The original poster said they were in original Drake packaging. If they
were in fact in original sealed Drake boxes I wouldn't worry about using
them together as Drake would have sold them at different times as
compatible spares. I have mixed all sorts of 3-500z tubes in many amps
over the decades and my test is to see if under color if the look about the
same they are ok to use together. Your are correct in theory but perhaps
not so much in practice. Right now my Henry 2K3 has one original NOS stock
Eimac from my shelf bought by me over 25 years ago and a brand new RF Parts
Chinese graphic tube. THey were both unused when I installed them six
months ago. They show about the same color, a nice dull red. when
appropriate and have normal plate current, grid current and put out about
1200 watts on 20 meters. Best practice?,no, but works fine. My buddy, a
retired broadcast engineer, doesn't approve either so I told him to buy me
a nice new matched (at least for a few hours!!) set.
On Sat, Dec 24, 2022 at 8:14 PM John Lyles <jtml@losalamos.com> wrote:
> The author originating this discussion said that they had one 3-500Z
> that was the vertical thin fin plate, Eimac original, and one 3-500G
> that has the graphite anode presumably with the horizontal heat
> radiating structure. This would look like the Amperex/Covimag tubes that
> are no longer made, but are cloned in China. For that matter, the
> original 3-500Z from Eimac is long out of production as well.
>
> I think it would be more risky to pair these two tubes together as they
> come from different manufacture with different plate physical design.
> There is no guarantee that the cutoff bias voltage is identical and that
> the mu is the same or within a tolerance. Operating this way might cause
> one tube to hog more current, have higher dissipation due to the idling
> current (Zsac), if they both share the same cathode bias setting.
>
> It would be optimal to put the older style NOS tube into the amplifier
> with one of the original (weak?) tubes to see if it works better. Then
> try it with the other original tube. If the two tests show large
> difference in power capability, then I would suspect just one of the old
> tubes. At least start with this plan, and see how it works, report back
> here.
>
> If anyone is interested in Amperex/Covimag 3-400Z or chimneys for either
> triode, get in touch.
>
> 73 and Happy Holidays
>
> John
>
> K5PRO
>
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