[Amps] EIMAC vs Amperex 3-500

[John Lyles]

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 at outlook.com 
<john.keating at 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 at outlook.com" <john.keating at outlook.com>
> To: "gudguyham at aol.com" <gudguyham at aol.com>, Amps at 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
>