[Boatanchors] Ohms Law

[Ian Wilson]

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