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Author Topic: 3-400 vs 3-500  (Read 4597 times)
K9FON
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Posts: 1012




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« on: October 11, 2008, 10:38:58 AM »

Besides the size of the tubes what is the differences in the tubes? My MK II direction manual says the amp is supplied with either/or 3-500 or 3-400 tubes. Mine of course, has 3-500 tubes.
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WB2WIK
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Posts: 21836




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« Reply #1 on: October 11, 2008, 11:17:38 AM »

Other than the fact that you can't buy a 3-400Z so this is kind of moot (they were discontinued by Eimac in about 1968 and haven't been made since), the 3-400Z was closer to a read "zero bias" triode in that at full plate voltage it ran about class AB2 without any bias applied at all.  A 3-500Z requires some bias to keep it under control with full rated plate voltage, although is about a "zero bias" tube with reduced voltage.

Because of this difference (depending on the plate voltage used) they were not truly interchangeable.

The 3-500Z's also a "taller" tube so there were some enclosures where it just wouldn't fit, if the amp was designed for 3-400Zs.  Ditto the chimneys.

WB2WIK/6
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K9FON
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Posts: 1012




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« Reply #2 on: October 11, 2008, 08:24:40 PM »

I read on another post that a ham found a couple of 3-400 tubes NOS. Thats quite a rare find IF the tubes are still good that is!
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WB2WIK
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Posts: 21836




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« Reply #3 on: October 13, 2008, 08:41:09 AM »

I really doubt they'd be good for anything other than mantelpiece trophies.

3-500Zs and similar have short shelf life and last longer if used frequently.

WB2WIK/6
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WW5AA
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« Reply #4 on: October 13, 2008, 08:46:27 AM »

The 3-400's in my Henry Tempo still put out full power!

73 de Lindy
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K3JVB
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Posts: 837


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« Reply #5 on: October 14, 2008, 05:31:09 AM »

Yep...

I have a pair on my Swan Mk-2. They are dated April-1964. Legal limit plus !

73
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WB2WIK
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Posts: 21836




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« Reply #6 on: October 14, 2008, 09:35:57 AM »

If you have 3-400Zs still doing great, I assume you must be *using* them.

Big difference between 3-400Zs that are being used (and get hot enough to getter properly) and ones that have been in storage for 40 years.

The ones in storage are not likely to be so good; the ones that have been in use may last forever.

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W8ZNX
Member

Posts: 1




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« Reply #7 on: October 21, 2008, 08:52:10 AM »

few points

3-500Z has 100 watts more plate dis
max plate voltage is 1,000 volts higher
and it will not fit
in a Hallicrafters Loudenboomer
unless you start cuting the top cover

3-400Z needs less drive
has higher amplification factor

for all the dope
see late 60's to mid 70's League Handbook


dit
dit
mac
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WA8UEG
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Posts: 810




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« Reply #8 on: October 24, 2008, 07:43:46 AM »

Steve,
As you know I rebuilt my MK II, I saw 2 NOS 3-400's very cheap ($125 for both) and bought them. I had no idea they could be bad from age however, they do perform as new. I might have just got lucky but I have been using them daily, well almost I am in China and have been for 3 weeks, but they have 6 months of use with no problems yet. Just an FYI.
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