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[AMPS] What (modern) tube to use?

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Subject: [AMPS] What (modern) tube to use?
From: (David A. Pruett)
Date: Fri, 19 Feb 1999 10:14:45 -0500
Terry, W6RU wrote:

> Consider the 3CX1200A7. This IS a 3-1000Z in a ceramic-metal package. A
> very rugged, instant on, tube.

Joe, W8IK wrote:

> The 3CX1200A7 (instant on, reasonable drive/gain, etc.) is certainly
> an interesting tube but I'm concerned that the only commercial user
> seems to be Ameritron and I have my doubts about the long term
> availability of a tube designed for the amateur market that is not
> getting a lot of use.

I agree wholeheartedly with Terry about the 3CX1200A7.  It has all the
best characteristics of the 3-1000Z.  Surprisingly, its an easy retrofit
for a 3-1000Z amp too.  Its not readily apparent unless you check, but the
hole pattern for the 3-1000/4-1000 socket (sorry, can't remember the Eimac
number) and the smaller 3-500-style socket (can't remember that either off
the top of my head) are the same.  Using the A7 tube (not the D7) the
filament requirements are the same.  When Frank, W3LPL was looking for
tubes earlier I meant to offer this as a solution.  Hopefully he was able
to round up sufficient stock.

Joe's comment is not quite correct.  Henry has been using 3CX1200s in 
several of their HF amps for years.  I know W4AN has had good luck with
these.  However, Joe's point should be kept in mind.  A tube without
widespread commercial use might suddenly disappear from the market.  On
the other hand, this didn't help the 4-400A...

The point of my earlier post was that even the recent new tube designs
from Svetlana have not done much to address the issue of load impedance
sensitivity.  My Alpha 91B is a great amp once its tuned up and
delivering power, but it doesn't appear to have any better tolerance for
load switching than my former Titan.  I wonder if George, K6GT could offer
any experiences with his homebrew 4CX1600A (great article George) or maybe
W4PA with one of the Titan II prototypes?

Rich Measure's recent post about running a 8171 at 2.5 KV and 1.5 KW out
is interesting too.  His comment about cooling at amateur power levels
and the chassis space of a 4-1000 frames the question nicely.  So what if
you can heat your shack with the filament power (pun intended).



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