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Thomas is correct. One end of your filament is at 5V of bias, and the other end is at 12.5V of bias. The average bias of the filament is 8.75V of bias.
Your voltage measurements agree with this.
I guess I am too thick to understand this. All I know is that what I measure agrees with Ale Moglia's plate curves and not with the RCA data sheet plate curves. And what the heck is "average filament bias"? Sorry to have wasted everyone's time.
Imagine that you set the tube up with a hum pot and put the cathode resistor between the center lug of the hum pot and ground. Let's use your case as the example, then you'd have 5V at the center lug of your hum pot. Assuming you're using a DC filament supply, you'll have 5V + (7.5V/2) on one end of the filament and 5V - (7.5V/2) on the other end, so the average bias is 5V.
This is part of the design process of filament bias, since there's no hum pot and bias isn't applied to a virtual center tap.
Not to be dismissive but -5v filament bias from a 4 ohm resistor and 1.25A of current gets me 10 ma current and 170 volts plate just as Ale Moglia's plate curves allow me to predict. For me it works and all this discussion about average bias I find difficult to understand. But thanks for taking the time to try and explain. Perhaps I should just buy a nice transistor amp, which I understand even less ;)
Ale probably referred the bias to the negative end of the DC filaments. That's why they exactly fit your application. But in any case, never expect every tube to behave exactly the same as the plate curves in the data sheet are only typical and tubes can vary considerably from sample to sample even when new.
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