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Author Topic: HF AM???  (Read 44631 times)
W2WDX
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Posts: 214




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« Reply #15 on: September 29, 2013, 07:11:18 AM »

My SSB rig can AM, but pretty low power, like 25 watts max carrier.  What is the point of being hi fi if your weak and in the noise?   

You do understand of course that 25 watts of carrier on AM also has a PEP component? So the actual output of you radio is the same or similar. In the recording I posted the louder station, W2NBC, is using only 20w of carrier and he is in New Jersey and the station that recorded this was about 400 miles away. Like anything in Ham radio it's not about power, its about radiation efficiency. NBC was using a long and high random length dipole using ladder-line and a link coupled tuner. A very efficient antenna system.

Here in New York City there is even an open 2m AM repeater. There are numerous people who use AM regularly on VHF, 10m 6m & 2m. I am fortunate that I live here since there is a great deal of AM activity on most bands in this area. My Clegg Zeus gets a work-out regularly here. I also used some retired and converted ITT FAA AM transmitters on 2m.

AM operation has evolved into a very relaxed mode and is about casual conversation and discussion. It is one of the few modes where you will find all types of Hams; collectors, gear heads, restorers, audio aficionados, people from the broadcast industry and most importantly experimenters.

If any of you have seen many of my posts, you can tell I operate AM. I am not known for brevity, on the air nor in writing.
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G3RZP
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Posts: 8079




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« Reply #16 on: September 29, 2013, 07:31:30 AM »

John,

I suspect that a lot of amateurs have forgotten that the PEP of a 100% modulated DSB AM signal is four times the carrier power. So a pair of plate and screen modulated 6146Bs running an output of 100 watts of carrier would be 400 watts PEP.
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WA2OLZ
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Posts: 110




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« Reply #17 on: September 29, 2013, 03:57:52 PM »

Back in the days before SSB, many of us worked the world with low powered AM transmitters. It took patience, technique and was a whole lot of fun.  Nobody was running lists to hand out contacts, no Internet based callouts of who was where, just listen and QSY.

My best DX on AM was 5A2TC on 10 meter AM. The transmitter was a WRL Globe Scout 40A feeding an indoor 10M dipole.  It took several tries before I realized his call wasn't WA2TC???. Oh, well! It would be a tough QSO to get today, I imagine. 5A2TC was at Wheelus AB, Tripoli, Lybia! I sure wish I still had that QSL card.
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KA5N
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Posts: 4380




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« Reply #18 on: September 29, 2013, 05:59:47 PM »

John you say you are going back to school, I can't help thinking about the six years you
have had a ham ticket and how much you could have learned in those years about all phases
of amateur radio and care and feeding of older ham gear.  The gear you have is certainly
not in the "boat anchor" category.  Find a local elmer and get some advice and help and
you will soon close the knowledge gap about your equipment and what wonders await.

Good Luck and 73
Allen KA5N
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W2WDX
Member

Posts: 214




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« Reply #19 on: October 01, 2013, 07:28:06 AM »

John,

I suspect that a lot of amateurs have forgotten that the PEP of a 100% modulated DSB AM signal is four times the carrier power. So a pair of plate and screen modulated 6146Bs running an output of 100 watts of carrier would be 400 watts PEP.

Actually you're half right. A pair of 6146b's will run a 25w carrier with 100W PEP when modulated, either screen or plate.
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G3RZP
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Posts: 8079




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« Reply #20 on: October 01, 2013, 08:11:27 AM »

RCA rate a pair in plate modulation ( which must mean screen as well) at 120 watts of carrier output - which would be  480 watts PEP.

With just screen grid modulation, you would want to keep to about 25 watts of carrier, though. The disadvantage of efficiency modulation.

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KC4MOP
Member

Posts: 955




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« Reply #21 on: October 01, 2013, 05:22:19 PM »

The real boatanchor transmitter Heathkit DX100 uses a pair of 6146B for a 100 watt carrier.
http://heathkit-museum.com/ham/hvmdx-100.shtml

Fred
Modern SDR's can make some nice audio on AM. Feed that 25 watt (or whatever drive the linear needs) carrier into a big linear and you have legal limit 375 watts of carrier which equals 1500W PEP
Fred
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W2WDX
Member

Posts: 214




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« Reply #22 on: October 01, 2013, 05:38:03 PM »

No ... on the DX-100 that's 100w input, not carrier.

At a 23w max plate dissipation at 750v max plate voltage in Class C the maximum power output is 62W for a single 6146b. That's it. You could not get 450W out of a pair of 6146's. That's 4 times higher than its maximum output capability in Class C. A pair of 572b's would be needed to get that kind of power, at 1800Vdc plate voltage at 300ma.

Let's do the math:

W = V x I

Watts = Volts x Amperage (in amps)

750Vdc x 112ma = 84w x .74 (~efficiency of Class C) = 61.16W

For two 6146B:

750Vdc x 224ma = 168w x .74 = 124.32w

And that efficiency is an estimate on the high side; 65% to 70% is more typical for real world Class C circuits. And this does not take into account real world things like voltage sag in the power supplies at peaks and tank circuit losses.

So a 25W carrier is typical with a modulation factor of 1 producing peaks up to around 110W to 120W with most Class C transmitters using a pair of 6146B's.

By your thinking my Johnson Valiant would be able to produce 675W peak!!! Holy shnikees!!!

John, W2WDX
« Last Edit: October 01, 2013, 06:38:03 PM by W2WDX » Logged

K1ZJH
Member

Posts: 3210




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« Reply #23 on: October 01, 2013, 10:02:18 PM »

A pair of 6146B tubes should easily provide 120 watts carrier power. At 100% modulation the plate voltage doubles, which is a 4 times increase in power. Even at 100 watts output, the PEP would be 400 watts for 100% modulation. For example, a B&W 5100 is rated for 135 watts input on phone. with a 70% efficiency that is still 95 watts output; or 380 watts PEP output power. Only a true Class C stage will provide 4X pep power when the plate voltage is doubled.

http://faculty.frostburg.edu/phys/latta/ee/6146amp/6146b/6146b_big.pdf

Even my  two B&W 5100 AM transmiiters have 150 watt modulator sections...  that would be rather silly to modulate a 50 watt carrier, per your estimation Smiley

Now, a 6146 running as a linear might be limited to 25 watts carrier, in order to provide PEP headroom... but in high level plate modulated Class C the tube is run at CCS or ICAS ratings. PEP is always 4 times carrier power for 100% modulation and a symmetrical modulation waveform for Class C.

G3RZP is absolutely correct in his analysis.

Pete k1zjh
« Last Edit: October 01, 2013, 10:15:24 PM by K1ZJH » Logged
W2WDX
Member

Posts: 214




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« Reply #24 on: October 02, 2013, 03:26:09 AM »

I think I had a complete DOH! moment there.  Embarrassed

Looking at this this morning, I ran the calculation for carrier power, didn't I? Of all people, I know better.

Remind me not to post anything after a day out with the guys. Gotta love Tequila! Hey, I got the math right regardless! Go figure.

Never mind, I'll shut up now.

« Last Edit: October 02, 2013, 03:32:57 AM by W2WDX » Logged

KC4MOP
Member

Posts: 955




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« Reply #25 on: October 02, 2013, 03:32:38 AM »

No ... on the DX-100 that's 100w input, not carrier.

At a 23w max plate dissipation at 750v max plate voltage in Class C the maximum power output is 62W for a single 6146b. That's it. You could not get 450W out of a pair of 6146's. That's 4 times higher than its maximum output capability in Class C. A pair of 572b's would be needed to get that kind of power, at 1800Vdc plate voltage at 300ma.

Let's do the math:

W = V x I

Watts = Volts x Amperage (in amps)

750Vdc x 112ma = 84w x .74 (~efficiency of Class C) = 61.16W

For two 6146B:

750Vdc x 224ma = 168w x .74 = 124.32w

And that efficiency is an estimate on the high side; 65% to 70% is more typical for real world Class C circuits. And this does not take into account real world things like voltage sag in the power supplies at peaks and tank circuit losses.

So a 25W carrier is typical with a modulation factor of 1 producing peaks up to around 110W to 120W with most Class C transmitters using a pair of 6146B's.

By your thinking my Johnson Valiant would be able to produce 675W peak!!! Holy shnikees!!!

John, W2WDX

No................I owned a couple of DX 100's and even the scratchy the Apache.  They made 120 watts carrier. I saw it over and over on a Bird watt meter. 10M they would be sweating a little but, 100 watts out to the antenna.
Fred
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W2WDX
Member

Posts: 214




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« Reply #26 on: October 02, 2013, 03:37:35 AM »

I got caught up in the moment. I normally run my boatanchors rolled back, to reduce plate dissipation and drive amplifiers. Somehow in my Tequila driven mindset at the time, I translated that into ...

Well anyway ...

There are times I wish one could just remove a post from a message board. DOH! Now people are gonna be clicking "reply" on my bogus post without scrolling down to read this retraction and the thread is gonna be filled with now pointless flames.

John

« Last Edit: October 02, 2013, 03:51:30 AM by W2WDX » Logged

KC4MOP
Member

Posts: 955




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« Reply #27 on: October 02, 2013, 07:17:56 AM »

I got caught up in the moment. I normally run my boatanchors rolled back, to reduce plate dissipation and drive amplifiers. Somehow in my Tequila driven mindset at the time, I translated that into ...

Well anyway ...

There are times I wish one could just remove a post from a message board. DOH! Now people are gonna be clicking "reply" on my bogus post without scrolling down to read this retraction and the thread is gonna be filled with now pointless flames.

John



That is no problem........It is really being able to shift gears. Today's Amateur is not familiar with a plate modulated tube transmitter. The solid state stuff and class D E F whatever, and linear devices gets into that carrier mode and Peak Envelope Power mindset.
A class C power amplifier will output a lot more RF watts because of the bias setting on the tubes. To make AM we need one half of the input/output power of audio to PLATE modulate the transmitter. Hence a 100 W class C transmitter will need a 50 watt audio amplifier with the proper transformer matching to modulate the Class C transmitter.
Most people today are equating the use of linear amplifier with its Peak Envelope Power limitations.
To make what we call legal limit AM we need a linear to output easily 1500 W PEP or there will be flat topping and distortion of the RF output. Linear amps that claim 2500W are the ones that have the necessary headroom.
375 watts carrier fully modulated will make 1500 W PEP.
There are readers and nay sayers who will say phooey to me, and claim that they can make a 700 watt carrier, and with some inaccurate peak reading watt meter, claim that they will adjust the modulation to only produce 1500w PEP.
A.M operation is a lot of fun and a challenge to get that nice audio. Better antennas and keeping close to legal limit is necessary to calm down MaNature. A.M. reception will be lot noisier with the wider bandwidth.
I have used old broadcast transmitters outputting 250 Watts of carrier and did just fine on 160M. Very little difference in signal level from 250W and 375. Antenna Antenna Antenna is the key.
Fred
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G3RZP
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Posts: 8079




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« Reply #28 on: October 02, 2013, 01:48:11 PM »

Fred,

Of course you can have a 750 watt carrier and not exceed 1500 watts PEP. But you mustn't exceed on peaks, something around 50% - or maybe less, I haven't done the sums - modulation depth.

My suspicion, again without having done the sums, is that you are better off with a higher percentage modulation and lower  i.e. 375 watt carrier.
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W2WDX
Member

Posts: 214




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« Reply #29 on: October 03, 2013, 10:54:09 PM »

And let's not forget about asymmetrical peak limiting! Negative peaks ... baaad, positive peaks ... gooood.

I'm still hurting from my Homer moment! I've owned about 30 or so boatanchors, many using 6146's, restored professionally literally hundreds, owned and converted two BC transmitters (one Gates and one RCA).

I have even designed and built a commercial design for a new AM plate modulated boatanchor type, using 6146's in high level Class C plate modulation with two EL-34's (AB1) using a toroidal modulation transformer and a DDS based VFO. It even had front panel switchable carrier output (20w to 100W). The fully analog audio section even had built in variable compressor with variable asymmetrical peak limiting, and a five band fully parametric EQ with high and low pass filters, and both standard communications mic input (hi-lo imp) & XLR balanced low impedance mic input (with phantom power) and a balanced line level input. It operated at full 8K00A3E bandwidth (or less by using the filters in the audio section). I would have been manufacturing now if the financing world hadn't gone belly up. Funny thing, as a result I am still sitting on about 300 new 6146's purchased for that project.

So for me, going Homer was really embarrassing!

John, W2WDX
« Last Edit: October 03, 2013, 11:10:56 PM by W2WDX » Logged

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