Re: Tubes

To: tesla-at-pupman-dot-com
Subject: Re: Tubes
From: FutureT-at-aol-dot-com
Date: Thu, 23 Jan 1997 18:54:00 -0500 (EST)
Subscriber: FutureT-at-aol-dot-com Thu Jan 23 22:43:18 1997
>In a message dated 97-01-23 01:15:14 EST, you write:

<< John -
 
>   I just had a flash(?) of inspiration - some fuzzy part of my brain
 >started screaming "load impedance". Perhaps the 304TL gave maximum
>bang due to tube's lower internal resistance???? (plate volt/current)
 
 >  I recall that there is a bit of an art to matching the plate load
>impedance to the tube's internal resistance. Too high a load impedance
>and you don't get full poop. Too low a load impedance causes the same
>thing, only the excess energy goes into heating the plate in the
>tube. This might explain the one post about how the tubes ran nice and
>cool with the incandescent (resistive) load and really warmed up with
 >the Tesla tank circuit (*highly* reactive.)
 
> Brent
  >>

Brent, all,

I agree.  Also for the benefit of the person who requested info on expected
spark-lengths:  I obtained a 15" spark using one 845 tube; the 845 tube has a
graphite plate and is rated for only 1200 volts, it doesn't even have a plate
cap, the voltage is applied to the base.  Because of this, the useable
voltage is limited to about 3500 volts before things start arcing in the
base.  Using 2 tubes, I obtained  a 17" spark (more length could have been
obtained).  Using 4 tubes, a 26" spark was obtained,  eventually the stresses
destroyed the tubes however.  It's a good idea when building a tube coil,
especially if you're going to be "overvolting" it, to place a 56 ohm resistor
 and a parallel choke in series with the HV lead to limit the current through
the tube in case of a internal arc.  Otherwise the internal arcing tends to
destroy the tube. 

Another interesting tube is the HF300, or EE300 which also has a graphite
plate.  It is about 3" in dia. by 11" tall, and has a large "top-hat-like"
tube cap, the grid connection is also a large terminal that protrudes from
the side of the tube.  This tube can also give at least a 15" spark.  

Also, the 3-500Z is available in both sheet metal, and graphite plate
construction, I never tried the graphite one but it should be a good
performer.

John Freau