Re: Toob coil questions

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

Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 

deleted your original message where you said Vfil was around 6V... if the
tube wants 6.3, you'd better give it 6.3..
I can't recall the exact temperature vs voltage characteristic (life is
V^12, though)...but, electron emission is very, very strongly dependent on
temperature. If the fil is too cold, the "gain" of the tube won't be high
enough, and you'll wind up using too much grid drive to get it to oscillate.

Too much grid drive >> very short tube life... the grid's a very fragile
structure, typically some real fine wires. Dissipate too much power in the
grid and it melts, or, just gets soft and deforms.

Grid dissipation comes from several things:
1) forward biasing the grid (evil... makes grid think it's the plate, and if
your grid source has too low an impedance, much current can flow..)
2) backscatter from secondary electrons from the plate (probably not an
issue for your tube)
3) energy from electrons coming off the cathode hitting the grid and
stopping (generally minimized by using fine (small cross section) grid
wires, and fancy ion-optics design (Today at work we were discussing a radar
pulse tube, the 4CX5000A7 (I think..), which claims such design.. grid
creates the field, but the cathode/plate design channels the electrons
between the grid wires.. such is the "magic" of Eimac's designers... This is
the tube you want for your mondo VTTC... 6kV plate voltage, 67Amps peak
Iplate.. designed to drive low impedance loads.. oh baby, I want to see that
plate power supply!)
4) RF current flowing through the parasitic Cgk ... this can be quite
significant..The current has to flow through the grid wires to the parasitic
C..

The usual sorts of designs have a fairly high source impedance for the grid
(after all, it shouldn't draw any current when biased negative), which
limits the current in the forward direction if the grid voltage goes
positive.  However, if there is energy storage or biasing components in the
system that can support grid current, #1 can be a significant problem.  I
don't have the circuit here in front of me, but part of the idea behind the
grid leak is to pull the grid negative, no?




----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, September 18, 2003 6:12 PM
Subject: Toob coil questions


 > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>