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Re: [Amps] EIMAC vs Amperex 3-500

Subject: Re: [Amps] EIMAC vs Amperex 3-500
From: John Lyles <>
Date: Mon, 24 Oct 2022 01:26:11 -0600
List-post: <>
Rob and all,

I apologize for the wierd WEtransfer link. On my desktop it works fine and is a simple storage location like Dropbox or Googledrive, for a temporary document or image file. On my Android tablet, however, I see the ads and question about my cookie preferences. I did answer them by making sure all the options were switched off and clicking save, but that still didn't get to the file. Here is OCR version of the text from the PDF document instead:


314R-l/828C-l 3-500Z TUBE PROBLEM

Some brands of 3-500Z tubes will run very hot in the Modulator position but will operate properly in the RF position. It is normal for the Modulator tube anode to appear red when operating at full power without modulation and can be expected to be yellow with full modulation. However it should never appear white hot with or without modulation. The output pulse from the Switching Modulator Card has a fixed rise time going in the positive direction. If this rlse time is too slow for the particular modulator tube used, there will be excessive anode dissipation. If the rise time is too fast, you can not achieve 100% modulation in the negative direction. It appears that somevof the tubes that are available now. have different characteristics from those that were available when the transmitter was designed and this rise time must be changed in order to use these tubes in the Modulator

Continental has checked two commonly available brands of 3-500Z tubes in a 314R-l and recommends the following changes to the Switching Modulator Card, A2, circuits: 1. Change R-12 to 2.7K, 2Walt Carbon Composition. Continental PIN 745-5669-000 2. Change C-13 to 1200pf, SOOVolt, Plastic coated silver mica PIN CM06FD122J03 3. Verify value of R-I0. May be two parallel 1500 ohm, 2 Watt carbon composition. 4. Verify value of R-l1. Should be 3.9K if Q6 is IRF-330 or 1.5K if Q6 is 2N6S7S. NOTE: It is important that only non-Inductive, carbon composition resistors be used at R10, R11, and R12. 5. Add a Parasitic Suppressor in series with Modulator anode lead. Continental Part
Number 650-2969-001

Please contact Continental Field Service if you have comments or questions about these changes. We would appreciate any comments about your experience with any brand of tube that you may be using in this transmitter and your results. Since this transmitter is out of production. we must depend on your input to us to help provide useful information to all users.

Message: 2
Date: Sat, 22 Oct 2022 15:46:39 -0600
From: 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.

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: 5
Date: Sun, 23 Oct 2022 05:30:07 -0500
From: Rob Atkinson <>
To: "" <>
Subject: Re: [Amps] EIMAC vs Amperex 3-500
Content-Type: text/plain; charset="UTF-8"

Maybe the Amperex tubes have lower emission.  Maybe they've been used
a lot more?

The link to the Eimac switch tube note gave me some kind of crazy page
wanting to install cookies or something.  No thanks.  No Eimac data
there at all.

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