I think the only problems the 6W4 had was heater-cathode leakage.I
would imagine that the Sparton has a seperate heater winding dedicated
to the 6W4 so the leakage would not be a problem.You could temporarily
sub a 6AX4 or even a 6AU4 in it's place just to see if it is the problem.
The pinout is the same on those tubes.Very loud whine usually means
the horiz. osc is running way off frequency.I would start troubleshooting
there first.Don't forget the flyback depends on resonance with the
yoke to work properly.The entire stage needs to be at resonance
at the proper frequency to get maximum efficency and output
power.Just like the output tubes in a rf linear amplifier.Check for the
usual leaky caps and way off value resistors in the horiz. osc. circuit.
Do you have a scope to look at the horiz. waveform with?If you do not
have one and since you are seriously into fixing old tv's you should get one.
It will make the troubleshooting a whole lot easier.
Regards,
Swanson