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Power question for the gurus...




 
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K5WLF
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« on: January 09, 2010, 06:21:39 PM »

Hi All,

I'm a newbie to high-power ham AM operation. I'm looking to acquire (buy or build) a rig that
will give me an honest KW out. However, I'm having trouble coming up with consistent numbers for
output in AM mode.

As an example, I emailed Ameritron and asked for their recommendations for operating the AL-811H
in AM mode.

They replied:"No more than 100W output for less than 10 minutes, then give it 5-10 minutes
to cool."

That seemed fairly pitiful for an amp with an input power of 1,040 watts (Ep x Ip). It's a worse percentage of rated PEP than my Yaesu FT-897D, by a factor of ~2.

In contrast, the Collins 20V-2 (my dream rig) with an input power of 1,550 watts (Ep x Ip) puts
out 1100 watts according to the factory manual.

My question is: To what do we owe the huge disparity in the relative capability of these units?

Is it a difference in power supply capability, cooling, overall quality of construction or ??

This leads to a second question: As I'm looking through schematics and articles about various
pieces of transmitting gear, what is a reliable method of determining the AM carrier output of a
unit which is described in "PEP watts" or "CW watts"? At one time, I thought "CW power/4" was a
good rule of thumb, but now I'm starting to wonder.

All enlightenment appreciated. Thanks.

73,
ldb
K5WLF
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W1GFH
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« Reply #1 on: January 09, 2010, 06:26:43 PM »

I ran an AL-811H for years at 200 watts of carrier on AM with never a problem. I never hit the audio too hard, and I figured 800 watts PEP on voice peaks was well within the unit's ratings. Looking back on it, I would say I was probably very lucky.... Grin
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KE6DF
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« Reply #2 on: January 09, 2010, 06:33:25 PM »

I think the general rule is that output for AM from a linear would be about 1/2 the total plate dissipation of the tubes.

But that assumes the tubes are well cooled.

Plus it's the limit on the tubes.

The power supply would have to be able to put out three times that amount (or 1,5 times the tube dissipation) under fairly continuous duty cycle to be safe for long transmissions.

There are plenty of threads on amforum about using linears for AM so try searching a little.

To reach the legal limit output on AM requires a very hefty linear -- and such a linear makes a wonderful space heater.
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N3DRB The Derb
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« Reply #3 on: January 09, 2010, 07:12:04 PM »

Quote
My question is: To what do we owe the huge disparity in the relative capability of these units?

linears are very low efficiency devices. they waste a lot of the energy they produce as heat because of the way tubes have to operate to be "linear", that is faithfully reproduce but amplify the input signal. For a given DC input power, the RF output can only be  roughly somewhere around 33 to 45% of the DC power applied depending on variables such as automatic bias voltage controls and such,  used on am.

the Collins uses a 'Class C' final amp, which is able to turn more of the DC input power you feed it into useful RF output power because it's inherently a more efficient mode of generating RF, but it's not able to operate as a linear amplifier because of how the tubes operate.

Most of todays linears just dont have the power supply capable of doing AM. AM puts a lot more demand on gear than SSB or CW does. A good rule of thumb is that you have to increase the power generating capacity (meaning how long a supply can deliver a given output )  by 4X over that of a supply made for SSB. Note that this is my personal rule of thumb, and others may say more or less.
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K1JJ
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« Reply #4 on: January 09, 2010, 07:17:07 PM »

Quote
"As an example, I emailed Ameritron and asked for their recommendations for operating the AL-811H
in AM mode.

They replied:"No more than 100W output for less than 10 minutes, then give it 5-10 minutes
to cool."

That seemed fairly pitiful for an amp with an input power of 1,040 watts (Ep x Ip). It's a worse percentage of rated PEP than my Yaesu FT-897D, by a factor of ~2."



Apples to apples:

Ameritron meant ~100 watts output of dead carrier and 400-500 w peak output.  In comparison, your FT-897 is good for only about  ~20-25watts out dead carrier and 100 watts pep out.   The reason they are derating the linear to 100 watts dead carrier for AM use is simply because of low efficiency and heat. On ssb with low duty cycle, (no constant  dead carrier) the pep output can be pushed to twice that, (800w pep out) as you observed.

T




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« Reply #5 on: January 09, 2010, 07:37:38 PM »

For a linear amplifier, depending upon the details of how you load it, the efficiency at carrier will be around 25-33%. The efficiency at the instant of a modulation peak will be around 66%. This is fairly straightforward to calculate, but I won't do that here.

So, for example, if the r.f. output is 200 watts at carrier, and the efficiency is (optimistically, if you load the amp properly for AM operation) 33%, then the tube(s) is dissipating 400 watts. [33% of 600 watts total electrical input power = 200 watts of r.f. output. The remaining 400 watts of input power is dissipated by the tube(s)].

If the efficiency at carrier is less than 33%  (for example, if you load the tube to allow enough headroom for more than 100% positive peaks)... say 25% ... then at 200 watts of carrier, the tube(s) is dissipating 600 watts of heat!

For a 3-500z, as an example, running at 25% efficiency at carrier... one would not want to run more than 167 watts of carrier (which implies 500 watts of plate dissipation]

All of the above is a consequence of using a linear amplifier.

Also of note, when you modulate, you do not increase the plate dissipation. For example, consider the case where the amplifier is loaded to accommodate 100% positive peaks (and no more). At carrier, it will be operating at around 33% efficiency. At the instant of a positive peak, it will be operating at 66% efficiency. Also, by definition, the rf output at the instant of a 100% positive peak will be 4x the rf output at carrier.

So lets do a calculation for the case where the rf output, at carrier, is 200 watts:

At carrier, we have 33 % efficiency, so the plate dissipation is 400 watts.

At the instant of a modulation peak, we have 800 watts of r.f. and 66% efficiency. This means that the plate dissipation is 400 watts!

The plate dissipation is the same at carrier as it is on modulation peaks.  Shocked

So... whether you talk or not, the tubes are dissipating the full 400 watts (in this example)



Best regards
Stu
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« Reply #6 on: January 09, 2010, 08:08:24 PM »

Unless a really "big" linear is in your budget I would suggest considering a decent controlled carrier transmitter (Drake 4 line, Heathkit DX-60) which is more linear friendly or go with a decent medium power transmitter like a Johnson Valiant.

The controlled carrier rigs can sound very good and since they vary the carrier output based upon the audio input power they are much easier on a linear amplifier.   For example, most of the "classic" 2 KW amps (Drake L-4, Kenwood TL-922, probably the Heathkit SB-220) can be run at their rated PEP input without excess strain for the power supply or cooling system.  The receiving operators S meter will vary with the carrier differences but the receiver AGC will take care of that issue.

Otherwise, think about a Johnson Valiant or similar which will get you within about 3 to 4 db of the legal limit without adding an external amplifier. 

For a full continuous carrier transmitter, it takes a very capable linear to obtain a worthwhile power gain.   I have experimented with one of my Heathkit stations (TX-1 transmitter with KL-1 amp) and even with this pretty solidly built amp with a VERY heavy external supply the additional effective power on AM is insignificant. 

The old ARRL Understanding Amateur Radio series had a nice bit of text and associated photos which wonderfully got the point across about using various amplifier classes for phone.   

Have fun with whichever path you take! 
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« Reply #7 on: January 09, 2010, 08:44:24 PM »

I just built a 4x813 amp, that has a total of 500 watts of plate dis.
I run it at 150 to 175 watts or carrier out and about 800 watts pep on peaks.
The air coming out of the top of the rack exceeds 90F, with 65F room temp.
It DOES make a good heater!

The power supply is CCS rated for 2500 volts at 500ma, and just the power supply weighs more than any ham amplifier made.

I have a PAIR of 813's in class C plate modulated that runs 600 or 700 watts carrier out, and can do 3kw pep.
And it generates much less heat than the amp.

Say I had four 813's plate modulated, that would be 1200 watts of carrier!
150 watts to 1200 watts is a BIG difference!

Now, for ssb, the duty cycle is so low, the poor efficiency does not cost much.
On my amp, I can crank the voltage up to 3000 volts and get lots of power, maybe 2000 watts pep and it does not get hot....

I enjoy running the flex into the amp, but its really stupid...


Brett
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WA1GFZ
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« Reply #8 on: January 09, 2010, 10:41:39 PM »

Now look at how much power and hardware it takes to build a modulator that will generate enough clean audio.
Valiant is a good choice to get started though.
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« Reply #9 on: January 09, 2010, 11:05:48 PM »

Well....
A pair of 811a's, run them off the rf power supply, drive the grids with an 8 ohm to 5000 ohm trans, 340 watts of audio, good for a 500 watt carrier.
2 tubes, two sockets, a filiment trans, a driver trans, and you are good to go.

To get 500 watts carrier out of an amp, I think you need:
1200 to 1500 watts plate dis (three 3-500z), a 220volt line into the shack, and loads of cash to pay the electric bill.

Brett
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« Reply #10 on: January 09, 2010, 11:15:21 PM »

22 11N90s PDM modulated 1 KW carrier >5  KW PEP. stone cold
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« Reply #11 on: January 09, 2010, 11:50:41 PM »

I suggest you get an ARRL Handbook or if you own one, Start reading up on AM.  You can really learn alot in just a few nights with that book. 

As others have said,  You are confused as to the power ratings of different operating modes. 

In AM speak,  We talk about power output of the Carrier of the transmitter or sometimes the full modulated RMS power.  Am people almost never use Peak power ratings.

The rule of thumb is to full modulate a 100 watt carrier, you would need 400 watts Peak.   Even though you would be outputing 400 watts to the antenna, We would call this 100 watt transmitter.

On SSB, you are talking about the peak output power.  1000 watts on SSB amplifier is 1000 watts Peak. 

The Collins you speak of is a 1000 watt AM broadcast rig.   This rig can do 4000 peak watts.  A totaly different story then the 1000 watt Ameritron SSB amplifier.

Most SSB Amplifiers will croak during AM use.  There can be many different reasons.  Light weight power supplys that where made for SSB use, Low plate disipation ratings on the tubes is another.  Some units are just not biased correctly for AM use and can be modified to work better for AM. 

Consider that a full power Side band amplifier that makes 1500 watts on SSB, is only going to be good for 350 to 400 watts AM at max output power.  The Current max legal limit for AM is 1500 PEAK watts.  This means about 375 watts of AM carrier power.

Use what ever you have for AM. You dont need alot of power.  I used my Ranger for two years at 65 watts AM and had a blast.  I talk to a guy with the same radio as you on AM once in a while.  When the band is in, He comes in just fine. When the band is not in, Its not in.. I am sitting here right now keying up a 350 watt AM Transmitter and have been calling CQ for an hour off and on.. I have yet to make a contact tonight. The band is horrible.

Clark
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WD5JKO
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« Reply #12 on: January 10, 2010, 12:59:15 AM »

I'm looking to acquire (buy or build) a rig that will give me an honest KW out. However, I'm having trouble coming up with consistent numbers for output in AM mode.

ldb,

   I think at this point you meant PEP output power when you said "an honest KW out". If as others already said, AM'ers talk about carrier power, and IF you really want a carrier KW out, then were talking serious boat anchor material.

  To get 1 KW out, linear mode, figure a class B 3cx3000a7 final tube dumping 2kw heat to make 1kw carrier. Alternatively, about forty 811's could replace the one 3cx3000a7. Smiley

  For class C, plate modulated service, just get your dream transmitter, a Collins 20V2.

Neither combination would be legal to rip full bore since you would be peaking to 4KW PEP or higher when the FCC says 1500 PEP is maximum allowed.

  The terminology, and efficiency of differing modes can be confusing. For example, take the common 6146 beam power tube. In plate modulated service, one of these can make 50 watts carrier output and about 200 watts PEP output @ 100% modulation. Take that same tube and run as an AM linear amp. Figure 12 watts carrier out, and 50 watts PEP output. That is 6 DB difference or about 1 S-unit. When the band is good, a measly 'S' unit isn't too significant.

Jim
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« Reply #13 on: January 10, 2010, 10:11:52 AM »

The nice thing about glass tubes is that you can see what you are doing right or wrong.

A DX-60, or any similar rig does make a nice start especially with a good mike and a few simple audio mods.

For a cheap linear its hard to beat an old 80-10M Henry console as they provide a heavy duty PS and cooling system. There are a few other brands using a 3-1000Z.

For a home brew linear, if you are up to it, a grid driven 4-1000A is hard to beat BUT they do like high voltage. While using that collect the parts and build a healthy modulator.

Or get a Valiant and be satisfied.

Carl
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« Reply #14 on: January 10, 2010, 12:59:45 PM »

One thing to remember is that a linear amplifier is just as efficient on AM as it is on SSB.  If operated properly, it simply amplifies whatever signal is fed into it, and the output is a near-perfect replica of the input signal, only at a higher power level.

The efficiency of a linear amplifier is a function of the amplitude of the signal, up to the maximum output capability.  Theoretically, the maximum efficiency should be somewhere around 67%, but in practice, most linears peak out at about 50% efficiency.  This maximum efficiency occurs just at the point below saturation.  Any additional signal causes no further increase in output.  On the scope, this point is visible as "flat-topping".  For maximum efficiency, the linear should be run so that the maximum positive peaks just barely fall short of the flat-topping point, or the point of saturation.

At the opposite end of the scale, is zero signal input.  At that point, the linear amplifier stage draws a certain amount of static plate current to result in a power input, but since there is no power output, efficiency is zero.  Thus, the efficiency varies between zero and some maximum value, directly in proportion to the signal amplitude.

With AM, the amplitude of the carrier is 50% of the maximum output at 100% positive modulation peaks, so therefore the carrier efficiency of the amplifier is midway between zero and maximum.  If the maximum measured efficiency is 67%, the carrier efficiency rests at about 33.5%.  Since the duty cycle of the carrier is 100%, the carrier efficiency is the most obvious factor with AM.

With modulation, the DC input to the final with an AM signal should remain steady, regardless of modulation.  The extra output to accommodate the the sideband power must come from somewhere, so the efficiency improves with the presence of modulation.  If the plates glow with the unmodulated carrier, the  glow should decrease with heavy modulation, since more of the DC input power is being converted to rf instead of heat.

For this reason, low level modulation such as grid modulation and linear amplification are known as efficiency modulation. At 100% modulation, the instantaneous peak output power is 4 times the resting carrier power.  The average DC input to the final is constant, but the DC input is doubled at positive modulation peaks and falls to zero on negative peaks; if the amplifier is truly linear, this averages out to the same constant value with modulation as it is with the unmodulated carrier.  This doubles the output power on positive peaks, but we still need to further double the output power once more.  This is achieved by doubling the efficiency of the amplifier on modulation peaks, along with doubling the instantaneous input power.

Another way of looking at it is that with AM, you have to leave enough power output capacity, or headroom, with the unmodulated carrier, to accommodate the positive modulation peaks.  If the amplifier is run beyond about 33% carrier efficiency, there isn't enough headroom left to accommodate the positive modulation peaks, and the signal will flat-top, producing exactly the same kind of distortion and splatter as overmodulation on negative peaks.

With SSB, exactly the same variation of efficiency between zero and maximum output occurs as with AM, but there is no 100% duty cycle carrier to contend with, so the tube runs cooler.  However, the overall average efficiency of a SSB linear is hardly any greater than that of an AM linear.  With the typical human voice, the average amplitude is about 30% the peak value, or somewhere around 10 dB below peak value.  This can be seen on a VU-meter at a properly operating broadcast studio, or at the input of an audio recorder that is not being overdriven on voice peaks.  The meter will ride somewhere around 30% modulation and only occasionally hit the "0" (100% modulation) mark on the scale.  So for SSB, with no "processing" or over-drive, the average signal amplitude runs about 30% of the maximum peak, which is actually lower than than the 50% amplitue level with the unmodulated AM carrier.  This means that most of the time, the efficiency of the SSB linear runs lower than that of an AM linear, with the higher efficiency occurring only on major voice peaks.  But the peak output is more obvious with the SSB linear because the meter can be seen to swing upwards on voice peaks, whereas with AM, the DC input meter is supposed to stand still, and the rf power meter (set to read real average, or mean, rf power output) just barely kicks upwards from the unmodulated carrier level on modulation peaks.

The only commonly available, economical means of measuring true mean rf output, is with a thermocouple rf ammeter.  A thermocouple meter reads true r.m.s. radio-frequency current, since it functions by the heating effect of the rf current passing through the thermocouple element.  According to Ohm's law, power delivered into a known impedance load can be calculated  using the formula P=I˛Z, where Z is the impedance.  For a pure nonreactive resistive load, the formula becomes P=I˛R, where R is the load resistance.  

A diode type power meter, such as the Bird 43 or the cheaper Hammy Hambone "wattmeters" sold on the ham radio appliance market, are actually diode rectifier type rf voltmeters with the scale calibrated in watts output, under the assumption of a pure 50Ω resistive load. Unfortunately, the rectifier type voltmeter is an average-reading instrument, not a true r.m.s. reading instrument.  With AM, the carrier reading remains steady with or without modulation.  With SSB, the output reading is considerably less than the real power output.  So the so-called "peak-reading" function was devised to indicate peak rf voltage, with the  dial scale calibrated to "peak power output", which may have  little to do with the actual effective power output.

A better type of meter is now available, that reads true power output.  This type of meter uses electronic circuitry to integrate the waveform of a varying signal to actually read effective, or r.m.s. voltage, instead of average voltage, allowing the power scale to be calibrated in mean/average power.  One example is the Bird APM-16 Thruline® RF Wattmeter. As you can see, these meters are not cheap, but they are about the only alternative to the thermocouple ammeter for calculating real power output.

For more discussion about the sometimes confusing distinctions between average and r.m.s. voltage/current, and how this relates to average (mean) power, see

http://amfone.net/Amforum/index.php?topic=20410.msg146386#msg146386
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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« Reply #15 on: January 10, 2010, 01:34:00 PM »

I agree. I think you should buy yourself a boat anchor AM Transmitter.  Then a Decent boat anchor reciver and setup a vintage station.

A Valiant is not a good First Rig in my opinion.  They are notorious for failing and having problems.  I have owned 4 or 5 of them and currently own 2. The rest where sold off after repair.  I worked on the valiants and talked on the ranger.

C
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« Reply #16 on: January 10, 2010, 04:46:01 PM »

I agree. I think you should buy yourself a boat anchor AM Transmitter.  Then a Decent boat anchor reciver and setup a vintage station.

A Valiant is not a good First Rig in my opinion.  They are notorious for failing and having problems.  I have owned 4 or 5 of them and currently own 2. The rest where sold off after repair.  I worked on the valiants and talked on the ranger.

C

Clark,

The Valiant should be very reliable as long as a couple of issues are addressed in addition to the typical vintage gear concerns.  Beyond replacing the electrolytic caps and the VFO resistor typical of many of the Johnson rigs there are only a couple of additional concerns for the Valiant.

If it is a very early rig then the wiring from the LV transformer to the 866A cathodes should be replaced or sleeved and failure here is the most likely cause of a LV transformer failure.  The other trouble spot is the turnstyle loading cap and these should be replaced with quality modern replacements when trouble occurs; you can think of this as the "cost" of having a range with very large built in antenna matching capability.  I don't think it is a good idea to "hot switch" the fixed loading caps because this does stress the switch.  You can do the initial tune up into a new antenna by switching the HV off before changing caps and once you have established the proper setting then preset this for subsequent operation.  Early Valiants may also have been built with two small an output coupling cap which will cause reduced output on 160 (and to a lesser extent on 80), later units typically have either a pair of 1,000 or 1,200 pf units in parallel.

I had a Valiant for my first rig and I own two of them now and they both spend a lot of time AM and CW on 160 through 40.  I had an 866A fail in one rig and so those got solid stated like the other rig but other than that the rigs are stock and I haven't had the case off either one in the past 4 years.  I just found a Valiant II which will match the SSB adapter I picked up a few years ago so that will be Valiant #3.

For reliability, the Viking 1 or 2 probably is near the top for vintage gear but they are a bit different style than the later Ranger and Valiant.  I have actually had more difficulty with my Ranger than the Valiants but this particular Ranger came with a curse.  The Ranger also runs a whole lot hotter with everything crammed into a fairly small case.

So, if you were depending upon a single transmitter for communications from a desert island and your only two choices were your Valiant or Globe Champ which one would you take  Smiley  (personally in this case I might try to sneak a spark gap aboard).

Rodger WQ9E
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« Reply #17 on: January 10, 2010, 04:58:00 PM »

By far the Champion.  Its very reliable. I have used it for 3 years now.  Its only needed maintenance. I have upgraded it along the way and in that process, the SR failed. 

The trouble with the valiants are many. Yes, You can upgrade them. Alot of them have been through hell and or CBers hands.  Alot of them where Kits and this brings up a whole host of unknowns. 

Clark
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« Reply #18 on: January 10, 2010, 10:14:04 PM »

Any of the CCS Telephony levels will be fine, but never use the ICAS ratings.

ICAS was fine back in the days when tubes were cheap and plentiful.  The idea was originated by RCA as a compromise that gave more power output but shortened tube life.  At the time, most tubes were cheap enough that you would come out ahead in the long run financially by running a smaller less expensive tube but replacing it more frequently. It was useful in amateur service where a tube might be fired up only a few hours per week as opposed to commercial service where a tube worked 18-24 hours a day and down time could not be tolerated.

But now that tubes are getting hard to find and expensive, it pays to treat a tube in amateur service with TLC in order to squeeze out every possible hour of life, even if it means a dB or so less power output.  Amateur service is about the only kind of service that vintage tubes see these days, if you don't include audiophoolery.
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Don, K4KYV                                       AMI#5
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K5WLF
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« Reply #19 on: January 10, 2010, 11:11:20 PM »

A lot of good information, guys, and I thank each of you who commented. It's going to take me a bit of study to get all this locked in my head. And I've got some reading to do.

I'll probably have more questions as I learn more.

Thanks again, y'all.

73,
ldb
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« Reply #20 on: January 11, 2010, 12:43:31 PM »

Ive worked on a few Valiants of both models but have never "owned" one. They were for customer repair or to flip and as Roger stated the problems are well known and documented.

My Vikings that are "owned" are a I and II CDC plus an Adventurer with HB modulator somewhere on a shelf. The I was a kit and the CDC naturally factory built. A bit of Internet reading and a few hours and both have been running fine since. Ive done a few mods but those are optional after simply getting it on the air.

Its getting hard to find a reasonably priced 100W or more plate modulated rig. Some controlled carrier ones are also going pricey lately with the DX-60 and earlier Heaths plus the Knight T-60 about the cheapest. I have a Knight T-150A at our summer cottage and after minimal work it does a decent job considering how cheaply it is designed; its a throw away if the place gets broken into.

Carl
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« Reply #21 on: January 11, 2010, 01:31:11 PM »

You just have to look around and let people know you are looking for a rig.
I sold two 32V3 transmitters, new looking, working without problems, with manuals for $250.00 each, which in my book is beer and pizza money, and they were COLLINS!!!.

I sold a very nice Gonset G76 for about $100.00 (with power supply), plate modulated, 70 watts carrier out, and a VERY nice receiver built in. The 262KHz IF could be set up for hifi or narrower. Thats an entire AM station (you dont really need an antenna tuner with it) in one box smaller than a ranger with more power output.

RE: old Johnson rigs reliability, none of the old rigs will be very reliable at that age without work, and working on and improving them is the fun part.

Dont want to fool with old unreliable stuff, buy a flex and an amp, and run it light. 200 watts (carrier) is ok.
The receiver is great, and the transmitter can be hifi right out of the box.

There is something for everyone on AM, and loads of ways to get on the air without spending a lot of coin.

And there are LOADS of AMers around with lots of old nasty looking DX100's they should pass on to someone who would actualy use it....

Guys, if you are 80 years old and have a pile of old rigs in the basement, think about moving one or two on to someone who would use it.

Brett


 
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« Reply #22 on: January 11, 2010, 04:27:37 PM »

Dont go the Flex route.  Nice SSB radio, but They just dont have the sound quality that a Class C am Rig produces. Some think they do and they are better then other Solid State modern radios for one reason.. The easy to adjust bandwidth filter. Its still a solid state radio though.

 Its AM. Get a Real AM radio from the 50s era and have fun. Thats a major part of this hobby.. Getting and old station working, then getting it to work better. We are all going to have our opinions on which old gear you should buy.  Go to a local Ham fest or swap meet and it will be destiny.


Clark
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