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KA9VAA
08-24-2004, 02:29 AM
I have not used am for thirty five years, I would like to start again, I have a kw 930 which puts out about 30 watts, I wonder if anyone has any suggestions as to a good am rig? Tube or solid state. Also, can I use my sb200 on am?

Thank You.

W8AMD
08-24-2004, 10:07 AM
A Heathkit Dx 100 or a Viking Valiant are my choices. This comeing from a man who still works AM with a ricebox and an amp. I'm not familar with your amp but it should do fine if you limit the plate current to half what the cw specs for the amp are. My amp it is 3/4 the cw plate current, not knowing your amp I'm adding a safety factor.

Someone here should know a bit more about that amp. If no better responses here try the forums at http:www.amfone.net These are the guys to speak to with all things about AM.

W8JI
08-24-2004, 12:12 PM
Quote[/b] (ka9vaa @ Aug. 23 2004,19:29)]I have not used am for thirty five years, I would like to start again, I have a kw 930 which puts out about 30 watts, I wonder if anyone has any suggestions as to a good am rig? Tube or solid state. Also, can I use my sb200 on am?

Thank You.
You can get an old rig, but plenty of new rigs sound great on AM. Some sound better than the best old rigs.

The FT1000D and my Orion are exceptional.

DX100's are cheap and by adding an electrolytic and a resistor to the screen supply and increasing a few capacitor values sound excellent.

As for the amplifier, the rule is you can run 1/4 of the PEP rating or less on AM carrier as long as you don't go over dissipation limits of the tubes. Usually the tube dissipation rating sets the the power limit on AM.

Here's a rough rule of thumb for clean safe AM use of an amplifier.....

you NEVER want to run more input power than the dissipation rating of the tubes and NEVER want to exceed the SSB PEP output rating on audio peaks.

Assume you have an SB200. It has 320 watts dissipation available in the two 572s and 1200w pep. Multiply tube dissipation rating times 1.33 for input power.

The SB200 plate input power would be limited to 320 watts. Output carrier would be 80 watts. Tube anode heat would be about 240 watts.

Peak envelope power will be four times carrier power, or about 320 watts PEP. So you are safe and linear.

Now look at a SB220. 1000 watts dissipation is available and the amp limits at about 1200 watts PEP output.

Input power could be 1000 watts. Output power on carrier would be about 250 watts. PEP output would be 1000 watts on audio peaks.

You also need to watch amplifier heat. Unless the amp has a lot of airflow, you can overheat components.

73 Tom

K9STH
08-24-2004, 03:46 PM
You also need to remember than under the "new" FCC regulations that you CANNOT run more than 375 watts carrier output with 100 percent modulation. FCC regulations now allow 1500 watts peak output. This means 1500 watts of carrier with CW, 1500 watts carrier output on FM, 1500 watts peak on SSB, but since the peak power output of a 100 percent modulated AM signal is 4 times the carrier you can only run 375 watts of carrier.

This is unlike years ago when you were allowed 1000 watts DC power input for the carrier. On 75 meters it was not unusual to get 75 percent efficiency so the carrier would be 750 watts output which would mean 3000 watts peak power output. As such, many of the older AM rigs have to be cut back in power to meet the present specifications.

I have a Heath Apache, Heath DX-100, Heath DX-35, Globe Champion 350A, Hallicrafters HT-20, and a Globe Chief 90A with the SM-90 screen modulators that I can use on AM. It is a "toss up" between the Globe Champion 350A and my slightly modified Apache as to which is the "best" sounding. You can modify the Apache by just changing 3 capacitors. Before changing those capacitors I got the usual "scratchy Apache" reports. After changing them I get "rave" reports even from the VERY critical operators that hang out on 3880 KHz in the period just after dawn.

You can find that modification on either

http://home.comcast.net/~zcomco

or

http://home.comcast.net/~k9sth

As for using a linear, W8JI is correct in that you can only run 1/4th the power input for your carrier that you would on CW. Also, you definitely have to watch the power dissipation of the tubes and the carrier output power. Again, remember, that you cannot put out more than 375 watts of carrier.

Glen, K9STH

AG3Y
08-24-2004, 04:04 PM
I never could figure out how all that power was generated, until I put all the VECTORS together. We're talking about INSTANTANEOUS power here !

http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif 73 from Jim

W8JI
08-25-2004, 12:22 AM
Quote[/b] (AG3Y @ Aug. 24 2004,09:04)]I never could figure out how all that power was generated, until I put all the VECTORS together. #We're talking about INSTANTANEOUS power here !

http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif # 73 from Jim
It's pretty amazing how it all adds up.

I can legally run 800-900 watts carrier with a stock Globe Scout, because it has such poor positive modulation peaks it only makes 1500 watts peak!

I agree about the Apache. My DX100 and Apache were both very easy to clean up. Not like my Valiant and Globe rigs.

Honestly, the Orion is the best sounding rig I have on AM. Both receive and transmitting.

K9STH
08-25-2004, 01:09 AM
Tom:

How long does that 6146 in your Globe Scout last! A "stock" Globe Scout (either the 680 series that have 80 through 6 or the 65 series that have 160 throuh 10) is only rated at 65 watts INPUT. The earlier versions were rated at even less input.

To get 800 or 900 watts of carrier out of it your 6146 final can't last more than a few seconds!

Now I agree that the Heising modulation that was used in the Globe Scout did not have enough audio power to fully modulate the single 6146. My Globe Chief 90A with the SM-90 screen modulator has more modulation percentage than the average "stock" Globe Scout and the power output is pretty close to the same (Globe Chief 90A uses a pair of 807 tubes versus the single 6146 of the Globe Scout) since if you "push" the transmitter you can get between 110 and 120 watts CW input which means approximately between 55 and 60 watts CW input when you run screen modulation.

Are you trying to say that when you drive a linear amplifier with your Globe Scout that you can run 800 or 900 watts of carrrier? That would be a different "animal", but that wouldn't be running a "stock" Globe Scout!

Some people use a rig with controlled carrier modulation with a linear. That way they can get more output without the problems that a continuous carrier produces. But, they still cannot get full CW output. However, the average carrier power is substantially greater than with "normal" AM. The older rigs like the Heath DX-35, DX-40, DX-60, Johnson Navigator, Hallicrafters HT-40, and so on are what those people use.

Glen, K9STH

WA2CWA
08-25-2004, 10:20 PM
Quote[/b] (ka9vaa @ Aug. 23 2004,19:29)]I have not used am for thirty five years, I would like to start again, I have a kw 930 which puts out about 30 watts, I wonder if anyone has any suggestions as to a good am rig? Tube or solid state. Also, can I use my sb200 on am?

Thank You.
The SB-200, with a "soft key" addition, or equivalent, #will work fine with the TS-930. I've used my Kenwood Twins, Icom IC-756 PRO II, and an Icom IC-706 MKIIG to drive my SB-200's. One of the amp's still has the original 572's in it. It's been running AM and SSB since the mid 80's.

As pointed out, 375 watts carrier output with 100% modulation are the rules. However, you can run more carrier power, (can provide better receiver capture of your signal at the other end) as long as you run less % of modulation. Running your modulation at 90% or 85% is hardly, if at all, noticable at the receiver end.

Pete, wa2cwa

K3UD
08-26-2004, 01:24 AM
I seem to remember several hams who used rigs like the Knight Kit T-60 which had screen modulators as exciters for amps like the SB-200. I think the carrier output of the T-60 was not much more than 10 watts or so in AM mode.

73
George
K3UD

W8JI
08-27-2004, 12:06 AM
Quote[/b] (K9STH @ Aug. 24 2004,18:09)]Tom:

How long does that 6146 in your Globe Scout last! #A "stock" Globe Scout (either the 680 series that have 80 through 6 or the 65 series that have 160 throuh 10) is only rated at 65 watts INPUT. #The earlier versions were rated at even less input.
Well, I mean into an amplifier of course!!

I actually only run about 15 watts carrier, and even at that can't get any more than 30W envelope peaks on a good day. One 6L6 in Heising doesn't do well, even with the HV dropped way back on the PA and a full 550-600V on the 6L6!

I can run 60-70 w out on CW. I rebuilt everything in the driver and PA and even added grid block keying and a new way of getting the 300V supply from the same transformer.

I don't know how long the tubes last, because none have failed.

73 Tom

K9STH
08-27-2004, 01:44 AM
Tom:

I hate to "nit pick", but you certainly are not running a "stock" Globe Scout considering all of the modifications that you have made to improve it! However, I definitely agree about the Heising modulation. Also, many of the Globe Scouts really do not have anything that resembles symmetrical modulation. I believe that this was even pointed out in the QST review of the Globe Scout Deluxe version that came out in the very early 1960s. The ARRL said that the modulation "sounded OK", but definitely was "not symmetrical".

Another thing that really shows up on FM much more than AM (but it can there as well) is that the vast majority of male voices are not symetrical while most female voices are. That is, when observed on an "scope" type deviation monitor the peaks on one side do not equal the other side. Typically when the deviation is, for example, +5KHz on one side the other side is between -4 KHz and -4.5 KHz. When a female modulates the FM transmitter the deviation is pretty much the same on both sides of the carrier.

I don't know if anyone has really done any real investigation as to what causes this. However, many two-way radio technicians have noticed this over the decades. Of course the vast majority of technicians are male. But, when they are making "off the air" measurements, or having a female dispatcher do a "test count", etc., the fact that the female voice usually is more symetrical is immediately apparent.

Glen, K9STH

W8JI
08-27-2004, 01:14 PM
Quote[/b] (K9STH @ Aug. 26 2004,18:44)]Tom:

I hate to "nit pick", but you certainly are not running a "stock" Globe Scout considering all of the modifications that you have made to improve it! #However, I definitely agree about the Heising modulation. #Also, many of the Globe Scouts really do not have anything that resembles symmetrical modulation. #I believe that this was even pointed out in the QST review of the Globe Scout Deluxe version that came out in the very early 1960s. #The ARRL said that the modulation "sounded OK", but definitely was "not symmetrical".

Another thing that really shows up on FM much more than AM (but it can there as well) is that the vast majority of male voices are not symetrical while most female voices are. #That is, when observed on an "scope" type deviation monitor the peaks on one side do not equal the other side. #Typically when the deviation is, for example, +5KHz on one side the other side is between -4 KHz and -4.5 KHz. #When a female modulates the FM transmitter the deviation is pretty much the same on both sides of the carrier.

I don't know if anyone has really done any real investigation as to what causes this. #However, many two-way radio technicians have noticed this over the decades. #Of course the vast majority of technicians are male. #But, when they are making "off the air" measurements, or having a female dispatcher do a "test count", etc., the fact that the female voice usually is more symetrical is immediately apparent.

Glen, K9STH
The positive peak is limited because the 6L6 modulator is class AB and single ended using only a choke. It can pull down on the HV line, but not up. Any upward swing in HV is caused by "flyback" action in the modulation choke and the limited dc drop in the choke when the 6L6 is resting. My rule of all stock above chassis parts and no new holes prevents me from putting in a new modulation transformer.

You can't tell my Globe from a stock Globe, except it has a few wires moved around and three terminal stips with parts mounted on existing holes below the chassis.

Operationally it is totally different, with regulated supplies and grid block keying for click-free CW and twice the output with no more PA heat than stock. The tube is class C now, and has no cathode bias resistor.

73 Tom

AG3Y
08-27-2004, 02:16 PM
This is reaching back a loooong way into the past for me, but I seem to remember that in the professional broadcast field there were audio processors for A.M. stations that took advantage of the asemmetry ( sp ? ) of the modulating waveform, and flipped the phase around so that the positive peaks could go up to wherever they wanted to, and the negative peaks would not exceed 100%. Some stations were able to achieve 150% positive modulation without clipping off the carrier, and sounded "loud and clean" !

Since the unmodulated carrier was the reference level for the station's licensed power, and there were no distortion products produced, everything was square and legal!

73 from Jim

W8JI
08-28-2004, 01:42 AM
Quote[/b] (AG3Y @ Aug. 27 2004,07:16)]This is reaching back a loooong way into the past for me, but I seem to remember that in the professional broadcast field there were audio processors for A.M. stations that took advantage of the asemmetry ( sp ? ) of the modulating waveform, and flipped the phase around so that the positive peaks could go up to wherever they wanted to, and the negative peaks would not exceed 100%. #Some stations were able to achieve 150% positive modulation without clipping off the carrier, and sounded "loud and clean" !
Jim,

It's been too long since i worked at a BC station to recall exact rules, but I think there was a limit of around 125% for positive peaks.

Some audio processors watched the input waveform and switched phase to minimize the amount of unnecessary modification of waveform, while others actually processed the signal and made waveforms nonsymetrical.

I believe the OptiMod had provisons for both. Maybe someone else knows for sure. I know there was a legal limit for positive peaks at one point in time.

73 Tom

KL7AJ
11-18-2004, 07:14 PM
Quote[/b] (K9STH @ Aug. 24 2004,08:46)]You also need to remember than under the "new" FCC regulations that you CANNOT run more than 375 watts carrier output with 100 percent modulation. #FCC regulations now allow 1500 watts peak output. #This means 1500 watts of carrier with CW, 1500 watts carrier output on FM, 1500 watts peak on SSB, but since the peak power output of a 100 percent modulated AM signal is 4 times the carrier you can only run 375 watts of carrier.

This is unlike years ago when you were allowed 1000 watts DC power input for the carrier. #On 75 meters it was not unusual to get 75 percent efficiency so the carrier would be 750 watts output which would mean 3000 watts peak power output. #As such, many of the older AM rigs have to be cut back in power to meet the present specifications.

I have a Heath Apache, Heath DX-100, Heath DX-35, Globe Champion 350A, Hallicrafters HT-20, and a Globe Chief 90A with the SM-90 screen modulators that I can use on AM. #It is a "toss up" between the Globe Champion 350A and my slightly modified Apache as to which is the "best" sounding. #You can modify the Apache by just changing 3 capacitors. #Before changing those capacitors I got the usual "scratchy Apache" reports. #After changing them I get "rave" reports even from the VERY critical operators that hang out on 3880 KHz in the period just after dawn.

You can find that modification on either

http://home.comcast.net/~zcomco

or

http://home.comcast.net/~k9sth

As for using a linear, W8JI is correct in that you can only run 1/4th the power input for your carrier that you would on CW. #Also, you definitely have to watch the power dissipation of the tubes and the carrier output power. #Again, remember, that you cannot put out more than 375 watts of carrier.

Glen, K9STH
Rats! I guess that puts the kibosh on "supermodulation" eh?

Yes...any clean A.M. signal can be amplified with a linear amplifier, if you keep the carrier level down to where the peaks never saturate. (I'd never run A.M. through a linear without an oscilloscope, though. But that's just me...I always felt naked wifout a scope)
My point is...WHY? Linear amplification of A.M. is very inefficient....and totally out of the "spirit" of Ancient Modulation. The way to do it is with high level modulation...There's something sacrilegeous about using anything but High Level modulated A.M. Get yerself a Globe King or Johnson Viking and have at it!

By the way...for more on the joys of A.M. read my November '93 QST article, "Solder to Talk."

Eric, KL7AJ

KL7AJ
11-18-2004, 07:21 PM
Quote[/b] (AG3Y @ Aug. 27 2004,07:16)]This is reaching back a loooong way into the past for me, but I seem to remember that in the professional broadcast field there were audio processors for A.M. stations that took advantage of the asemmetry ( sp ? ) of the modulating waveform, and flipped the phase around so that the positive peaks could go up to wherever they wanted to, and the negative peaks would not exceed 100%. #Some stations were able to achieve 150% positive modulation without clipping off the carrier, and sounded "loud and clean" !

Since the unmodulated carrier was the reference level for the station's licensed power, and there were no distortion products produced, everything was square and legal!

73 from Jim
YES!! The Gates Solid Statesmen limiters did this...(with a pair of rather audibly evident mechanical relays, by the way!) Current FCC rules allow 130% positive peaks on A.M. transmitters. By the way, Leonard Kahn, ironically, developed the Symmetra Peak to get RID of asymmetry in male voices.....a lot of Symmetra-Peaks were installed in TV and FM stations in the late 60s

Incidentally, although some stations WERE going to 150% on positive peaks, it was never legal. The FCC also specified modulation percentages as well as carrier power....and always has.


Eric



Eric