[R-390] NEED HELP R-390

[Barry]

Mark,

Just curious why you suggest a 25W resistor?  300mA through 47 ohms is just
a little over 4W, right?  Bigger is better, I suppose, but you can get by
with less, I think.

Barry - N4BUQ

----- Original Message ----- 
From: "Mark Huss" <mhuss1 at bellatlantic.net>
To: <dhallam at rapidsys.com>
Cc: <R-390 at mailman.qth.net>
Sent: Wednesday, June 15, 2005 6:12 PM
Subject: Re: [R-390] NEED HELP R-390


> David is right.  I repeated Dallas Lankford's work and found that the
> receiver PTO is much more stable with a 42 Ohm resistor than the
> origional 3TF7.  Unless you have AC power that varies more than about 7
> volts, use a 25 watt 42 ohm resistor.  It generates some heat, but no
> more than the origional 3TF7.  And if you have a fetish about drift of a
> few hertz, use a Voltage Regulator and a good heat-sink.  You have to
> add a ground wire to the socket, but it was staple to the point that it
> was unmeasurable on my equipment.
> I tried the 12BA6 with a 1 ohm resistor in series to measure current
> rush, and saw quite a spike in current on turn-on.  Makes sense as
> filiment resistance goes up as the filliment heats up.  Enough that I
> decided to stick with the resistor to keep the filliment surge down.  It
> helps, I noticed that the PTO takes about two seconds longer to come up
> than normal.
>     If anybody is interested, I did the design work up to Breadboard for
> a plug-in Voltage Regulator.  it is designed for 40Volt Peak input
> voltage and 80 degrees C. ambiant at 20% overcurrent.
>
> David C. Hallam wrote:
>
> >Why replace the ballast tube at all?  Dallas Lankford's work showed that
the
> >ballast tube didn't do a very good job of stabilizing the PTO filament
> >voltage anyway.  The resistor or 12BA6 substitutions require no
> >modifications and do just as good or better job.  Besides they cost less
> >than the $20-$30 you have to pay for a new ballast tube.  Unless a person
is
> >a fanatic about original condition, I don't see any reason to keep the
> >ballast tube.
> >
> >David C. Hallam
> >KC2JD
> >
> >
> >
>
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