[Milsurplus] LV Tube operation.... was half-voltage dynos

[jcoward5452 at aol.com]

Hi All,
  The November issue of Electric Radio has an article on a vacuum tube 
curve tracer.This looks like a useful tool if you are into design.
  In tech school we had to generate characteristic curves for each half 
of a 12AU7, create the transfer characteristics and then build various 
configurations of small signal amplifiers.We just had power supplies 
and meters all cliped together,but it was fun if you only had to do it 
once!
  Anyway,the setup in the ER article may be the ticket to look at what 
happens to the tube characteristics as a function of filament voltage 
and effective heating of the cathode.
 Jay KE6PPF

-----Original Message-----
From: jfor at quik.com
To: milsurplus at mailman.qth.net
Sent: Sun, 3 Dec 2006 11:50 AM
Subject: [Milsurplus] LV Tube operation....  was half-voltage dynos

  Hi,

  There are some low power receiver applications that run the filaments
or
  heaters "starved" -half voltage or less. They also run at very low
plate
  voltages - maybe 8 volts or so. The writers have not said anything
about
  tube life except that the tubes loafing along and should last
"forever".
  I am going to try it eventually.

  There are different materials used for filaments and indirectly heated

  cathodes. Some of those are very sensitive to reduced voltages and
  especially when used at the full rated plate power. Some transmitting
  tubes come to mind. I have some old surplus radios and I don't think
the
  tubes would be particularly sensitive to reduced heater/filament
  voltages. Obviously they need enough heat to produce some emission. I
  have limited personal experience with this and I won't be able to try
it
  in a timely manner for this thread. Apparently other people have
  successfully done it beyond the extent proposed here.

The problem is related to cathode poisoning. There are apparently
several different things going on  cathodes, including diffusion of
Thorium from deep within the cathode to the surface, where the action
is. Such processes are temperature dependant.

Also, the heat loss from the cathode by radiation is a 4th power of the
cathode temperature, so the temperature is NOT in direct proportion to
the filament voltage (they will be hotter than 1/2 temperature at 1/2
voltage).

I think all bets are off when you run the tubes outside their design
envelopes.

FWIW,

-John




______________________________________________________________
Milsurplus mailing list
Home: http://mailman.qth.net/mailman/listinfo/milsurplus
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Milsurplus at mailman.qth.net

________________________________________________________________________
Check out the new AOL.  Most comprehensive set of free safety and 
security tools, free access to millions of high-quality videos from 
across the web, free AOL Mail and more.