Was I wasting my time , rather than throwing out some ECC83, a 6V6 and some EL34 and EL84.? All various degrees of C/H leakage causing vibration/tinging sensitivity so useless despite useful gain.
All measured 5 to 20 M ohm on C/H insulation test on valve tester. Decided to power the heaters from a bench ps and try C/H leakage with a 500V Megger insulation tester. Increasing the heater volts from normal 6V , for short durations then back to 6 , and monitoring on the Megger. Eventually taking up to 15V for 1 second , no longer orange glow of course, and vey low C/H Megger resistance. Now measuring them for the C/H leakage on the valve tester better than 30 meg (maximum measurable on my tester).
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on