On Fri, 19 Feb 1999 00:47:44 -0500 "David A. Pruett" <k8cc@ix.netcom.com>
writes:
>
>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
Nice station Dave!
When KQ2M used to flog my station in contests the run amp was a pair of
3-500's that took all the abuse he gave them. My antenna combinations
were similar to yours and other contesters.
The multiplier amp was a pair of 3CX800's that I had drive attenuated
down to around 1400W max and had marked the dials for heavy loading. That
was the only way it survived all those years.
I use a grid driven NCL-2000 on 6M with multiple switched antennas, and
had one on HF from 1964 to around 1986. The screen current was/is very
critical of the loading.
On 6M I wound up adding short coax jumpers to force equal loading between
the 3 antenna choices.
The 3CPX5000A7/YC-156 seems to be an ideal tube since it would be
indestructible at the 1500W level and the 225W filament is a manageble
66% increase over a pair of 3-500's. Medical pulls are appearing
regularly at $250 since there is little market for them. The CBers dont
like them!
I suspect that the 3-500Z will be around for a long time. Building
monoband amps using 3 tubes would give you plenty of reserve overhead and
long term reliability. Build them with low Q tanks for less loading
susceptibility.
73 Carl KM1H
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