Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on May 18, 2014, 04:14:44 pm

Title: Raised heater CT voltage?
Post by: Voxbox on May 18, 2014, 04:14:44 pm

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

Title: Re: Raised heater CT voltage?
Post by: HotBluePlates on May 18, 2014, 04:19:54 pm

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.

Title: Re: Raised heater CT voltage?
Post by: Voxbox on May 19, 2014, 01:50:34 am

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

Title: Re: Raised heater CT voltage?
Post by: sluckey on May 19, 2014, 07:28:20 am

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

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

Title: Re: Raised heater CT voltage?
Post by: HotBluePlates on May 19, 2014, 10:04:02 am

Quote from: Voxbox on May 19, 2014, 01:50:34 am

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.

Title: Re: Raised heater CT voltage?
Post by: jojokeo on May 19, 2014, 03:56:18 pm

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.

Title: Re: Raised heater CT voltage?
Post by: Voxbox on May 19, 2014, 05:05:58 pm

Gents,
Great replies!  I can now talk intelligently to my customer  :smiley:
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!
 :icon_biggrin:
Cheers, VB