If you look closely at the Amperex tube, you may note that the anode
(plate) shape is different than the CPI/Eimac 3-500Z. Most of the
Amperex/Covimag tubes have a wavy horizontal structure with a dull flat
textured plate whereas the Eimac has vertical fins around the plate and
is a more shiny thin metal. They are not precise clones of each other,
and Amperex took some license from Eimac long ago to produce theirs, but
there are differences. Noteworthy are some mechanical differences inside
between grid/cathode/plate - possibly spacing as well as the pitch of
the grid and design of the filament. This will change the mutual
conductance which is related to the grid voltage needed for a particular
plate current. A service note for the Collins 828C1 and Continental
314R1 1 kW broadcast transmitter indicates there is a difference in a
critical application for the 3-500Z switch tube in that PDM rig. It can
be viewed as a small PNG of the page on this link, good for one week.
https://we.tl/t-lvcbHr0qyu
I recommend NOT mixing an Amperex and Eimac triode in parallel in the
same amplifier. One will run hotter and hog a share of the
amplification. If you want to make them more alike, you would need to
tweak the cathode bias diodes.
I have some 3-400Z that are also from these two sources, and I am
willing to pass them on with a very good deal. They are slightly
different from 3-500Z, and require a shorter chimney to get same cooling
performance. I have both types of chimneys if you need any.
73
John
K5PRO
On Friday, October 21, 2022, 9:30 PM, john.keating@outlook.com
<john.keating@outlook.com> wrote:
Spec sheets for these tubes appear identical. Yet they behave
differently in my dual 3-500 amp. I can drive the Amperex at 90w for
near-legal limit out (power gain of about 16x) without exceeding 200mA
grid current; Plate current is just about 800mA for the pair. With the
Eimacs, I can't exceed 75W drive without exceeding 200mA grid current
(running about 720mA plate), and they do give the same 16x power
amplification at that drive. At 90W drive, the Eimacs exceed the
recommended grid current limit. Wondering if the amp brain trust can
enlighten me on the difference and cause.
73, John / AI6LY
_______________________________________________
Message: 4
Date: Sat, 22 Oct 2022 15:06:35 +0000
From: "john.keating@outlook.com" <john.keating@outlook.com>
To: "gudguyham@aol.com" <gudguyham@aol.com>, Amps@contesting.com
200mA is the total for two tubes, that's in the amp tune condition,
single tone key down.
My question is more about why the Amperex and Eimacs draw such different
grid current under identical operating conditions, rather than concern
over the maximum ratings.
John
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