[AMPS] Fil voltage al-1200

[Phil Clements]

> I checked my voltage at the outlet for my amp. and it measured 235
> volts.  This was measured with a B&K 20k ohm voltmeter.  Factory setting
> for the AL-1200 filament xfmr line voltage is setup for 245v.  There are
> taps for 232v and less, would it be advisable to switch my tap to the
> 232v setting???  Thanks de Phil W2GE

The filament voltage must be measured at the tube sockets in order to set
the taps properly. There is an exponential increase in tube life when the voltage
is set slightly below specs. At +/- $600 per copy, it is well worth the trouble to
do this. A good mod while you are in there is to insert a power resistor in the
primary of the filament transformer to reduce the inrush current. A cold filament
looks almost like a dead short for a few ms. There are naysayers out there that
maintain that indirectly heated filaments do not need this protection, but this is
hogwash. Such good protection for a couple of bucks. The value of the resistor
is adjusted in combination with the transformer taps until the desired voltage is
obtained @ the tube sockets. I use a 50 ohm 100 watt adjustable (slider) resistor
to do the set-up, and then replace it with a fixed resistor of the proper value and
wattage when done.

(((73)))
Phil, K5PC




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