[Amps] 3-1000Z Followup

[Dennis Sokol]

Thanks to all who provided input re my 3-1000Z filament voltage question.

I did manage to increase the filament voltage at the tube pins of the BTI LK-2000 by .5 volt by using the boost winding on the filament xformer and then using a dropping resistor to get about 7.25 at the tube with full load. The effect on power output was a gain of 50 watts to 1300 indicated. This is about 60.5% efficiency so raising the voltagefrom 7.08 to 7.25 did not do much.

The real effect comes, of course, from boosting plate voltage ideally to 3500 volts or more. I am working to reduce the voltage drop under load which ranges from 2825 to 2910 volts @ 750 ma.

Clearly, from the inputs, there is a great advantage in tube life by having some way to adjust the filament voltage in your amplifier.
Homebuilders can easily do this with a variable resistor in the primary of the filament transformer. It would be great if the commercial manufacturers would include some way of measuring the filament voltage so a person would know whether they are cooking their expensive tubes to death. On the other hand, maybe they don't want youy to know!

73, Dennis W0JX