>
> 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?
A couple of choices:
Three 4-400's in swamped grid or GG will give a reliable 1500W with plenty
of overhead even at 2500-3000V. In GG they require 120+W drive for that
power if down in emission. Pulls are often cheap and even NOS military can
be found very reasonable. Use the Amp Supply LK-550 circuit for GG, I can
supply input board values. Of course you can also use 3-500's. Buy some
parts from Ameritron.
3CPX15000A7 medical pull from reputable seller $250-350 often with full
output even on 10M. Use one of the available triode boards and the rest is
simple. A regular 8877 is an alternative. Same comments about Ameritron.
Chinese 8877's are reported as very reliable at $650 new.
3CPX800A7 medical pull, about same prices as the 8877 but you may get lucky.
With 3 of them and modest air they will run 1500W 24/7. Guys with the dual
voltage LK-800's are getting 5000W PEP with 150W drive and the blower on the
high speed position. With the single voltage power pak and 100W drive its a
reliable 2400-2500W as they arent even coming close to being pushed.
If I was building and didnt want to go real HV it would be a pair of either
8877 variety, no more than 3000V and low and quiet air flow if its really
going to run 1500W even in contessting and digital modes or RTTY.
A medical pull YC-156 will easily run 1500W out with only 3KV and 100W
drive, about $350. The high output C requires some creative L-Pi or L-Pi-L
outputs for 10M. No socket and bolts to the chassis. It will be the last
tube you will ever need....even as an AM linear (-; For those not familiar
with this tube it has 3CX15000B7 internals and a 3CX5000 anode; lots of
reserve emission and fantastic IMD performance.
If I was modfying an existing amp it would be an Alpha 76 3 holer which can
be found cheap with bad tubes, get the one with the optional Dahl
transformer. Go with a pair of 3CX/CPX800's and you have a fine amp with
minimal retrofit work.
Obviously I dont care for Russian tubes.
Carl
KM1H
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