The Collins 828C1, and Continental 314R1 1kW medium wave transmitter
used a single 3-500Z as a switch tube running at 70 KHz switching
frequency for pulse width modulated AM. It was very picky of the type of
tube, and required component changes if switching to a graphite anode
tube to prevent disasters. So apparently there are differences in the
tubes. Maybe not subtle when paired in parallel in ham amplifiers?
http://www.jtml.info/
73
John
K5PRO
On 12/24/22 11:27 PM, David Gow wrote:
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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