RE: 4-125A, vacuum tube do's and donts

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "T. David Cohen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tdavid26-at-cox-dot-net>

Hi Matt,

My experience with Tesla Coils is very limited but I've used lots of
transmitting tubes over the years.  The 4-125A is a great tube and ought
to work very well for you.  The filament supply is fairly critical
however -- it needs to be 5 volts +/- 5% maximum and the transformer
will need to supply 6.5 amps.  I've never had any experience with
microwave oven transformers so I don't know about their ability to
supply filament voltage.  If the filament winding is accessible and you
know what voltage it was designed to supply under load, you can rewind
it to work with the 4-125.  Just wind a proportional number of turns
based on the ratio of the design voltage to 5 volts.  #12 enameled wire
is the minimum I'd use.  You can probably get some suitable wire from a
motor rebuilding shop if you can't find it anywhere else.  Even if the
30 volt/amps that the tube requires is somewhat more than the original
filament, it will be a very small percentage compared to the normal
transformer load.

Good luck,

Dave Cohen

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com] 
Sent: Saturday, April 13, 2002 1:48 PM
To: tesla-at-pupman-dot-com
Subject: Re: 4-125A, vacuum tube do's and donts

Original poster: "Herwig Roscher by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <herwig.roscher-at-gmx.de>


Fucian-at-aol-dot-com wrote:
> any idea how much current this could do?

Matt,

Usually MOTs drive a magnetron, which delivers about 800 W to 
the food to be warmed.

>Its filament winding is only 3.18volts :-((((((((( I need 5volts
- Sometimes it is possible to remove some of the turns. So you 
could add another MOT with less turns to your existing one, if you 
cann't find a suitable filament transformer.

>Is the core the return?
- Yes, one end of the HV winding is tied to the MOT's core. You 
would like to connect the core to the mains ground for safety.

Take care!

Herwig


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