[Amps] Price per Watt Conversation

[Roger (K8RI)]

Efficiency is not high on my priority list "as long as the heat 
generated is easy to manage" and ceased to be a concern when the legal 
limit went to output instead of input..  OTOH predistortion, or Adaptive 
predistortion would appear to allow both efficiency and quality.

At close to 200W per device, it would require 8 devices in a circuit 
much like the Yaesu Quadra, but with a legal limit out for less money.
BTW I saw several posts praising the Quadra, but the Quadra has several 
inherent problems in the basic design according to the Yaesu news 
groups..  One is the PS is designed for 220 VAC and not the current 
standard of 240 leading to a number of failures. Another is the way the 
power supplies feed the individual amps. OTOH it is a nice amp and I do 
see a number of hams claiming to be running 1200W out for many years.  I 
have two.

73, Roger (K8RI)

On 4/25/2017 8:37 AM, Jim Thomson wrote:
> Date: Sun, 23 Apr 2017 22:43:31 +0000
> From: Manfred Mornhinweg <manfred at ludens.cl>
> To: amps at contesting.com
>
>
> Jim,
>
>> ##  why not use   4  x  BLF188XR    or    4 x MRFX1K80H ?
> Cost is a powerful reason against that, in my opinion. It would be
> expensive overkill. I do think that using 4 devices (or more) is a good
> idea, but then these should be cheaper devices.
>
> A few days ago I was looking at the IXZ210N50L2, which should be
> suitable for a multi-device legal limit linear class AB amp. Each of
> these individual (not Gemini) MOSFETs should be good for close to 200W
> RF output in linear service, and they are 28 bucks apiece at Digikey, in
> single quantity. An added benefit is that they run from 100 to 150V
> instead of 50V, which should allow achieving better efficiency and
> linearity.
>
>> Is it even  possible to use 2 x devices in parallel in each half of a
>>   push-pull amplifier ?
> At 50V supply, the drain impedance for a single 1500W amp is so low that
> it's really hard to handle properly. It's much better to make separate
> lower power amp modules, and combine them.
>
>> If it is possible, then the heat could be extracted  over 4 x
>> devices.
> Yes. Or 8 or whatever. It makes for a much simpler, smaller, lighter
> cooling solution.
>
>> The TX imd, with 4 x devices run at 1.5 kw, on paper, should be good,
>>   since each device is running at 375 w pep output.
> Don't let this fool you! If you put each of the 4 devices in the typical
> circuit designed to saturate at 750W (1:9 transformer at the output),
> then indeed you will get good IMD at 375W, but also you will get an
> absolutely lousy efficiency. And if you instead use a lower ratio
> transformer, to make each module saturate at 400W or so, then you are
> back at square zero, with "normal" efficiency and IMD! The only thing
> you gain that way is the easier heatsinking, and/or robustness against
> overheating damage.
>
> ##  I was afraid of this.   We are back to square one.   50%  eff and lousy imd,
> or better IMD and lousy 35% eff..take your pick.  My  1000-d and also 1000-MP-MKV
> run 50%, when run at 200w out.  back po down to 100watts..and eff drops to 35% ..but
> imd improves a bunch.
>
> ##  so say a pair of  MRFX1K80H in PP. run at 750w out.   Then an identical 2nd pair in PP,
> also run at 750w out.   Then combine them for 1.5 kw out.    Lousy eff and good  imd
> ...or normal 50% eff..and lousy imd.   We just cant win.
>
> ##  You may well be able to put 2 x  50-60 vdc switching  supplies in series to obtain your
> 100-120 vdc  for your proposed
>
> ##  baffles me how SS  marine xcvrs all have superb IMD..and also run on 11-15 vdc.
> Are they using loads of NFB ?
>
> Jim   VE7RF
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