Re: Toob coil questions

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

Original poster: "Adam Horden" <electronicmadman-at-blueyonder.co.uk> 

Hi Terry,

I am looking into designing some protection into my dual 833a power triode
VTTC I am currently building. Hopefully I should get time this weekend to
upload a few photos and info on the project so far.

At the moment I have made a quick mock up of a timer that turns on the mot
when a saftey intelock is activated then waits 2 mins and triggers in the
filimant supply.

Also looking into useing a large fet to ramp voltage from 0V to 240V on the
filimant supply when the 2 mins is up to stop the V surge damaging the
toobs.

Its all based around a ne555.

Steve can you send me a copy of your faq as well? I would like to take a
look at it,

Adam

----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, September 19, 2003 2:46 PM
Subject: Re: Toob coil questions


 > 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>
 >
 >