As I said, I am a believer, but the tradeoffs are more real than you allow
While the LDMOS are more rugged than the MRF150 style parts, and a I have a jar
full of 150s to show for it, they are still not as tolerant of faults as tubes.
The 65:1 SWR metric is not under CW conditions. Likewise the open/short You
Tube videos are not CW either. 3:1 is likely too high for most fully
available SS AMPs, not just for the transistors but the filters and matching
networks as well. Beyond survivability, you will see big variations in output
when you switch between antennas with 1:1 and 3:1 SWRs. You are correct on the
issues of the filters. I have had more filter failures than RF module
failures, with the most common component failure being the caps. If you are
going to build single band amps, use heavy duty power caps. With today’s
available SS amps with switched filters, and smaller parts, 3:1 SWR would be
pushing it.
Protection circuitry is important, but exhaustingly testing the many fault
scenarios is a time consuming and expensive process where pure economics
preclude completeness. A vendor with significant volume outside of the ham
market will be more likely to fund this activity. However, nothing will be
foolproof given the inherent small geometries of the LDMOS parts. Also, there
are surely failure modes that need to be protected with analog hardware since
microprocessor response times will be too slow.
Using multiple parts improves IMD, but lowers efficiency. The dynamic VDD
modulation from 4O3A sounds like an interesting way to keep the efficiency up.
I am curious to see how effective this is.
Overall, my approach to this may be conservative, but this is what it took to
stabilize the amps in my environment. All antennas have SWR’s below 1:5:1.
This helps with reliability and minimizes differing power levels when switching
antennas on the same band. Lots of airflow and temperature monitoring with
automated shutoff. All antennas selection is automated and interlocked with
PTT to keep from changing while transmitting. Manual antennas selection will
surely result in occasional errors. With all of that in place, antenna, coax,
and switch failures can still occur. I protect the amp the best I can hope for
the best. This is the risk I take for the operating efficiency they create.
I am excited by some of the value add being brought to market on top of the
traditional designs. 4O3A’s team is doing some interesting work and I expect
some good work from Elecraft. K3LR will team with top engineers and produce
top notch work that we can all benefit from. SS Amps are clearly the future,
but there is a lot of overpromising going on now and people with marginal
installations will experience failures and be disappointed.
Tom
From: Ria Jairam [mailto:rjairam@gmail.com]
Sent: Tuesday, December 26, 2017 7:08 PM
To: Tom Georgens <tomgeorgens15@gmail.com>
Cc: Jeffrey Maass <jmaass@k8nd.com>; cq-contest@contesting.com
Subject: Re: [CQ-Contest] Solid State Amps in Multi-Multi Stations?
I’ve used three models of SS amps here.
The dirtiest of them all is the Tokyo HyPower 2.5. Next up is the SPE 1.3K,
which uses a single LDMOS. Cleanest is the Power Genius XL (2 LDMOS) which is
under beta test here.
I think the concern about SWR and ruggedness is based on outdated info. The
latest LDMOS devices by design can tolerate high SWR, up to 66:1. With
de-rating the devices can handle around 3:1 SWR continuous at reduced power.
The limitation is in the output filter which are limited by saturation of
toroidal inductors. The newest amps are also microprocessor controlled and
protected to the hilt. I am more concerned I will blow up a tube amp than I
will a modern solid state.
IMD is less of a concern with two devices as there is plenty of headroom and
again - microprocessor control of biasing and other parameters such as is done
with MEffA in the PGXL produces a very clean signal, even without adaptive
predistortion.
I do agree that in a K3LR type of environment there is little benefit to change
out all of those existing, working tube amps for SS ones. That is, other than
as a trial run, or experiment to see how well they can do. However if designing
from scratch or expanding, amps can be shared between run and mult stations in
M/S or M/2. Even in M/M it provides some redundancy.
Ria
N2RJ
On Mon, Dec 25, 2017 at 9:07 PM Tom Georgens <tomgeorgens15@gmail.com
<mailto:tomgeorgens15@gmail.com> > wrote:
I have been using home brew SS amps at my 8P5A station exclusively for the
last two years. The flexibility and increased operator efficiency are a
big advantage. As such, I am a true believer for this use case.
However, I am not sure that they are a better solution than single band amps
for the pure Multi Multi. With the operating efficiency of instant band
change largely moot, there is less to offset the down sides. With
potentially 6 full power signals at once, and in band receivers, signal
cleanliness is a key factor. The LDMOS parts simply do not have the IMD
specs of tubes. K3LR has supreme engineering skill and may be able to
design around some of these limitations on a per band basis. Plus, the
predistortion techniques are very exciting, but not limited to SS amps.
Overall, if it can be done, K3LR will find a way, but I would not be sure
that current off the shelf solutions would be the cleanest for M-M.
Overall, I think creative engineering can help with IMD, but it is not
standard today. Also, the parts have come a long way, but the LDMOS
transistors are not as rugged as tubes in most fault situations. If kept
cool, reliability will be fine with well matched antennas, but will not be
as tolerant of antenna system faults. Without the need of the flexibility
of instant band switching, I am not certain that SS amps are a better
option than single band tube amps for the dedicated M-M station.
Situations other than M-M can be good matches for SS Amps, but the need for
a well controlled environment and well matched antennas is still there.
Just to be clear, I am not claiming to have solved any of these problems,
just that I feel the advantages outweigh the risks in my case. YMMV
73, Tom W2SC 8P5A
-----Original Message-----
From: CQ-Contest [mailto:cq-contest-bounces@contesting.com
<mailto:cq-contest-bounces@contesting.com> ] On Behalf Of
Jeffrey Maass
Sent: Friday, December 22, 2017 10:13 AM
To: cq-contest@contesting.com <mailto:cq-contest@contesting.com>
Subject: [CQ-Contest] Solid State Amps in Multi-Multi Stations?
Is anyone using solid-state amps at a competitive multi-multi station?
Which? Any problems?
73, Jeff K8ND
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