[Amps] triode or tetrode

[Jim W7RY]

Agreed Carl... This is why I'm building another 8877 amplifier. 

73 
Jim W7RY 


> -----Original Message----- 
> From: km1h at jeremy.mv.com 
> Sent: Thu, 26 Aug 2010 11:26:36 -0400 
> To: sub1 at rogerhalstead.com, amps at 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 
> 
> 
> 
> 
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