Garey, et al,
I tried the "fake ALC" method with my old KWM-380 and an Alpha 91b after
fiddling with ALC on the 91b with little success. I had no more success
with the fake ALC circuit I found on a site. While it DID work at lowering
the output (that could be witnessed in the CW mode with carrier turned
down), the Collins had a leading edge spike that was a killer. The Alpha
faulted on that spike, EVERY time. I have read a few places on the net
hinting that the same might be true for the TR7. Anyone had experience with
that?? Also, I was reading some time ago that the output of the driver
board has been successfully used as a "0dbm" input for transverters. My
thought was to remove the original PA in a TR7, build a small 30 or 40w amp
to take it's place and drive the Alpha from that. I have since sold the
Alpha and the KWM-380, and have a nice 2 tube, 3-500z amp whose ALC works
well. I have NOT tried it as yet with the TR7, hence my question about the
"leading edge spike", althought the amp would take it, 150w into this amp
will give you 2k into a dummy load, hi.
73 and thanks for your input Garey,
Don, WB5HAK
----- Original Message -----
From: "Garey Barrell" <[email protected]>
To: <[email protected]>
Sent: Sunday, March 01, 2009 12:12 PM
Subject: Re: [Drakelist] TR7 with Modern Linear
Jim -
Some good points there. However, most are related to the "dynamic"
action of ALC, NOT a fixed gain adjustment as I described. I and
others have used this technique with success for a long time, but there
is always new information!
Of course there's always the argument that says "all this for less than
one S-Unit?!??"
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line & TR-4/C Service Supplement CDs
<www.k4oah.com>
Jim Shorney wrote:
On Sun, 1 Mar 2009 09:26:20 -0000, John Stringer wrote:
The linear requires only 20watts drive for 400 watts out (our limit).
In SSB the only output control on the TR7 is the Mic Gain.It is a real
fiddle to adjust this for 20 watts and more so to keep it there. The
smallest touch on the gain control can double the output and also it
cannot
cope with peaks.
Modern radios have SSB output level separated from Mic Gain.
Any ideas on how to get around this situation?
A couple of things come to mind. Does tha amplifier have an ALC output?
This can be used to limit your drive. You don't mention what kind of
amp it is, but if there is no ALC output, you might be able to look at
how Drake implemented the ALC in the L-4B/L-7 and add it to your amp.
A power attenuator could be added inline. The difficulties there are
that you have to find non-inductive resistors of sufficient power
rating to soak up the extra exciter power, and provide T/R switching
somewhow.
A *good* external RF speech processor (SP75 or equivalent) could be
used to limit the output level. I've found that my SP75 does a good job
of limiting the peak output to a certian level. Or you could try
adjusting the ALC control inside the rig for 20 watts maximum output.
You might have to change some component values in the ALC circuit, but
it should be do-able.
The problem with reducing the output from a 150 watt amp to 20 watts is
that you are operating closer to the crossover point of the
transistors, and you could have higher IMD on your transmit signal.
There can also be also other problems associated with using ALC to
control your output level as is done in "modern" rigs:
http://www.nitehawk.com/sm5bsz/dynrange/alc.htm
From a spectral purity standpoint, it's probably better to have an
attenuator in the input side of the linear than use any of the other
methods. The National NCL2000 comes to mind here. The grid circuit is a
50-Ohm dummy load with two taps - one for high power and one for low
power (~20 watts) drive. But that's a grid-driven amp, you may need
something a little different if your amp is grounded-grid.
73
-Jim
--
Ham Radio NU0C
Lincoln, Nebraska, U.S.S.A.
TR7/RV7/R7A/L7, TR6/RV6, T4XC/R4C/L4B, NCL2000, SB104A, R390A,
GT550A/RV550A, HyGain 3750, IBM PS/2 - all vintage, all the time!
"Give a man a URL, and he will learn for an hour; teach him to Google,
and he will learn for a lifetime."
HyGain 3750 User's Group - http://groups.yahoo.com/group/HyGain_3750/
http://radiojim.exofire.net
http://incolor.inetnebr.com/jshorney
http://www.nebraskaghosts.org
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