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If you seriously do not have a meter, there's a 'quick & dirty' way of dipping the efficiency coil. Connect a #44 bulb (which is rated at 250 ma) in series with either the plate or the cathode. Adjust the eff. coil for minimum brightness. The bulb should glow at signifigantly less than full brightness, indicating the current to be well below 250 ma., and in the "safe" range.
We used to carry a 'magic light bulb' adapter in the tube caddy for this purpose; just clip it in the plate lead of the tube. It was surprizing how many sets in the field were 'off the dip'. In the shop we always used a proper (analog) meter.
There was never any observable correlation between eff. coil setting and linearity.
Red plating of the tube should be avoided at all cost. It can kill the tube quicker than it can kill the flyback due to the tube's small thermal mass. I've seen HO tubes with holes melted thru the glass and the flyback was still good.
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