[Boatanchors] (no subject)

[Paul Baldock]

The way you are doing it is the perfect solution as you can adjust 
the filament voltage to exactly 5V and not overstress the tubes. I do 
exactly that with my 813 Amp.

How much voltage do you measure across the new 15 Ohm resistor?

- Paul


>Thanks, Drew, but in this case I think that's a bit overkill.
>
>73, Barrie, W7ALW
>----- Original Message -----
>From: "Drew P." <drewrailleur807 at yahoo.com>
>To: <boatanchors at mailman.qth.net>
>Sent: Sunday, April 18, 2010 7:42 PM
>Subject: [Boatanchors] (no subject)
>
>
>
>Barrie Smith wrote:
>
> > I've been juggling 5 volt transformers all day, trying to
> > find one that would actually produce close to 5 volts for a
> > pair of tubes.
> >
> > They were all either too high in voltage, or too low.
> > One did give me almost exactly 5 volts, but it ran way too
> > hot.
> >
> > So I've selected one with plenty of current capacity and am
> > playing with resistance in the primary to get the secondary
> > voltage down to 5 volts.
> >
> > Using a 25 ohm, 150 watt rheostat, I find that 11 ohms gets
> > me real close. The rheostat runs quite warm, which
> > leads me to my question.
> >
> > The voltage drop across the rheostat is a measured 13
> > volts. Knowing that and the resistance, Ohm's law
> > tells me that I have 1.18 amps running through, which equals
> > 15 watts.
> >
> > I don't want to install the rheostat in the transmitter
> > because it is quite large, so I thought I'd substitute a
> > fixed resistor.
> >
> > I tried a 15 ohm, 30 watt fixed resistor. It got
> > very, very hot real quick!
> >
> > I must be making a mistake. I thought I should use
> > the "dropped voltage" in Ohm's law. Perhaps I must use
> > the full 115 volts, instead.
>
>Another method of reducing the secondary voltage would be to remove turns
>from the secondary winding, assuming that the transformer construction is
>amenable to this type of modification.
>
>I've done this to low voltage transformers on several occasions with good
>success.
>
>Drew
>
>
>
>
>
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