[Amps] Sad News for Eimac Users

[Gene May]

In some earlier postings, someone pointed out the lack of cleanliness and apparent linearity in the output of many of the solid state transceivers on the market.  This person, IMHO accurately, also pointed out an apparent decline in the ARRL's standards concerning higher IMD in their test reports.
 
I think Manfred is probably correct in his predictions, but I hope that both amateur and commercial design improves on the linearity of transmitters, high and low power.  At least now, it seems that properly designed tube amplifiers are still cleaner.  It's nice to tune around the 30 meter band and not hear so much crud between signals.  While that is hardly a definitive test of the cause, could the relative lack of QRO in that band be part of the reason for the narrower signals -- because power output is limited in this band in the USA?
 
I am puzzled as to why the solid-state devices seem to be more linear than tubes ("fire-FET's"?).  As I understand it, FETs are square-law devices that generate mostly low-order and second-order distortion products, while tubes are 3/2 power-law devices that generate more and higher order distortion products.  Does anyone out there with a better comprehension than I have of the physics in this matter have an explanation? 
 
Gene May
WB8WKU

++++++++++++++++++++++++++++++++++++++++++

> Date: Mon, 24 Sep 2012 21:33:05 +0000
> From: manfred at ludens.cl
> To: amps at contesting.com
> Subject: Re: [Amps] Sad News for Eimac Users
> 
> 
> Neither would I.  Unless I needed one as a replacement for an existing transmitter or other device, that is otherwise still good, and too expensive to replace completely.
> 
> But for new design, I think one would need to be blind not to see the writing on the wall.  Take the hint!
> 
> When in the 1970s receiving tubes started to get more expensive and harder to find, while transistors were already dirt cheap and very good, there were still many designers who were reluctant to learn their trade anew.  They kept designing with tubes, until they simply lost touch with the real world.  Those in the industry lost their jobs, while hobbyists continued to design with tubes until they died, as part of they hobby, in which there was no pressure to be competitive.  Then in around 1980 the same happened with transmitting at the 100 watt level.  And then, in the 90s, it reached the kW level.  The lower power tubes are mostly gone from production, except for some used by the esoteric audio gang, which is large enough to sustain some small factories in China.  The industry has since mostly dropped tubes in the 1kW class sector, be it transmission, medical, or industrial uses.  They are using them for replacement purposes, in some cases, and this is dying out.  Many hams are moving to 100% solid state up to legal limit power right now.  There still IS room for home builders to build tube amps just for fun, but this group will hardly remain large enough to sustain factories like the audio gang is doing.  That means that 1kW class transmitting tubes WILL be discontinued, and most of them rather soon.  Probably existing stock will last nostalgics for several decades, so don't cry, but the technically sensible thing to do is to leave this old technology behind and take advantage of the modern developments.
> 
> Manfred
>