On 8/26/2010 9:33 AM, Jim Hoge wrote:
> I am kicking around the idea of a new amp ( I want 160m capability which my
> current amp doesn't have) and have a few questions for the gurus and those
> that
> have theses beasts. Am I better off with a triode or tetrode based amp? Does
> it
> really matter? How about a one or a two holer amp? How much headroom should I
> look for in a legal limit amp? Some designs are nearly at the limit of the
> tube(s). How about auto tune capability? Do the amps that have this feature
> find
> the best match or will an override/manual tune do a better job? Any other
Jim, you are probably going to get as many opinions as there are people
willing to post.
IF you are going to purchase or build makes a big difference.
I prefer Tetrodes as they take less drive, BUT Triodes make for a
simpler and easier to build circuit.
That's not a problem if you are purchasing a new, well designed amp. I
think you will find Tetrodes typically have better IM products, but is
that a problem with modern designs and tubes?
Headroom? Now we get into some sensitive territory, particularly if you
want to run contests and digital. Many amps that are advertised as legal
limit have to be derated when running digital. Those same amps are
likely to get pretty hot when contesting. OTOH that's not much of a
problem for casual rag chewing, or casual DXing unless you tend to be
long winded like some of us<:-))
A good manual tune amp that has enough headroom for contesting and
digital is going to be pushing the limits of FCC certification. There
are some, but if running CCS or CAS I'd prefer something that could
easily hit 2 to 2.5 KW in normal use (without overheating) . Something
like that should do well at the legal limit in contests or digital. You
probably won't find many with those ratings that have the FCC's blessing.
OTOH Alpha does have an auto tune unit they *advertise* as continuous
duty at the legal limit. We need some Alpha owners who are also
contesters to chime in on that. I understand that although the amps
have the head room it warns you when you hit the legal limit. It just
depends on how much you are willing to spend.
Most, but not all, FCC approved amps that can run the legal limit do not
have a lot of headroom. More often than not it's limited by the
transformer rather than the tube. 8877s have the dissipation, but only
IF they have enough air. For instance at 2500 watts input at an
optimistic 60% efficiency that is 1500 out with only 1000 watts of
dissipation. To me that's a small tube that takes a lot of air. 1000
watts makes a good space heater. OTOH if it does have enough air it
should last a very long time. Still SSB is figured at only about a 20%
duty cycle.(depending on which book you read) The tube should loaf
along at that.
73
Roger (K8RI)
> tidbits of wisdom?
>
>
> Tnx,
> Jim W5QM
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|