Johan Potgieter fra diyaudio skrev her en dag:
A few months ago I tried operating ECC88s with series heaters, dc, higher voltage, better smoothing in the power supply and all that. Did I get a shock! (No, not electrically from the 48V odd). Some lit up like a christmas tree while others were dull. Dashing for the turn-off switch, I went back to data: 300mA acording to some, 365mA (!) according to others. So I measured the heater current for some 8 valves at piously 6.30V on the heater. The currents varied from 290mA to 420mA! - some of the same make. I was forced to shelve my efficient series design and go to a rather hot 6,3V output supply.
I have some 25 x 6L6GCs. I dread doing the same measurements on those - not that I would use series heaters there. But wth is going on here with quality control? Or has it been like that all the time (for me that will be over 50 years)? One never measured heater current; the ubiquitious 6V winding did service and that was that. I am also using a dc heater supply for ECF82 input tubes in a power amplifier, and found the same thing there, though not nearly as severe. Still, again I was forced to stick to a 6,3V (hot) heater supply. (All the above are regulated in a pious attempt to cope with the varying mains supply, load-wise and from town to town, in RSA.)
I would like to know whether others had any experiences in this direction. I cannot imagine this is typical of the heater world; I do not have enough examples of other types to conduct a similar experiment.
Var kanskje derfor du måtte avgårde å måle på hva italienerne hadde beregnet av marginer ;D.