FS: 2m Linear 90w
Edward R. Cole
90w for 25w drive, or
60w for 10w drive
operates on 13.5 vdc at 13amp
Originally the amp was spec for 150-170w but I'm guessing one output
transistor is gone. It seems to work fine with one transistor at 90w
as I have been running it there for about three years.
Has internal preamp which is functional.
external PTT or RF sensed T/R
SSB/FM
5-foot dc power wire
no manual
$105 with FREE shipping to USA. PayPal or money order
I will have a photo, tomorrow on
http://www.kl7uw.com/fs.htm
73, Ed - KL7UW, WD2XSH/45
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BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-?
DUBUS Magazine USA Rep dubu...@gmail.com
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WB7RSG
As a group, we're a bit more "technical geeks" than the average
bunch of hams, so please don't take my questions and comments as an
attack on you sale item....... It's a great opportunity to touch on
amplifier performance and repair issues...... this is how we learn
stuff......
A very quick search didn't produce a schematic of this amp for
me.... What type transistors are used in this amp? It may be costly to
repair if the transistors are hard to get.
If 10 watts drive makes 60 watts output, this is only 7.7 dB
gain.... and if 25 watts makes 90 it's only 5.6 dB..... 2.1 dB into
compression, assuming that it's "linear" at 60 watts output..... it's
unlikely that this amp will be useful for SSB service as is.......
certainly not at 90 watts PEP output
Have you been able to observe it's gain compression point? How
much power output does 1 watt of drive produce? How hard can you drive
it before the gain starts to fall off? How bad does this look with a 2
tone signal?
Doing a righteous job fixing this amp will probably require at
least the ability to sweep it's input power to find it's gain
compression details..... A two tone signal generator and scope with
enough bandwidth to observe it, or a spectrum analyzer with enough
resolution to see the tones would be very helpful figuring out optimal
bias points for the new transistors ....... We all want to listen to
nice sounding SSB signals..... Nasty amplifier distortion is a bad
thing for all of us....... of course, if you only need it for CW and
FM, you can get away with class C amps and really nasty distortion,
but in this case, only 6 or 7 dB gain may not be worth the effort.
Sorry to rant, I'm not trying to be snotty..... just a geek :-)
73 DE WB7RSG
Greg.
On Dec 13, 12:44 am, "Edward R. Cole" <kl...@acsalaska.net> wrote:
> I have a RF Concepts 2-315 2m linear amp for sale.
>
> 90w for 25w drive, or
> 60w for 10w drive
> operates on 13.5 vdc at 13amp
>
> Originally the amp was spec for 150-170w but I'm guessing one output
> transistor is gone. It seems to work fine with one transistor at 90w
> as I have been running it there for about three years.
>
> Has internal preamp which is functional.
> external PTT or RF sensed T/R
> SSB/FM
> 5-foot dc power wire
> no manual
>
> $105 with FREE shipping to USA. PayPal or money order
> I will have a photo, tomorrow onhttp://www.kl7uw.com/fs.htm
>
> 73, Ed - KL7UW, WD2XSH/45
> ======================================
> BP40IQ 500 KHz - 10-GHz www.kl7uw.com
> EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-?
> DUBUS Magazine USA Rep dubus...@gmail.com
> ======================================
Stephen Hanselman
Ed mentioned that there was probably one device blown so it was running on
only one device. Given that he had to remove/modify the splitter/combiner
to remove the bad device there would be some performance losses. I'm not
familiar with this unit but most of those 160/170W designs used Bi-Polar
technology so 7dB of gain is fairly believable for a single transistor.
Where modern MOSFETs such as the NXP BLF578 are good for 20+dB of gain (5 W
for 1000 in my case).
Should be a good base to experiment with solid state power.
Steve KC4SW
Ed (and all)
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Edward R. Cole
Greg,
Thanks for your comments. RF Concepts has not manufactured these VHF
and UHF linears for several years, but they are very good amps. If
you google RF Concepts you find the site for the maker of Alpha HF
amps. But the VHF amps are still seen often on used sales (three are
current on e-bay).
To answer the question on transistors I would have to open the amp
but from memory they are something like MRF-141 (I think). I had
given thought to repair the amp one day but instead bought a
replacement RFC 2-317, from K7MDL. I'm sure that it would cost more
than used amps sell to repair both output transistors. One could be
replaced if one is industrious. However, keep reading:
The original amp design ratings were 150w with 30w (7-dB gain) or
170w with 40w (6-dB gain). That is not unusual for VHF
transistors. There is no driver stage in this model so it takes fair
amount of drive.
I tested it at 10w as this is minimum level I can achieve with my 50w
transverter. I do have another transverter that outputs 2.5w so I
can test at that level. I would expect the same gain as I got with
10w: 7.8-dB gain would result in 15w output. Most amplifiers exhibit
higher gain a low drive levels so your compression conclusion is not
quite accurate (see the original linear gain figures for this
amp). All RF measurements were in CW since I do not have a
peak-reading power meter.
I have owned the amp since about 2003 or 2004 and I currently run it
on both SSB and FM at 90w with no linearity issues (good on air
reports). I suspect the bad transistor is acting as a load for the
input so probably only half the drive is being amplified.
The gain compression point is about 85-90w. Increasing drive above
25w does not result in higher output. If both transistors worked this
would imply an output of 170w. When new the amp saturated at
170-180w with 45w drive.
So my point is the amp is working fine as-is (having used it in
current condition for over three years). I realize it is not in
original new condition (which would have a price of $350-400 or used
at $200). But neither have I mislead anyone on its current condition
- a usable 60-90w linear amp, depending on your drive. A good value
at $105 (FREE shipping).
73, Ed - KL7UW
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73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-QRT, 1296-?, 3400-?
DUBUS Magazine USA Rep dubu...@gmail.com
======================================
WB7RSG
Yes, the bipolar amps tend to have less gain per stage than the
cool new MOSFETS. If the RF Concepts amp is similar to ones like the
Mirage (MFJ) amps, it's output power is probably advertised at
saturation..... I haven't fooled with RF concepts amps myself, but
this is what I've observed in other commercial amps....... If one of
the output devices is "bad" and the amp isn't blowing fuses, the
collector is most likely open. It depends on the amplifier topology
how the amp will behave with a bad device, and how easy it would be to
rig around the output combiner and still show the remaining device a
reasonable collector impedance....... Without knowing the details,
it's hard to guess. With one bad device, without looking on the bench,
all bets are off for distortion performance.
There are several reviews of this amp on eHam, one fellow claims
that he sees about 60 watts output for 3 watts drive, 13 dB gain, this
sounds about right for a single stage bipolar VHF power amp to me. The
specified output power is 170 watts for 40 watts drive..... only 6.3
dB gain...... and they claim it's a linear amp! All of these
amplifiers require far less drive than the manufacturers recommend. In
my view, the rule of thumb should be to never drive any of these amps
harder than about 1 dB gain compression.
I use a Mirage B-1018G (it was cheap at a ham fair) This is a 2
stage amp, so it's overall gain is higher, but it's still a good
example to talk about. The manufacturer claims this is a 160 watt
output amp, and in one sense it is, it will make 160 watts output when
saturated, and that's fine for CW or FM...... It's 1 dB gain
compression point is around 100 watts output, and it requires around 1
watt of drive to get there..... This is about 20 dB of overall
gain...... The manufacturer is recommending that you operate this amp
with 8 dB of gain compression..... This is a bad idea! I would think
foul things about anybody that purposely operates RF power amps in SSB
service anyplace close to hard compression....... going from 100 W PEP
output with reasonable third order distortion too 160 W and sounding
like hell is only 2 dB difference, it's not worth the QRM.
As I'm typing, Ed has beaten me too the response.... He's right,
$105 is a good price point for this damaged amplifier, but a buyer
should be cautioned that without making careful linearity
observations, CW and FM should be all it's used for. Good "on air"
reports from the guy you're working 200 miles away aren't the whole
story...... the measure of your amp's performance are what the guys on
the next hill top, 5 KHz up the band, are saying about you, they
shouldn't even notice you...... I have cursed folks out loud, during
contests, for noising up the band, and being clueless.
Plotting gain compression is pretty easy, SSB transmitters are
linear up converters, insert a steady tone into the microphone jack, 1
kHz is nice and traditional, adjust the tone level to make the drive
power you want, when it stops going up, you have found the level that
your speech compressor starts working. Most of us have power meter's
around the shack, observe the input and output power of the amp.....
do the math...... 10 X LOG ( Output power / Input power ) = Gain in
dB...... when the gain has fallen off by about 1 dB, that's as hard as
you should drive the amp. If you have a two tone generator and a scope
you could observe the envelope shape, or even better with a spectrum
analyzer, you could see just how bad your distortion products are 5
KHz up the band....... Keep in mind, if your running 100 watts, and
the adjacent channel distortion products are down 30 dB, you're still
making 100 mW on the next channel....... If I'm 10 miles away, on the
next hill, and I'm hearing all of your voice peaks while you call CQ
all day, I will think foul things about you....... and I will not
answer you for contest points.
Sorry to rant even more, but bad sounding transmitters are one of my
favorite things to argue about
73 DE WB7RSG
Greg.
Edward R. Cole
which are no longer available substitute 2SC2782 $35.25 or $70.50 for
a MP from RF Parts
datasheet: http://www.rfparts.com/pdf_docs/2SC/2SC2782.pdf
rated for 80w to 175 MHz with 18w drive (6.5 dB gain).
I measured the current amp:
1w-in 10w-out 10-dB
10w 68w 8.3 dB
15w 80w 7.3 dB
20w 83w 6.2 dB
25w 88w 5.5 dB
I have a pending offer.
73, Ed - KL7UW