[AMPS] tube anode cap, etc.

[John Lyles]

First, I have to comment on Jon's stark question:

>Is the reflector working?  Been kinda quiet the last day or so?
>
>73,
>
>Jon
>KE9NA


Isn't it nice? The way a mailing list ought to be....
This reflector is much like tuning across 20 meters sometimes. I suppose
that's good, but it sure is difficult to find time to filter through when
ya gotta work for money.


>- --------------------------------------------------------------------------

>Date: Wed, 13 Jan 1999 23:26:25
>From: Dick McDuffie <ny1e@server.nlbbs.com>
>Subject: [AMPS] 4-1000A plate caps
>
>I'm not sure how to explain this, I have a couple of 4-1000's with only the
>1/4" rod sticking out of the top, I need the cap that brings it to 9/16".
>does anyone have an old tube that the allen screws will turn on or can I
>mickey-mouse something right on the 1/4" shaft? how important is the heat
>sink of the plate connector?
>Thanks, Dick NY1E

Contact Reid Brandon at CPI/Eimac for the answer, and maybe suggestions on
getting a cap thing. It is very important to have the heat sink cap on the
tube if you plan anode dissipation! It's part of the heat exchanger from
the structure inside to the outside air. Radiation and convection cooling.
Its also important to force air over it, to keep that top seal from failing
or the glass from suck in. I once saw an 833 triode in a MW broadcast rig
(1KW) with a suck in, it lost cutoff bias and somehow ran TOO much idling
current. I suppose the 833 wouldn't make a good zero bias triode like a
3-500Z, but it sure is a pretty bottle to look at.

By the way, the 833 did not have a heatsink on the stud either.
>------------------------------

>Date: Wed, 13 Jan 1999 21:41:22 -0600
>From: Jon Ogden <jono@enteract.com>
>Subject: [AMPS] HV Bypass Question
>
>Hi y'all,
>
>Got a question regarding the plate RFC and bypass caps.  How near to the
>B+ bypass caps should the plate RFC really be?  The way my layout is
>coming together, the B+ feed and bypass caps are several inches away from
>the plate RFC.  I was thinking of running the B+ through a piece of
>shielded line and bypassing it additionally at the RFC.  Is this
>unecessary or a good idea?
>
>73,
>
>Jon
>KE9NA

I believe that you are speaking about the low RF (cold) side of the RFC,
right, Jon? At HF, in a kW level amplifier, you will pick up a little bit
of RF on the wire, but it's not enough to consider adding more Cap to gnd
to compensate, if your bypass cap is already there and capable of shorting
out most of the conducted RF that got past your RFC. It is my experience
that you don't want a foot of wire in the PA compartment to the B+ feed or
the bypass, it can couple magnetic fields like a loop if oriented in the
worst way to the RF currents in the walls and the tube. A few inches is
nothing though. It hardly warrants a shielded cable unless you really want
to go to the trouble, it certainly would be the ultimate approach to do
that. I have used shield braid pulled over a silicone HV wire in a modified
Thermex 30 kW 27 MHz RF heating system, where I needed to route the cold
side B+ lead down and out a hole in the bottom. It must have been at least
a foot or more of wire needed.

In the 100 KW 2.8 MHZ amp that I just finished, the plate RFC is back in a
corner, pointed out parallel to one wall, then a wire leads over
perpendicular to the anode of the TH555A tetrode. It is about 6 inches
long, but it already at the high RF potential being a shunt fed DC
connection. I tend to put the loose wire in the high RF, and keep the cold
end close to the bypass feedthru cap, in my case it was a 20 KV Draloric
ceramic cap  that looks like a small mortar shell. Its 3000 pF and then I
have an added Matroc/Unilator 5000 pF disk in parallel at the feedthru,
that looks like a small Frisbee saucer.


Finally, I would like to post JPEG photos of my 2 x 100KW pushpull
amplifier layout, if anyone is interested. But where to put them? Anyone
have server space for this?

John
K5PRO









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