Phil, K5PC says
>The use of variacs with capacitor input power supplies is not good
>engineering practice. It can reduce a nice, stiff 240 volt line to mush
in many >cases.
Depends on how far up the variac you are. If you aren't using much of
it, they are pretty good. We used one on the test rig for the Marconi
P2000 tx in the mid 60's - that was for a 18Kv 3 Amp supply. If you're
talking about using say 60% of the output, they can be, as Phil says,
awful. If you are using it to just gently wind up the input from zero to
the input line voltage, that's a different matter.
>>#6. Tank circuit with appropriate RF current rated components.
>>This surely comes down to deciding what the application is. If you
want
>>RTTY at 1500 watts for half an hour or more, then it's very different
to
>>non compressed speech with one minute messages every hour. If you
want
>>a contest amplifier, then you're better off with the RTTY approach.
>IMO, this is the most flagrant violation of good engineering practice
in
>today's "store bought" amps. Tank circuits that were used in 500 watt
amps
>15 years ago are now passed off as 1500 watt units. 1/4" tubing on
10-20
>meters, tapering off to at least #10 wire for 40-160 meters should be a
bare
>minimum. A 100% duty cycle 1500 watt amp just cannot be tucked under
>one arm and walked away with, and it is time the newcomers were
educated >about this.
I guess this should apply to newcomers in the professional design area
as well, Phil. I must admit I only use 3/16 tubing, though! Having said
that, amongst my prize possessions is a book I inherited from my father
- Wireless Valve Transmitters, of 1925. This has a photo of a 10 watt Tx
for 160metres, and the tank coil is 3/8 inch tubing, 6 inches diameter!
Those were the days....
>>#7. Good input swr with any transceiver.
>>
>>I must admit I don't understand this. If the input SWR is good
>>(<1.5:1?), then why should the driving transceiver come into it? Why
>>should the input impedance be dependent upon the driving transceiver?
Or
>>have I missed something?
>What a lot of folks do not realize is that the new broad-band solid
state
>transceivers do not have a 50 ohm output across all nine bands. The
power
>ratings are "into a 50 ohm load". A variable tuned input with controls
>brought out to the front panel would be convenient and produce a
cleaner
>output.
Hopefully, no tx has a 50 ohm output impedance. If it does, it's limited
to a 50% efficiency. Desirably, the output impedance of a transmitter is
zero ohms. Although this apparently violates Thevenin. To strictly apply
Thevenin, you need an infinite power capacity, or at least one high
enough that the matched power condition can be attained. The fact that
practical linears tend to come out about 50% efficient gives them an
effective output Z of around 50 ohms, give or take, but that's
fortuitous. A Class C or a Class D PA running at 90% efficiency doesn't
look like a 50 ohm source. So I still don't follow. If the amp input
looks like 50 ohms, where's the beef? This does mean by the way, that
the amp should preferably look like 50 ohms over a wide band, not just
the operating frequency, as a number of solid state PA's can go unstable
if provided with a load which is 50 ohms at the operating frequency, but
none 50 ohms elsewhere - the Swan 100Mx was a classic for this. And the
idea of input tuning is to make the source impedance look low for a GG
amp. Another reason why I prefer passive grid...........
>>#8. Glitch protection.
>>
>>I assume that by this, Rich means the 'spike' of energy of much higher
>>level that the transceiver can produce before its own ALC grabs the
>>output.
>snip.......
>What Rich is talking about is "big bang" HV arcs that blow plate IC
meters
>across the room and do other terrible things. Solid state circuitry is
now
>available at a reasonable price that will limit anode current, shut
down the
>AC mains, and begin discharging the filter cap. at a very fast speed.
Sorry, I totally misunderstood. I'm looking into using a hydrogen
thyratron as a crowbar. OTH, my company makes big thyristors (2000volt,
1200Amp sort of level) for controlling electric trains and the
like..........unfortunately, it's another plant, so it's not too easy to
get samples....but I'm trying! But certainly a bit of protection is
desirable........and it doesn't cost much.......even if its only the 50
ohm resistor in the HV and diodes across the meters.
Some of the simple things that can make a difference are really very
cheap to apply. Some of the others can be desirable, but possibly
expensive in a commercial amp. In a home brew amp, the lily can be
gilded as far as you like.
Thanks for the comments on the vacuum relays and PINs. The relays that
were available here were brand new, so no problems........although I
arced a couple in my ATU and rapidly turned them from changeover to
single pole break. Being mean, I recycled them in the next ATU where
they serve to open both legs of the open wire feeder when I use the
tower as a vertical, with very few volts on them.
73
Peter G3RZP
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