Re: Looking for a HV Vaccum Tube Triode

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Tesla list wrote:
> 
> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
> 
> Tesla list wrote:
> >
> > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <CoolCorals-at-aol-dot-com>
> >
> > okay, sorry.  Not trying to be sneaky or anything.  My name is Brad
> > Kretzmeier.
> >      You guys are great!  I have been looking for something like this
> > forever.  One more quickie about the tube Bert spoke of....  The 6C21?  Is
> > there a particular maker for that model number?  and is it something I
might
> > find easily or not?
> >
> > Thanks a lot TC guys!
> >
> > Brad Kretzmeier
> > INTRASPEK
> > Lafayette, IN
> 
>         You never said what you wanted to do with the tube!  As for the 6C21,
> it is a great big honker which takes 140 watts of heater power!  At a
> plate voltage of 20 kV the cutoff grid voltage is nominally about 650
> volts, so it doesn't exactly meet your criterion of low grid drive.
> 
> Ed

Hi Ed and Brad,

Unfortunately, there's not a huge selection of reasonably sized,
reasonably priced triodes that will handle 15-20 kV AND have hugh Mu.
The "flakiness" of oxide-coated cathode filaments can cause cathode
material to break off and redeposit on the grid, potentially causing the
grid to become an unwanted emitter and also disrupting the cathode-grid
E-field. This tends to limit the appearance of oxide-coated filaments in
higher voltage triodes, particularly those destined for hard modulator
use. Unfortunately, it takes ~3X higher filament power to get sufficient
emission from a directly heated thoriated tungsten filament as from an
oxide coated filament. Just the physics of the situation. 

If a tetrode could be used instead, the shielding effect of the screen
grid can significantly reduce the amount of grid voltage necessary to
control the tube from cutoff to full on. Possible affordable HV  tetrode
alternatives that are sometimes available on the surplus market at
reasonable prices include the 715B (15 kV), 5D21 (the modern replacement
from Eimac is the 4PR60C/8252W (20 kV)), or 6D21 (37.5 kV). A more
modern alternative may be a pulse rated (PR prefix) tube such as the
4PR1000A (30 kV in pulse modulator duty). 

Happy hunting and researching!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" (TM) Shrunken Coins!
http://www.teslamania-dot-com