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Author Topic: Which AM Transmitter?  (Read 38242 times)
KB5JO
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Posts: 66




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« on: September 29, 2011, 09:54:27 AM »

I have become interested in AM (again), presently using a Globe Scout 680 with Heising modulation on 40M.  Have been thinking about a plate modulated transmitter, initial thoughts being a Ranger or Valiant.  But, I have been reading that they aren't very reliable, difficult to keep on the air, and that a DX-100 is a better option.  I have owned both of the Johnson transmitters decades ago ( why did I sell them?! ), they seemed reliable then,  but maybe age hasn't been kind.  All of these seem to have become quite expensive, are there other options I ought to consider?

73m Curt KB5JO
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W1BR
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« Reply #1 on: September 29, 2011, 10:50:19 AM »

You could use a linear that is rate to handle the 375 watt carrier limit for AM service to boost
the WRL transmitter's power.

I have a pair of B&W 5100 transmitters that I am going to rehab for AM and CW. They are huge,
but the modular construction makes servicing a lot easier that the dogs' breakfast you will find
in unitized chassis transmitters.  My concern with many of the Heathkit products is that
the potted power and modulation transformers seem to be a bit more prone to failure than they
should be. There is a good group of AMers over on AMfone.net, you might search the discussion
groups over there for more ideas.

Pete
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AC5UP
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« Reply #2 on: September 29, 2011, 11:22:43 AM »

It's a two day drive and another $200 for gas & motel if you want to eyeball the goods before you buy, but this is exactly what you need...............

http://tulsa.craigslist.org/ele/2595237758.html

On eBay you'd pay about that much for either piece, solo, and still have to deal with shipping. Back in the 60's this was a PDG station.

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KA5N
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« Reply #3 on: September 29, 2011, 11:47:26 AM »

The Eico 720 with a 730 modulator (and of course a VFO) makes a nice high level modulated AM rig that is easy to work on if repairs are needed.  Also you won't get a hernia lifting it and moving it around.  Wish I had kept mine.

Allen
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N2EY
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« Reply #4 on: September 29, 2011, 02:29:21 PM »

I have become interested in AM (again), presently using a Globe Scout 680 with Heising modulation on 40M.  Have been thinking about a plate modulated transmitter

Heising modulation IS plate modulation. What makes it Heising is that the modulator is single-ended Class A.

initial thoughts being a Ranger or Valiant.  But, I have been reading that they aren't very reliable, difficult to keep on the air, and that a DX-100 is a better option.

The main issue with any older rig is that it's....old. All of the rigs you mention are more than 45 years old; many examples are well over 50. How reliable they are depends very much on how they've been treated over the decades.

Those old rigs may seem expensive today, but they're no more expensive than what they cost back in the day, when you adjust for inflation. Yes, we went through a period in the 70s-80s when the old stuff went for a song, but those days are gone.

Personal opinions:

- Any kit rig is as good or bad as the original builder.

- DX-100 is a good rig but a desk-crusher with a so-so VFO. With audio mods it can be rather good. Apache is a more-modern version that will cost more.

- All the EFJ rigs are good but again the Valiant is a monster. My favorite is the Viking 2; the VFO is external and you can use it on the WARC bands.

- The Eico 720/730 is about the same power as your 680, maybe a little more. Needs external VFO. Main problem is that the modulator uses audiophile tubes.

- Consider homebrewing an AM rig - or at least a modulator. If you focus on one or two bands it's not that hard.

73 de Jim, n2EY
 
 
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W8JI
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« Reply #5 on: September 29, 2011, 07:15:49 PM »

I have become interested in AM (again), presently using a Globe Scout 680 with Heising modulation on 40M.  Have been thinking about a plate modulated transmitter, initial thoughts being a Ranger or Valiant.  But, I have been reading that they aren't very reliable, difficult to keep on the air, and that a DX-100 is a better option.  I have owned both of the Johnson transmitters decades ago ( why did I sell them?! ), they seemed reliable then,  but maybe age hasn't been kind.  All of these seem to have become quite expensive, are there other options I ought to consider?

73m Curt KB5JO

All you ever wanted to know about Heising (constant current) modulation, and Globe Scouts:

http://www.w8ji.com/Heising%20modulation.htm

A little work and the Globe Scout is indistinguishable from any other plate modulated rig.
« Last Edit: September 29, 2011, 07:21:45 PM by W8JI » Logged
KX5JT
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Posts: 217




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« Reply #6 on: September 30, 2011, 08:29:35 PM »

Heising is an excellent way to go for AM.  Class A!!  Think very low distortion!  The first AM broadcast stations in the 1920's and 1930's used Heising modulation.  A lot of homebrew plate modulated rigs also use Heising modulation.  I would seriously think of using that Globe Scout.  I've heard Valiants have all the components pretty much maxed out, but I never heard that the Rangers are particularly unreliable.  I hear many Rangers and Valiants out there.

Anyway, Heising is a great way to go, I don't understand why you don't think it is plate modulation, it is indeed!
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W8JI
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« Reply #7 on: October 01, 2011, 05:09:41 AM »

The Globe Scout needs work for 100% symmetrical modulation. It is terribly distorted and non-symmetrical as manufactured. That work is documented at the link I gave earlier (heising modulation) and here:


http://www.w8ji.com/globe_scout_modifications.htm

The Valiant is not pushed to the max, contrary to what some people say. The transmitter is great as is with the exception of a few capacitor changes in the audio.

http://www.w8ji.com/Johnson%20audio%20mods.htm
« Last Edit: October 01, 2011, 05:12:01 AM by W8JI » Logged
N2EY
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Posts: 5081




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« Reply #8 on: October 01, 2011, 02:56:45 PM »

Heising is an excellent way to go for AM.  Class A!!  Think very low distortion!  The first AM broadcast stations in the 1920's and 1930's used Heising modulation.  A lot of homebrew plate modulated rigs also use Heising modulation. 

The bad thing about Heising modulation is that the modulator efficiency is low. You burn up lots of watts in the modulator tubes. You also need a rather enormous mod choke for the power involved.

However, if those things are OK, there's no reason not to use it.

73 de Jim, N2EY 
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KD0FAT
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Posts: 68




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« Reply #9 on: October 01, 2011, 03:34:49 PM »

Not trying to hijack the thread, but I wonder how the Johnson Viking One stacks up as an AM transmitter? I bought one a while ago, intending to check it out for AM use. It is certainly heavy duty (and just plain heavy). I have heard that the Viking One was guilty of TVI back in the day when everyone was watching channels 2-13 over the air. What do I need to pay attention to in order to bring this beast back to life? 73, Al
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W8JI
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« Reply #10 on: October 01, 2011, 06:32:10 PM »

We have to be careful with what people call Heising modulation.

Many AM operators wrongly call traditional modulation "Heising" because they add a modulation reactor.

Many AM operators call the modulation reactor in a traditional plate modulated stage a "Heising choke".

To be Heising modulation the transmitter must have a single modulator tube off the feed to the PA stage, NO TRANSFORMER.

If it has a choke and a modulation transformer, that is traditional plate modulation with a modulation reactor. The modulation reactor or choke simply keep plate current out of the modulation transformer to improve low frequency response. The plate current in a normal PA stage will reduce inductance in the mod transformer by DC biasing the core.

When I look at what most people call Heising, it turns out to be traditional plate modulation. This is a very common mistake.

Heising is, by definition, constant current from the supply. That current alternates between the PA tube and the modulator tube. It has only a modulation choke, no transformer at all.

If it has a transformer, even if it has a choke, it is not Heising. The choke is not a Heising choke.

73 Tom
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N4NYY
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Posts: 5224




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« Reply #11 on: October 02, 2011, 07:01:22 AM »

Are all AM stations old Boatanchors? It appears this is a hell of a lot more popular than I thought. At our club hamfest, these old AMers disappeared faster than a communist dissenter. There was a nice Valiant that a guy I parked for tailgating, was displaying. I went to park another car, and the damn Valiant was gone. I just don't seem many of these styles.

Do people do AM with modern rigs?
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G3RZP
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« Reply #12 on: October 03, 2011, 12:43:38 AM »

I'm not sure if many modern rigs still do AM.

If you've won the lottery and want an AM rig, then a 32V1 or 2 from Collins or even a KW1(?) - the rig with a pair of 4-250A in the PA and 872 rectifiers. I seem to remember they were priced in the thousands of dollars in the early 1950s...and you need a 75A2 to go with it.
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N4NYY
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« Reply #13 on: October 03, 2011, 07:38:43 AM »

Quote
If you've won the lottery and want an AM rig, then a 32V1 or 2 from Collins or even a KW1(?) - the rig with a pair of 4-250A in the PA and 872 rectifiers. I seem to remember they were priced in the thousands of dollars in the early 1950s...and you need a 75A2 to go with it.

Collins is big money here.
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G3RZP
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« Reply #14 on: October 03, 2011, 08:26:50 AM »

I've heard that a lot of Collins gear is snapped up at high prices by Japanese collectors. But I did say 'if you've won the lottery'!
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