"Sam Goldwasser" wrote in message
...
He said the plates are glowing red. That's not a filament voltage
problem.

I agree with Sam; it is quite possible that the OP is chasing something that
is not a problem.
And resistors do not 'generate' harmonics!
While a wire wound resistor may have inductance in most heater circuits this
is unlikely to be problem.
Red plates in transmitting tubes is most likely to be a different problem;
if it's a transmitter it may be 'off-tune'?
Or the transmitter drive may be low, (or too high) depending on the
operating class of the final/s?
Reluctant to start the OP on that angle because of the high voltages etc.
involved. Must be fairly high power?
We really don't have enough info! But really wonder if heater voltage is
REALLY the problem?
Wonder what the application is if it's a radio amateur transmitter would
expect the operator to be a little more knowledgeable? If it's something
critical such as a paging transmitter or something like that
................. in say a hospital etc. should be maintained by a qualified
tech.
Voltage 7.2 compared to nominal 6.3 is a bit (14%) high but perhaps
believable depending on the voltage of the AC supply, 115 or 230 volts? Or a
mains transformer may be set on the wrong voltage tap; e.g. if it is on the
100 volt setting (which I believe is the Japanese AC supply?) and the supply
is 115 that is 15% high to start with.
On other hand OP may be measuring the 7.2 volts off load, with the tubes
removed or something? With the tubes plugged in it might be spot on 6.3 and
at what point in the circuit is the 7.2 being measured? It all sounds rather
dangerous!