[Boatanchors] Ohms Law

[rbethman]

The problem with that idea is this transformer is for the filaments of 
an amplifier circuit.

So it usually isn't "practical" to start using variacs in permanent 
circuits.

Bob - N0DGN

On 4/19/2010 8:02 PM, J. Forster wrote:
> You might like to use a small Variac to drive the input of the bucking
> transformer, to allow essentially lossless voltage adjustment, especially
> if your line is not really stable.
>
> -John
>
> =====================
>
>
>
>    
>> 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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>
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