Agreed Carl... This is why I'm building another 8877 amplifier.
73
Jim W7RY
> -----Original Message-----
> From: km1h@jeremy.mv.com
> Sent: Thu, 26 Aug 2010 11:26:36 -0400
> To: sub1@rogerhalstead.com, amps@contesting.com
> Subject: Re: [Amps] triode or tetrode
>
>
>>
>> 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
>
>
>
>
> _______________________________________________
> 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
|