[AMRadio] 100 V filament transformer primary

Rick Brashear rickbras at airmail.net
Tue Feb 20 16:11:36 EST 2007

Hi John,

I suspected as much as to why they used the 100 volt primaries.  I also 
figured they were saving money by using only one meter for all the 
filament monitoring.  That's risky business to say the least.  In the 
case of the BT-20-A I have it may have been a contributing factor in the 
demise of the low voltage supply filament transformer as well as the 
plate transformer.  I feel sure a very low filament voltage on a 5R4 
would not bode well for the tube or the plate transformer.  However, the 
fused circuitry likely protected it to some degree, but over time it may 
have been a partial cause.  One never knows for sure...

Thanks for the insight.


John Lyles wrote:

>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. 

More information about the AMRadio mailing list

This page last updated 17 Dec 2017.