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Re: Vacuum Tubes



It was written:

>Ben:  I would reconsider using the 304s for your coil unless you can 
>get them free.  Sockets for them are getting very hard to come by.  
>Also, each tube requires 10 v at 12 A to light it up (you could wire 
>each one in parallel but that would require 5 v at 25 A, a huge 
>transformer!!.  That's a lot of power just to light filaments.  You'd 
>do better to fish around for

You can easily make great heater power supply transformers out of old 
MOTs.  Hack off the secondary (I used a hatchet) and insert a few turns 
of your choice.  My particular MOT would produce about 0.8 volts per 
turn, so for a 5V heater supply you'd need about 6 or 7 turns.  

Current will not be a problem.  My MOT will supply a magnetic flux on 
the order of 1500 amp-turns to the secondary.  So, if you used the right 
wire (THICK) you would get about 1500/5 = 300 amps, more than eoungh for 
most tube filaments!  :) I'd suspect #10 magnet wire would work for 25 
amps...

You can insulated this new winding from the MOT core by inserting poly 
sheets inside the MOT before winding, etc.

I've tried this.  It works.  The only possible problem is that my MOTs 
tend to start saturating at about 100 volts on the primary.  Thus the 
magnetizing current is somewhat high under no-load conditions.  If this 
is a problem you could either run it off a variac, or off a lower Ac 
voltage, as produced by perhaps a second "univerally-tapped" 
100-110-120-130vac type of transformer...  Either way, volts per turn on 
the MOT will be different so you must account for this, etc.
-Bill



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