I re-wired an amplifier heater circuit to an external current-limited
stabilised dc supply. We had to fit larger heatsinks and more fans to keep
the supply cool and arrange for this to be switched with the amplifier's
main supply. It also had a crowbar.
All this I thought necessary because of the large voltage fluctuations at
the farm where the station resided, where the load varied a great deal
during milking. Removing the heater load from the main transformer allowed
it to run cooler and produce a higher voltage to the EHT which was less than
required.
The amplifier has not died in the several years of high use.
David
G3UNA
> On Thu, Jan 28, 2010 at 03:10:46PM +0800, Alek Petkovic wrote:
>> Just as an aside. When the amp is run on 245V AC, here in Australia,
>> the voltage measured on the filament pins, with all 4 tubes lit up,
>> is 7.15V AC. A bit on the high side you could say. Could even call it
>> scandalous.
>
> I've noticed the same here with an AL-80B: On 240V, the filament
> voltage runs >5.5V on a 3-500Z (nominally a 5V tube). I spoke with
> Ameritron on the phone about the issue and their tech told me that
> voltages up to 5.6V are considered normal.
>
> I understand that high filament voltages is a contentious issue among
> hams and amp manufacturers. One camp claims it drastically shortens
> the tube life, whereas the other claims that in ICAS operation that
> other factors will kill a tube long before the excess filament voltage
> will do so.
>
>
> Rob / KD8WK
>
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