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Re: danger measuring antenna

 

If you look at the typical pi-network configuration of these vintage
transmitters, there is nothing for the final plate blocking capacitor to
work against. Therefore, it will never charge to the DC potential as a
dipole or other balanced antenna is a DC open circuit. Heaven forbid
should a bird light across the center insulator of such an antenna fed by
such a transmitter. That choke does two things: 1) gives something the
plate blocking capacitor can work against to charge so there is no DC
potential on the feedline, and 2) serves as a 'fuse' as Jim Lux has
described. Of course, the DX-40 has no fuse ! The unit I just received to
resurrect - my novice transmitter - has been modified with the addition of
a fuse - good work. My original from Heath some 61 years ago did not
contain a fuse on the chassis. Oh........., I forgot, it did, but it was a
bit dangerous. The AC plug contained cartridge fuses in both sides of the
AC plug. They were exposed to contact at the line cord end of the AC plug
- open to a rather "awakening" experience.

Dave - W?LEV

On Wed, Aug 18, 2021 at 3:48 PM Jim Lux <jim@...> wrote:

On 8/18/21 8:25 AM, Victor 4X6GP wrote:
Those chokes were rated at 125 ma or 300 ma at the most. If the plate
blocking capacitor were to short, they would burn out before the primary
fuse blew. Whatever the function is supposed to be, it couldn't protect
against a shorted blocking capacitor.

I'm not so sure - if you're building a 3kV, 0.5 A supply (1.5 kW DC
power) then you'd be putting in something that can suck up half an amp
for at least long enough to trigger the overcurrent protection. A 300mA
choke can probably take twice that for a short time. And remember, it
doesn't have to be a very good choke while the fault exists - it can
melt the form, it can saturate (if it has an iron core), etc. It just
has to last longer than it takes the fuse or breaker to trip.


I would think, for instance, that the choke could be comparable to the
plate choke from the power supply - both have to be able to potentially
stand off the entire HV power supply (if the tube or surroundings arcs
to ground).









--
*Dave - W?LEV*
*Just Let Darwin Work*
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