Re: Pass tube heater supply question



Robert McLean wrote:

> "Ned Carlson" <see-www-tubezone-net> wrote in message
> news:386a9sx81t16$.su1mr17sidu4.dlg@xxxxxxxxxxxxx
> > On Sun, 8 Jan 2006 11:21:50 +0100, Tom Schlangen wrote:
> >
> >> Which is the method you would prefer, and why?
> >>
> >> Thank you in advance!
> >>
> >> Tom
> >
> > If it doesn't affect the hum level in the output,
> > I'd just hook one side of the filament to the
> > cathode.
> >
> > You could use a 100 ohm hum-balance control, but it
> > had better be rated for the DC voltage involved.
> >
> > --
> > Ned Carlson www.tubezone.net
> > South Side of Chicago,IL USA
> > 1/8/2006 5:33:05 AM
>
> I finished building a tube based regulated power supply a couple of months
> ago, and I did as Ned says, just wired the heaters straight to the cathodes,
> and I have had no problems with it.
>
> One thing I thought about at the time was that doing this means your
> filament transformer winding will be seeing the high dc voltage. Now in my
> case I was getting the 6.3 volts from a combination plate and filament
> transformer, so I figured the transformer as a whole had sufficiently high
> winding to case voltage rating to handle the plate section AC voltage, so I
> didnt worry about it. I didnt think about the winding to winding voltage
> ratings, I just sort of hoped the transformer makers knew people might do
> this sort of thing sith tube circuits. I do not know what the winding to
> case voltage rating of a seperate 6.3 volt transformer might be, especially
> a modern one not specifically designed for tube heaters. This may or may
> not be an issue.

Many power trannies have 5V and 6.3V heater windings which are
designed to be run at +500V of the B+ supply.

One can easily wind an isolation 6.3 : 6.3 tranny if you are worried.

But usually the pass reg output is at low impedance, so any noise from a heater
winding referenced at one end to the output is supressed,
but connection of a hum adjust pot of 100ohms across the 6.3V and
taking the CT to the output B+ cathode voltage and adjusting for minimal hum
with a sensitive CRO will reduce hum to a minimum.

Patrick Turner.


.

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