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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