Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: billcreller on August 19, 2012, 03:08:18 pm

Title: PT voltage
Post by: billcreller on August 19, 2012, 03:08:18 pm

 I know that this has been covered a number of times.  I have this PT from Mojo, that I didn't use earlier because the HV secondary is really high.  Any simple way to drop the HV output voltage ??
  It has way over 500 volts coming out, like about 530 volts. and my filters are 450 volts, which promptly smoked the first node filter.
   I believe I remember something about a particular type diode to use on the center tap (??)
 If there's no way to drop it, I'll have to toss that PT & get another one.

Title: Re: PT voltage
Post by: tubeswell on August 19, 2012, 07:10:34 pm

Zener that is reverse-biased (banded-end pointing to ground) on the centre tap. Doesn't need to be a big zener (5W axial will do for anything between a 6V to 15V zener it its mounted away from the chassis on a terminal strip. String a few 15V zeners in series to get the desired drop.  But if you are going to bolt it to the chassis, you might want a bigger zener because of the 'reverse heat-sink' effect of the chassis heating up the zener).  If your'e using a FW bridge rectifier, then the zener goes on the ground side of the bridge (instead).

Title: Re: PT voltage
Post by: billcreller on August 19, 2012, 10:37:33 pm

Thanks...that's what I was looking for  :icon_biggrin:  So who supplies those types of diodes ?

Title: Re: PT voltage
Post by: tubeswell on August 20, 2012, 01:40:26 am

I get them from the local electronics store.

Title: Re: PT voltage
Post by: Willabe on August 20, 2012, 07:45:25 am

Quote from: billcreller on August 19, 2012, 10:37:33 pm

So who supplies those types of diodes ?

Try Mouser and/or Digikey.


                 Brad      :icon_biggrin:

Title: Re: PT voltage
Post by: billcreller on August 20, 2012, 04:59:40 pm

  OK.... :icon_biggrin: