Tom W8JI wrote:
>> In Field Day use, with more than one station hooked up to the same
>> generator, there can be violent swings in the line voltage as
>>stations
>> switch unpredictably between transmit and receive. A regulated
>> filament/heater supply could prevent damage due to insufficient
>>cathode
>> temperature.
>
>The cool down time is about the same as the warm up time Ian, momentary
>sags wouldn't hurt a thing.
Indirectly heated cathodes do have some thermal inertia which tends to
keep the temperature more constant... but the scenarios I've experienced
have involved sags in line voltage that were a lot more than
"momentary".
Typically, it involved three stations all running from the same large
generator (because that's all we had). The heater supplies in all the
amplifiers had been set up for average home-station line voltage. But
when all three were transmitting at the same time, they would pull the
line voltage far below that level. On a long CQ, that could last 30
seconds or more, which seemed to be enough time for the cathode to cool
down significantly. Then the other two stations go back to RX, leaving
one PA still transmitting - which suddenly has full line voltage, full
B+ voltage and a somewhat chilled cathode.
Even if that caused no damage, I think a regulated heater supply would
be justified to help reduce the uneven performance with the constantly
varying line voltage.
>Personally, I'd use a constant voltage transformer if it was that big
>of a deal.
CVTs are hard to find, and very expensive (at least, on this side of the
Atlantic). A linear regulator is obviously a possibility, but a
pre-packaged switcher would be smaller, lighter and much more
cost-effective.
>If I used a switcher or a current regulated/limited supply, I'd extend
>the warm up time to be safe if the filament supply was current limited.
Good point - I'd forgotten that. A voltage-regulated supply will try to
push a very large current through a cold heater or filament at
switch-on, unless it also has a slow start or a suitably low current
limit. I seem to remember hearing about a 5V DC switcher being modified
for use with 6V heaters, with its controlling voltage divider 'slugged'
by a large capacitance to give a ramp startup.
--
73 from Ian GM3SEK
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