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[AMPS] What (modern) tube to use?

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Subject: [AMPS] What (modern) tube to use?
From: (Terry Gaiser)
Date: Thu, 18 Feb 1999 23:09:43 -0800
Consider the 3CX1200A7. This IS a 3-1000Z in a ceramic-metal package. A
very rugged, instant on, tube.
Terry W6RU

"David A. Pruett" wrote:
> Joe Subich's note about which tube to use hit the nail on the head.
> W3LPL's recent post looking for 3-1000Zs illustrated how Eimac's exit from
> the glass tube market narrows our choices immensely.
> One amplifier characteristic I have never seen discussed is a particular
> tube's sensitivity to load impedance changes.  I suppose this is not an
> issue in commercial service, where the antenna is connected and the final
> is tuned up on one frequency and it runs forever.  (The recent note about
> the 8877 in KH6-land running 1900W out on 106 MHz day in and day out is a
> good example.)
> However, we amateurs change frequency routinely in the course of our
> operating, which means that the load the amp sees is changing too.  If the
> operating frequency is high, or the change in frequency is small, the
> effect on the amp is minor.  Most amps have some sort of trip circuitry to
> prevent damage in the event of operator errors during bandswitching.
> However, what about when switching antennas?  My station is set up here for
> contesting and we have more than one antenna on most bands.  Unless you're
> very meticulous about antenna tuning, and unless your feedlines are exactly
> the same electrical length (or with additional half wave multiples) it can
> be very hard to switch antennas without driving your amp nuts.
> Two examples from the K8CC log:
> 1. Ten-Tec Titan on 10M.  Three antennas: stack of widespaced 4Ls, a long
> boom 6L, and a small 4L fixed south.  Find a spot in the band where the
> Bird 43 says the SWR is approximately the same (pretty close to 1:1).  Tune
> up on antenna #1 to 1500W out, grid current under control.  Antenna #2 is
> 1000W out, no grid current.  Antenna #3 pins grid current, shut off before
> I bother reading the wattmeter.
> 2. Homebrew 8877 on 40M.  Three antennas: full size 3L, N/S dipole, E/W
> dipole.  SWR's are not perfectly matched, but reasonably close (all under
> 1.5:1). Setting up to use on 40M in ARRL SS.  Tune up on antenna #1, 1500W
> out, grid current under control.  After much trying I could not get the amp
> into a state where all three antennas could be used safely (i.e., not set
> off the grid trip).
> In situation #2, I pulled the 8877 amp and stuck in a Viewstar PT-2500A.
> No problem getting 1500W plus or minus 200W on all three antennas.  The
> 3-500Zs were saying "Did you change the load?  We didn't notice."
> >From these experiences I've concluded that the modern, high-gain power
> tubes are very sensitive to load impedance changes.  My friend K3LR gets
> around this at this contest station by loading his single-band 8877 amps
> heavier than normal, which keeps the grid current down and the tubes can
> take the extra power dissipation.  My experience with homebrew single
> 4-1000A and 3-1000Z "wart hog" amps (refers to construction methods, not
> power output) is that these tubes are very tolerant to load impedances.
> This is what a contester needs in an amp.  I don't want an amp that
> protects itself, which will take itself off line when I switch antennas and
> jump into a pileup.  I want an amp that doesn't need to be protected!
> Solutions with a 3CX3000, 4CX3000 or larger are OK, but the filament
> demands are excessive as well as the key-up idling current, even when
> running 1500W (yes, some of us do).  It might be tolerable for one amp, but
> I have six amps, one for each band.  Its hard to manage 2KW of filament
> heat in or near the ham shack.
> Does anyone have any experience with grid-driven tetrodes with regards to
> load impedance sensitivity?
> 73,
> Dave/K8CC
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