You could you connect a small filament xfmr (with a
appropriate rating) in series with the primary of the
"main" filament xfmr to buck or boost its primary
voltage.
73,
Charlie, N0TT
On Mon, 9 Feb 2009 10:04:45 EST Gudguyham@aol.com writes:
> The 3CX800 and 3CPX800 both use an odd ball filament voltage, 13.5
> volts.
> With 1.5 amps. If you try to find a filament transformer with that
> voltage,
> it is not that easy. If you do, it will probably be a lot of
> money. I had
> this problem myself and wanted to find an easy solution to it.
> Many of the
> seasoned amp builders know this, but some of the newer amp
> aficionados may not.
> A little lesson is in order here. Filament transformers like most
> low
> voltage transformers used for AC use, have ratings. For example
> you'll see sales
> of 12.6 volt transformers in many current ratings, like, 1,2, 3, 4,
> 5 amps
> etc amps. There is a good reason for this. If you take any of
> these
> transformers and measure the NO LOAD voltage, it will be somewhat
> higher than 12.6
> volts. once you load it down with its rated current rating, the
> voltage will then
> be 12.6 volts. Back to the 3CX800 filament voltage, I once bought
> 3 4 and
> 5 amp 12.6 vac transformers, I set up a test jig with a 3CX800 and
> tried each
> transformer until I found one that gave me "about" 13.5 v when
> connected to
> the filament. As I recall the 4 amp 12.6 v transformer got me
> right into the
> safe zone of filament voltage for the tube. A transformer of this
> type is
> less than $10.00.
> On another note, a filament transformer for a pair of 3-500z tubes
> is 5
> volts@ 30 amps and a filament transformer for one 8877 is 5 volts @
> 10 amps. If
> you tried to use a 30A 5V transformer for a pair of 3-500's on a
> single 8877,
> the filament voltage would be way too high.
> You must consider the current rating as well as the voltage
> rating.
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