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Re: Filament voltages

Original poster: "Ray von Postel by way of Terry Fritz <twftesla-at-qwest-dot-net>" <vonpostel-at-prodigy-dot-net>

Tesla list wrote:

>Original poster: "sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>"
>    The tranny for the Quad 833 is a separate tranny than the plate 
>tranny.  It's wound with 1/8" tubing on an old NST core.  Unloaded I get 
>10.2v on it, loaded that drops to around 7v (pushing 4 tubes).  Do I need 
>to correct the fully-heated filament voltage to 10v, or let it sit at 7v
Yes, you do need to correct the filament voltage.  It should be 10 volts 
at the tube.  I notice that your transformer has been wound with tubing. 
 You may be able to increase the output capability
of the transformer by replacing the tubing with the same size or larger 
solid wire.  It will have
less resistance than the tubing and the loaded voltage should go up.  

If the voltage at the transformer is 10 volts, fully loaded, then the 
drop in voltage is being caused
by the leads from the transformer to the tubes.  The solution is then to 
increase the wire size
between the transformer and tubes.  

If the voltage at the transformer is 7 volts, fully loaded, then the 
drop in voltage is being caused
in the transformer itself.  Since the unloaded transformer voltage is 10 
volts, then the turns ratio
between the primary and secondary is correct.  If the rewound 
transformer used the original
primary, 120 volt winding, and if the transformer was originally rated 
at 400 watts, then the voltage
drop is most likely happening in the secondary wound with tubing.  The 
fix for that would be to
use the same number of secondary turns, but use larger wire.  Tubing has 
higher resistance than
solid wire of the same size.  You might consider rewinding the 
secondary, 10 volt, winding with
1/8 " diameter wire.  I don't have a wire table handy so I won't suggest 
a gage.  If you can get
the same number of turns on the transformer with a still larger size 
wire, then I would probably
do it.  

If the original NSF was rated below 400 watts then I would suspect the 
size of wire in the
transformer primary.  The manufacturer would not use a larger gage wire 
than he had to.
You didn't say the transformer was getting hot, but if it is too hot to 
touch after running loaded for
a while, then the transformer is overheating because it is not designed 
for the load you are putting on it.  The 400 watt figure came from the 
minimum power the tubes would draw.  This is
just a ball park, rule of thumb, thing derived by  multiplying the 
voltage by the current, by the
number of tubes.   10 volts X 10 amps X 4 tubes  = 400

Within limits, you can play all kinds of games with the other voltages 
applied to tubes, but
filament voltage is sacred.  That doesn't mean you have to use meters 
accurate to 0.1%, but
it does mean if the manufacture says 10 volts, then he expects his tube 
to give good service
if the filament voltage (which determines the current drawn) is held at 
10 volts when read
with a decent quality analog type meter.  Current day digital meters 
read a lot closer than
do analog meters for the same voltage range.  10 volts plus or minus 0.1 
volt is satisfactory.
0.25 volt is beginning to stretch things.  To high a filament voltage 
and you shorten the tube
life dramatically.  If it is low, then emission is low and the tube 
doesn't work like it should.
You can also get a condition were the trace elements in the surface of 
the filaments is depleted
and the tube emission will be low.  This is also caused by  operating 
with a low filament voltage
over an extended time.  The cure for that is what used to be called a 
"tube rejuvenator".  What
it did was over voltage the filament (cathode) for a few minutes 
causing, usually thorium, to boil
to the cathode surface.  Some times it worked and sometimes you burnt 
out the filament.