Eimac quit making them over a decade ago, and they were superceded.
Then, of course, they got out of the glass triode business entirely.
3-400Z equivalents are still made, from the Covimag factory in France.
They are the heavy duty graphite anode variety, and work well. We use
them at work for a non-RF pulse amp application.
Date: Wed, 12 Jun 2013 15:01:46 +0000
From: Steven Katz <email@example.com>
I'm very familiar with the Mark I. It's not as good as the Mark II. :)
Two potential issues:
3-400Zs haven't been manufactured in over 40 years. 3-500Zs are a direct replacement, however they are
"taller" and the Mark I usually requires mechanical modification to accommodate the extra tube
height. Also, while the 3-400Z was a "zero bias" tube at 3000V (for AB2), the 3-500Z really isn't
and normally needs a bit of cathode bias to bring the idling current down to something reasonable. (They'll
"run" at zero bias, but dissipate a lot of power under zero-signal conditions, creating heat etc.)
The other issue is the Mark I has the power supply in the same enclosure as the
tubes. The tubes get very hot (of course) and make the transformer hotter, so
they are prone to failure. I've replaced power transformers in several Mark
I's over the decades - some didn't last more than a few years (back in the
early 70s) before they gave up, if they were used a lot. The Mark II doesn't
have this issue, as its power supply is separate and has its own cooling system.
But should be a good candidate for a monoband amp.
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