GeeWiz,
1KW comes from I was talking about high power which generally starts at
1KW and up Guess all the broadcast stations up here are behind the times
because they use tubes instead of SS the exciters are all SS power
amplifiers are hollow state. Channel 9 did have a 25Kw backup
transmitter which was SS As to your second point I think you proved
mine all the SS finals need a external matching network to protect them
from failure or they have protection circuitry which either shuts down
the final or progressively turns down the power as SWR exceeds 2.5 to 1
Think of this all NMR and PET scanners use tubes to generate the RF
fields needed. These are cost no object devices if semiconductors
could handle the antenna systems used in these devices they certainly
would as I think most hospitals would prefer not to replace a dozen or
so pulse rated ceramic tubes every 250 hours or so as the tube bill and
the concomitant downtime are quite expensive
I am not sure that having more manufacturers enter the market will
decrease the cost very much heck even MFJ has a SS amp and it's
expensive. Simple fact is that high power RF semiconductors are a low
yield process and so they tend to be expensive. I have a spare set of
transistors for my 2KL and I shudder to think what I paid for them they
were as expensive as a new US 4CX1000A for the collins and much less
forgiving of weird loads. I love the convenience of my 2KL no standby
heat, instant bandswitching but I cannot run it into some of my
stranger antennas like the NVIS setup for MARS. What will drive down
the cost is increased demand and improved fabrication processes for the
RF transistors probably on non-silicon substrates so that breakdown
voltages for the junctions will be in the low thousands of volts instead
of the low hundreds as they are now once the breakdown voltages are that
high non-optimal SWR should cease to be a issue as it will no longer be
able to cause the PA stage to fail.
What is interesting is that tube amps are now getting highly efficient
switch mode HV power supplies which removes one of the largest costs in
building a tube amp i.e. the linear power supply
I think that SS amps are now beginning to be practical but they are
still very much in a early adopter phase right now. Even the military
is usiing them for medium power <1KW check out the 400W brick amplifier
for the Falcon II radio sets AN/PRC-150 high power is still hollow state
Tubes will go into the history books when semiconductors become as
forgiving as tubes when operating into non-ideal loads and not until.
Rob Atkinson wrote:
> Let's see where to begin....okay two comments: First, please explain what
> you mean by "commercial practice." If you mean broadcasting, no
> broadcaster in his right mind today is running a tube rig unless he has to
> (where you came up with that 1 kw and above factor I have no idea), but "has
> to" is almost unimaginable now since you can get a 250 w. s.s. AM BC rig for
> less than a lot of ham transceivers, and any of them including 50 kw boxes
> practically pay for themselves in about 3 years because they are 90%
> efficient, need essentially no maint., and just sit in a rack or cabinet and
> tx night and day for years--no tube tx does that...and if an RF module does
> fail, no big deal they are hot swappable, you loose a few hundred watts, pop
> in a good module and on she goes unlike failure of a final tube where you
> hope the aux fires up okay or ur off the air....go to NAB, ask any broadcast
> engineer if he'd like to run a tube rig...I've never had any of them tell me
> yes to that question.
>
> secondly, all statements below were made pretty much 35 years ago when solid
> state exciters started coming out and the last time I checked on the bands,
> lots of hams were operating s.s. rigs into all kinds of loads albeit with
> matching networks, and not many are giving up and going back to tube rigs so
> they can tx into a bedspring or some bizarre load without a tuner. So, no,
> I think the market for what is basically a 1.5 kw 2nd PA on your exciter is
> going to soon spring open as hams retire their tube amps for one reason or
> another and the costs of these amps start to drop as more mfrs. enter the
> market. Now, that doesn't mean everyone is going to run out and get one,
> to be sure. I for example ragchew and park on a single frequency for hours
> sometimes and I don't mind manually tuning everything; in fact I enjoy it,
> so I won't be in any big hurry to get rid of my Centurion. But if Mother
> Nature got rid of it for me, and I had the $$$, sure, I'd go s.s. and never
> look back and I doubt if I'm the only one.
>
> 73,
>
> rob / k5uj
>
>
>
>
> Unsigned message posted below:
> <<<Commercial practice STILL uses tubes for 1KW and above. I have a Icom
>
> 2KL which works nicely IF the SWR is less than 2:1 - IT NEEDS its
> companion AT-500 (which I also have) to keep it happy Also have a
> Harris AM-7223 and a Collins 30-S1 both the Harris and the Collins will
> load up into a bedspring without a external tuner the Harris will tune
> automatically the collins we do the old fashioned way. I think tubes
> will be around for a while yet until truly HV junctions can be devised
> at a economical cost at which point the impedance mismatch issue will
> become moot and hollow state devices will pass into history
>
> Part of the issue here is that SS finals are designed to load up into a
> 50 ohm characteristic impedance ONLY where a tube anplifier is designed
> to compensate for a complex impedance and it considers the antenna
> system as part of the tank circuit since the tube can handle the high RF
> voltage created by a impedance mismatch it is possible to tune the
> circuit until it comes into resonance and a reasonable impedance
> value. Since the transistors junctions cannot handle a high RF
> voltage the protection circuits need to reduce power until the reflected
> voltage is within the junctions ability to handle the voltage.
>
>
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