[Boatanchors] Ohms Law

[J. Forster]

"a small Variac to drive the input of the bucking transformer"

That reduces the range of the adjustment from 0 to the secondary voltage
of the bucking transformer.

If the filament transformer can supply, say 5% more than the nominal
voltage and the bucking transformer secondary is 12 VAC on a 120 VAC line,
the Variac would allow an adjustment range of +/- 5%, enough reduction to
increase the tube life by roughly 4x, and yet allow an overvoltage for
emission increase as the tube ages.

-John

==============


> 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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>
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