[Amps] 3CX800 filament voltage

[Missouri Guy]

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 at 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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