Raised heater CT voltage?

[Voxbox]

Hi,
Is there a preferred voltage to apply to the heater centre tap?


I've just tried 70V dc and it works great, really reduces the 50Hz hum I've been struggling to get rid of. I've now got 10mVpp on the output which is below the resistor noise.
I tapped off right after the choke through a potential divider and smoothed the lower resistor with 22uF.
B+ is about 440V in 1960 Vibrasonic.


Intuitively 70V seems a bit high when there is a plate voltage of 170V on the pre-amp tubes. Or does it matter?


Oh, and why does it work?
Cheers, VB

[HotBluePlates]

I think the RCA tube manual suggested 20-50vdc. I would probably use 40-50v, especially given the elevated cathode voltage of the phase inverter.

[Voxbox]

Thanks HBP,
I thought there might be a limit to the voltage.
Any idea why?
And why does it work?
Cheers, VB

[sluckey]

Merlin has some easy to read info about elevated heaters on this web page...

     http://www.valvewizard.co.uk/heater.html

[HotBluePlates]

I thought there might be a limit to the voltage.
Any idea why?

The d.c. standoff voltage is limited by the heater-to-cathode voltage rating of the tube. So you ought to consider that you will need the heater positive of the cathode when you have stages with elevated cathode voltages (cathode follower, long-tail, split-load inverter, etc) and have the heater positive but without exceeding the rating for the stages with cathodes near ground.

[jojokeo]

The usual thing to do is simply connect the CT to the power tube(s)' cathode which is easy enough to try & see if it helps in the first place.

[Voxbox]

Gents,
Great replies!  I can now talk intelligently to my customer 
Sluckey, that's the perfect article. I would never have worked out the diode effect.
The main thing now is the amp so quiet. Just resistor noise from the Allen Bradley Plate Resistor Mojo.
The amp is not cathode biased as the power valve cathodes are tied to deck, the potential divider off the B+ will do the trick.
Once again, this Forum rocks!
 
Cheers, VB