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Re: [Amps] EIMAC vs Amperex 3-500

Subject: Re: [Amps] EIMAC vs Amperex 3-500
From: John Lyles <>
Date: Sat, 22 Oct 2022 15:46:39 -0600
List-post: <>
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.

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.




On Friday, October 21, 2022, 9:30 PM, <> 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: "" <>
To: "" <>,

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.


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