Rob, You have to remember that the tube ratings are for CCS not SSB or CW
service. CCS service is 100% duty cycle, SSB is 30% duty cycle and CW is
about 45% duty cycle. It is not the higher plate current nor the higher
grid current during the cycle that kills the tube, it is the constant over the
top plate dissipation that kills the tube. Running an AL-80B at 1000 watts
output on SSB will barely show much anode color compared to the same anode
color at CCS rating at 750 watts output. That amp was rated by it's
designer W8JI and he sure knows what he is doing. I tend to agree with him
since I pay close attention to the tube anode color with a pair of 3-500's
running at 1500 watts PEP on SSB. Although that would be in the rating of the
tube anyway. In fact henry I believe rated one of their 3-500 amps at 2000
watts output on SSB, so that is in line with Ameritron's claim.
In a message dated 5/23/2010 10:18:54 A.M. Eastern Daylight Time,
ranchorobbo@gmail.com writes:
you say you are getting 400 w. with 520 ma Ip. I think that's 120 ma
over the max Ip for a single 3-500 so you're possibly in trouble right
there.
you say you increase drive and get no more suds which means the tube
is probably saturated.
An AL-80B on any band putting out 900 w. is a bad idea regardless of
what Ameritron says. It is a lot easier to _sell_ a "1 KW amp" than
it is a 700 w. amp, which is what the AL80B really is (actually that
power output is optimistic) unless you don't mind getting a new 3-500
every few years. If you consider the max rated plate current of a
single 3-500 (400 ma) and look at the specified power supply v. in
that amp (2700 v. no load as I recall) and figure it sags down to 2400
under load and assume 70% efficiency (giving them the benefit of the
doubt) you get a choice of either abusing the poor single 3-500 or
discovering some promotional hype license is being taken (In ham
radio? You gotta be kidding me!). That amp is probably what is
keeping 3-500zg rolling out of some tube plant in China, so maybe we
should all be glad. Every time I work some ham operating that amp
and driving it over 700 w. I politely tell him what's happening and
get either anger or the guy never comes back to me. 3-500 used to be
sort of cheap for the watts but they are getting expensive now. Oh
well it's not my money.
But, back to your point, I'd drop the drive down to where you are
getting < 400 ma on the plate and be happy. How much power does it
take to have a QSO on 6 m. anyway. I have never operated that band
but I thought the "magic band" either let you have a QSO with 59 copy
and 1/2 watt, or no amount of power would get through. Nothing in
between.
As for 100% duty cycle, if the cooling is stock, you probably want to
put in a fan that moves more air. No matter what you do, 520 ma is
too much plate current.
73
Rob
K5UJ
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