[Amps] Dedicated RF

[Jim Thomson]

Date: Sun, 29 Jul 2012 13:44:45 -0700 (PDT)
From: "Jerry Kaidor" <jerry at tr2.com>
Subject: Re: [Amps] Dedicated RF

A lot of new rugged devices have hit
> the market
*** Indeed. I was quite excited by the new Freescale devices - they have
a 1200W output one, protected up to 60-1 VSWR, and just $250 from Digikey!
Each device has a PAIR of RF FETs, suitable for a push-pull stage.

- Jerry Kaidor, KF6VB

##  I see you just got sucked in by the BS hype.  No it won’t do 1200w out.  Well, it will
in radar pulse mode, and even then the duration of each pulse has to be really short.  And
no, it won’t handle a 60:1  swr mismatch,  that’s pure drivel. 

##  To get good IMD3  out of that device... and  I mean, like –40db below one tone, 9-46 db pep)
it has to be severely de-rated.   Run it in CB mode..and  IMD goes to hell.   Its maybe a 400-500w pep out device
at best. 

## My problem with these  SS amps is this.   You require an RF deck that covers 1.8 to 30 mhz.  Now
like Rob sez.. that’s a 16:1  ratio.... which is no small feat.   You also require a power supply.  You also
require a 2000 watt  CCS  rated automatic ant tuner, so the SS amp doesn’t blow it’s brains out. 

##  I don’t see any ferrite combiner’s  that will cover  160-10m..and work eff.   On top of all  that, the
RF deck also has to have a 2 kw CCS rated  LP  filter for EACH band !!   Then you gotta  switch the entire
mess  with 2 kw rated relays etc.   Well, you could get away with a minimum of   7 x  LP filters  to cover 9
ham bands.  

##  BIG difference  between a  1 kw CCS, class C  FM broadcast  TX... that sits on ONE freq , and has 92% eff
vs a 160-10m amp, operates in class AB, that barely runs 50%  eff, and MUST be a linear amp.   That’s a HUGE
difference  in design demands.    The single freq broadcast TX is a piece of cake, and is much simplified.   

##  any tube amp doesn’t  need  a 2 kw  LP filter, never mind  7 of the damn things.  A tube amp also doesn’t need
an automatic 2 kw tuner either.

##  ever tried   fixing  a SS  amp.....what a bitch of a job.    A  buddy had a  spike-transient come in from the street on his
240 vac line..and it took everything out, and I mean everything.... on his megabuck ICOM SS amp.    It had just gone out of warranty
and he was just livid. 

SS amps are not ready for prime time...not yet. 

Jim  VE7RF