 20 14:30:06 EST 2007

The reason that manufacturers install 100 V primary transformers, then add a rheostat in front of it, is to allow some lattitude to adjust a filament +/- around the nominal value. When the tube is old and emission starved, the filament primary can be jacked up a few % this way, and the tube continues to play until the next downtime. If a 120 VAC primary were used, there would be NO headroom to boost the voltage higher, only lower. Makes sense. 

Metering the filament via the primary AC power after the rheostat, is a bit cheap, but in this case (GE) it gave one meter which would globally be responsive to the filament setting. Its up to the engineer to correlate the reading from that AC meter to the individual socket voltages. Don't forget to use either a true RMS meter or iron vane movement when setting filaments on tubes to the correct voltage. 


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