30-08-2024, 09:11 AM
Variac would be ideal to be able to twiddle it so that the heater voltage is exactly 6.3V - then you would know how much to reduce the mains by, with a little dedicated transformer (that needn't be large).
A Variac permanently connected is a tie-up of an expensive item!
There isn't of course a hard-and-fast rule as to what is acceptable and what isn't, the standard manufacturer's tolerances is, on the low side, what still gives acceptable performance; and on the high side, what doesn't excessively shorten life. If you don't need maximum emission from the cathode, slightly under-running is probably preferable.
Over-running goes along the lines of an hour at 10% above nominal reduces valve life the same amount as an hour and a half at nominal.
A Variac permanently connected is a tie-up of an expensive item!
There isn't of course a hard-and-fast rule as to what is acceptable and what isn't, the standard manufacturer's tolerances is, on the low side, what still gives acceptable performance; and on the high side, what doesn't excessively shorten life. If you don't need maximum emission from the cathode, slightly under-running is probably preferable.
Over-running goes along the lines of an hour at 10% above nominal reduces valve life the same amount as an hour and a half at nominal.
