> On Sat, 29 Mar 1997 13:35:23 -0500 (EST) "David L. Thompson"
> <thompson@mindspring.com> writes:
> >Technology is is place to build solid state 1.5KW amps. It has been
> >since
> >the early 80's when QST published an article on an all band 1KW amp
> >(1982).
<SNIP>
> >Another amp the "SSKW" was published in QST about 1990 or 91. Part of
> >the preface to this article talked about all the difficulties in
> building
> >a solid state KW (poor IMD, poor regulation etc)
>
> The first real attempt in ham literature to use FET's if I recollect.
No, the two part QST article in the early 80's used Motorola
MRF-150s, which are FETs.
> >Another aspect required for the bigger solid state amps is that the
> >traditional 13.8V supplies could not product the 100 to 200A for these
> >amps. Research done in the commercial maket showed that 28 or 50V
> supplies
> >were more suited. I have not seen an ad in a while but Lucas
> Thansformer
> >was making high quality 28 and 50V models which is what Ameritron/MFJ
> used
> >for the 600W model they sell.
<SNIP>
> I fail to understand the reasoning behind archaic linear LV supplies. The
> technology is mature and parts readily available to build 28 or 50V
> switching supplies that are only a tiny premium over a well built linear
> of equal ratings. A ham friend of mine locally is a highly respected
> power supply designer of primarily Mil-Spec switchers. He threw together
> a 100A 13.8VDC unit for me that is smaller than a 20A Astron. I have
> him almost convinced to do a 50V 100A unit next.
>
> As 50V or higher FET's come down in price I am sure we will see more
> homebrew of 1500 ++ W SS amps. A key part of the pricing problem is all
> of the components required for the output filters. They add up to big $$
> real fast at those power levels.
50V and 100V (or higher) devices will give you better IMD products.
Years ago, I worked for a company that produced SS RF power
generators for 'sputtering.' We had a 2KW unit that would have
been great for 20M. But, how do you get rid of the fan noise ?
It would have been easily adaptable for all bands, with input
tuning networks, etc. The output filters from the 80's QST article
could be used. Another thing is that you start adding all this
stuff, and the thing gets BIG and BULKY.
Other problems with SS amps are VSWR and noise from the cooling
fans. You can probably remote the amp (ARD) or try some other
means of cooling to keep the noise down. VSWR can be a real
problem however. Think of a SS amp as an extension of your
solid state radio. When does its (the radio) SWR protection circuit
kick in ? The same 'rule' would have to apply for the SS amp as
well. Probably some sort of automatic antenna tuner would have
be incorporated at the output (like the JRD amp), to prevent
SWR from killing devices.
> The CB crowd use the 2SC2879 in a quad arrangement of 4 PP modules to
> generate 1200W as drivers for their big amps....no filters of course!
If you are going to use bi-polar devices, you would have to build a
'bias regular' circuit to prevent thermal runaway.
> Even with the higher cost of components today I
> still
> >see no reason for the pricing for the JRC or ICOM 4KW models. A SSKW1.5
> >model could easily be built for around $1,000 for the amp deck and a
> large
> >Lucas supply runs about the same. I cannot see much more than another
> $400
> >or 500 for control circuitry and cabinet.
>
> I think you are way underestimating the RF deck price Dave. I would
> figure $2000 minimum for a 1500W amp with a decent amount of breathing
> room (say 2KW capable) and 50V FET's.
MRF-150's in small quanities are not cheap. Our design used 16
devices. 8 per module. 2 modules, 1 KW output each. Kick in
2 broadband input transformers, 2 output transformers, power
supply, input/output vaccum relays, etc. We also incorporated
software/computer control to monitor Power Dissipation, Id,
Vd, Output power, etc.
As with anything in the amateur market, I guess it's pure
economics..law of supply and demand. How many would a
company REALLY sell ??
Tom, N2SR
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