I clearly do not fit the criteria of " savvy hands-on experimentally
inclined" but I have built two SS Amps and used them exclusively at my
contest station (8P5A) for the last two years. I have tried off and on over
the years to build one, and have bought a commercial version, but none of
them were satisfactory.
I don't claim to be an expert, buy I do have some empirical observations.
1. The LDMOS parts are a huge step forward. The prior parts, typically
needing matched pairs or quads were far more fragile and, failing one is
failing two (or four). This was expensive, frustrating and frequent. As
improved as the LDMOS part are, they are far from indestructible and no
match for tube reliability. The 65:1 SWR spec does not apply to CW where
far more modest mismatches will destroy the parts nearly instantly. I have
had one of the 1.25KW parts fail on me.
2. A key to reliability is headroom. The closer you are to their rated
limits the more delicate they are. I am running two 1.25KW parts at 1KW
(total) out. I believe it has helped reliability, but efficiency goes way
down when you run that far below max output. That means more heat
3. The good news is that you do not need a matching network. The bad news
is you do not have a matching network. Output varies quite a bit as the
load impedance changes and you can't do anything about it. My solution is
to flatten the SWR of all my antennas so I can switch between them without
seeing big excursions in the output power. The worst my amps will see is
1.5:1 and most of the operating occurs below 1.3:1. Manufacturers will
disagree, but I think if your SWR exceeds 1.5:1, you should use a tuner.
4. The filter has been a bigger challenge than the RF module. I have had
multiple cap failures early on. Some have been due to other design issues,
RF in the selection logic etc.. I have not had a cap failure this year, but
I really do not know how much headroom I have since the specs for RF current
are sparse. Another reason to minimize the mismatch at the output of the
amp. Also, I concur with the comment that a diplexer is a must. The third
harmonic is only about 10-11 dB down. It is good to know where that power
is going.
5. The splitter/combiners have been simple and reliable.
6. IMD is not good. There may be clever feedback schemes to help with
this, but it is over my head. My RF modules have been purchased as pallets.
I do not think these parts are optimized for linearity and they are nowhere
near as good as tubes. Generally the IMD is better at lower output. I
really am hoping that some clever "predistortion" algorithms will find their
way into the SDR radios to offset some of the deficiencies of these parts
7. Heat removal is manageable for CW and SSB. I have not tried RTTY, but
there is a lot of heat in a very small area so it needs serious heat
conduction and airflow. If I were to build one for home use, I would try
water cooling. Some of the pallets now come mounted on heat spreaders
milled for water flow.
8. While energy is still energy, it is a lot easier to work with the cover
off when there is no 3500V exposed. That said, it is still dangerous if you
are not careful
9. If all these limitation exist, why bother? It is simply too convenient
to ignore. In 48 hour contesting, efficiency is very important and it is
game changing to operate with what feels like a KW radio. This implies that
the amp filtering will follow the radio. This is a must. IF the amp does
not have automatic band selection, you will blow it up some day, protection
circuitry notwithstanding.
10. No doubt solid state is the future and, in a very controlled situation,
can be made to work well under demanding circumstances. However, will they
be as reliable and as linear as tubes for a wide range of load mismatches,
high duty cycles and operator errors? Not today
Like I said, I am speaking as an integrator, not as a designer. Many of the
shortcomings can be improved with better designs, but the underlying
technology still has its limits.
73, Tom W2SC
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Leigh Turner
Sent: Wednesday, December 07, 2016 4:42 PM
To: 'AMPS' <amps@contesting.com>
Cc: leigh.turner@ieee.org
Subject: Re: [Amps] SS amps and auto-tune. Is it even necessary?
I agree with the spot-on-the-mark sentiment summary expressed below.
A very fine QRO legal-limit S.S. amp with decent IMD specs can be homebrewed
by the savvy hands-on experimentally inclined (not many of us left?) amongst
the ham radio fraternity.
Jim is right about the complexity and cost of "everything else" particularly
implementing the requisite diplexer based harmonic terminating auto-switched
HF multi-band LPF capable of handling up to 2.5 kW under potentially high
VSWR conditions. In terms of dollars per Watt the modern LDMOS FET active
devices per se are very cheap.
If one is designing a monoband S.S. QRO amp then some complexity and
associated cost savings can be had.
Adequate heatsinking and dealing with the demanding thermal issues (the
Achilles heel of LDMOS and MOSFET RF power devices) and efficient heat
extraction is the other principal challenge. This has a big impact on
semiconductor MTBF.
It can all be done; but not cheaply :-(
Cheap, reliable / good MTBF, no time limit QRO, low IMD, are mutually
exclusive design goals. The pragmatist has to take their pick which
parameters they value most; choosing them all is an expensive exercise even
for the resourceful homebrewer; commercially we are talking over $10K ++ for
a high-end S.S. amplifier.
Leigh
VK5KLT
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Catherine James
Sent: Thursday, 8 December 2016 3:45 AM
To: amps@contesting.com; Jim Thomson
Subject: Re: [Amps] SS amps and auto-tune. Is it even necessary?
On Wed, 12/7/16, Jim Thomson <jim.thom@telus.net> wrote:
## The low cost of the LDMOS is the least of the issues involved. Its
everything else that?s involved with the SS amp.
## Jim VE7RF
Yep, that's it in two sentences. That's why I can't get excited about cheap
SS finals until I see a complete, commercialized SS amp using them, said amp
to be affordable and reliable. Right now they are for experimenters.
73,
Cathy
N5WVR
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