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MYSTERY SOLVED! thanks all...

HI,

The mystery is solved.

When adjusting the heater voltage (to be 6.3V), I wrongly measured between + and ground (since this was easier to do).

Of course I need to measure between + and the top of the filament bias resistor.

Now the filaments where at 4.2 volts (6.3 - 2.1 bias) .

So less electrons emitted as designed, so less current measured.

I needed to raise the voltage to 8.8 volts to get 6.3 volts between both legs of the heater (note, the WE437A stage has a 5.6R filament resistor, not the 3R resistor shown).

Now it makes more sense.

VCCS out = 8.8V
Bias = -2.5V
Heater voltage is 6.3V

B+ = 185V
Va = 160V

So, 25V drop over 1385 ohms is 17mA

Eyeballing the datasheet I should get over 20mA, but its close enough.

NC

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