[Boatanchors] Ohms Law

[Barrie Smith]

Thanks, Ian, and everyone else!

I looked today and found a 3 amp (a bit overkill) transformer, with 
secondary taps at 8, 10, 12 and 14 volts.

In buck configuration, I tried all the taps just to see the results.  The 12 
volt tap yields almost exactly 5 volts at the tube filament pins.

I'm running the setup on the bench right now just to see if there will be 
any overheating.

BTW, more than a couple of folks asked me if I was using a "true RMS 
voltmeter".  The answer is yes.

Also, I was asked if I was mesuring the voltage under the full load.  Again, 
yes.

I'm getting real excited to get this rig on the air.  It's for 10 and 15 
meters, only.  Uses a pair of VT-127As, which are vaguely similar to the 
100TH except they use the grid and filament from the 250TH.

They are a WW2 radar tube.  Look like a glass spaceman, with grid and plate 
pins coming out the sides.

A junior high-school buddy of mine (Scott,W7TLZ, now SK) had a pair of 
VT-127As on 40 meter CW back in the very early 1950s.  His ham-father built 
it for him.

I used to go over to his house and marvel at that rig.  Then I'd go home and 
look at my single 6L6 and pout.

Oh well, almost 60 years later I'll have my own VT-127A rig, as a tribute to 
my good friend Scott.

He looked like a miniature Groucho Marx, but was funnier.

Another BTW (as long as I'm rambling):  The rig will use a BC-610 plate 
transformer, a BC-610 modulation transformer and a modulation reactor from a 
Gates BC250GY.

The modulator tubes will be (drum roll, please) VT-127As!

Thanks again, guys & 73,
Barrie, W7ALW
----- Original Message ----- 
From: "Ian Wilson" <ianmwilson73 at gmail.com>
To: "Boatanchors Mail List" <boatanchors at mailman.qth.net>
Sent: Monday, April 19, 2010 5:46 PM
Subject: Re: [Boatanchors] Ohms Law


> Just to summarize the numbers..
>
> The tubes draw 2x14.4A =28.8A according to their specs (not sure if Barrie
> published that; may
> have been a private reply).
>
> If you needed to drop 13V at the primary, the original voltage to the 
> tubes
> should have been about
> (5/115)x13V = 0.6V too high.
>
> 13 volts at 1.18A is 15.3W. Your 150W rheostat (power specified assuming 
> the
> whole resistance, by
> the way) is good for about 70W at the 11 ohm setting.
>
> Your 15 ohm resistor would dissipate about 21W assuming the same current
> flow, so it would get hot
> but still within ratings. Metal mounting brackets to the metal chassis 
> plus
> some space to let the heat
> circulate should be OK.
>
> The ideal solution as noted by one poster is to use a 12.6V bucking
> transformer in opposition to
> the primary winding. A 50VA transformer would be plenty (perhaps a 12.6V 
> 3A
> filament transformer).
>
> To drop the excess 0.6V in the secondary would require about 0.02 ohm. 
> This
> will be required to
> dissipate the same power (15W or so) as the 11 ohm resistor in the primary
> winding.
>
> 73, ian K3IMW
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