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[AMPS] What (modern) tube to use?

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Subject: [AMPS] What (modern) tube to use?
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Date: Sat, 20 Feb 1999 12:58:36 -0500

On Fri, 19 Feb 1999 00:47:44 -0500 "David A. Pruett" <>
>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 
>the glass tube market narrows our choices immensely.
>One amplifier characteristic I have never seen discussed is a 
>tube's sensitivity to load impedance changes.  I suppose this is not 
>issue in commercial service, where the antenna is connected and the 
>is tuned up on one frequency and it runs forever.  (The recent note 
>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 
>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 
>very meticulous about antenna tuning, and unless your feedlines are 
>the same electrical length (or with additional half wave multiples) it 
>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 
>boom 6L, and a small 4L fixed south.  Find a spot in the band where 
>Bird 43 says the SWR is approximately the same (pretty close to 1:1).  
>up on antenna #1 to 1500W out, grid current under control.  Antenna #2 
>1000W out, no grid current.  Antenna #3 pins grid current, shut off 
>I bother reading the wattmeter.
>2. Homebrew 8877 on 40M.  Three antennas: full size 3L, N/S dipole, 
>dipole.  SWR's are not perfectly matched, but reasonably close (all 
>1.5:1). Setting up to use on 40M in ARRL SS.  Tune up on antenna #1, 
>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 
>off the grid trip).
>In situation #2, I pulled the 8877 amp and stuck in a Viewstar 
>No problem getting 1500W plus or minus 200W on all three antennas.  
>3-500Zs were saying "Did you change the load?  We didn't notice."
>From these experiences I've concluded that the modern, high-gain 
>tubes are very sensitive to load impedance changes.  My friend K3LR 
>around this at this contest station by loading his single-band 8877 
>heavier than normal, which keeps the grid current down and the tubes 
>take the extra power dissipation.  My experience with homebrew single
>4-1000A and 3-1000Z "wart hog" amps (refers to construction methods, 
>power output) is that these tubes are very tolerant to load 
>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 
>heat in or near the ham shack.
>Does anyone have any experience with grid-driven tetrodes with regards 
>load impedance sensitivity?

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

73  Carl  KM1H

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