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Author Topic: 6 METER AMPLIFIER  (Read 1384 times)
KE4CQW
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« on: July 22, 2003, 02:27:46 PM »

Hello all,

I guess you would call me a late blomer in the 6 meter craze, but I am looking for information on bulding a decent 500 + / - watt amplifier for 6 meters.  It could solid state or tube but I think a tube style will be much better to build and use.  I have a friend who is a tube nut so I will have an advantage there in making it.  I have small budget to put forth so buying a new one is out of the question.  Hence building one will be like the John Cash caddy amp, one piece at a time.  :-)  Thanks for all you help, 73 Moe Brewer  ke4cqw@knology.net  
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WB2WIK
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« Reply #1 on: July 22, 2003, 03:22:00 PM »

By far cheaper than building from scratch, unless you happen to have all the components on hand already, is to simply modify a suitable HF amplifier candidate.  One great example is the Ameritron AL-80, although most any HF amplifier using a single 3-500Z is a splendid conversion candidate.  They cost very little on the used market, and already include the tube; socket; fan(s) or blower; chassis; power supply; T-R relay; metering; and 90% of everything needed to make a viable 500W 6m amplifier.  

Typically, all you'd need to change is the input and output circuits, plate RF choke, filament RF choke, and parasitic suppressor, and all this can be done in one working day for about $50.

Alternatively, you could of course use a single 4CX250B, 3CX400A or any number of single VHF-UHF power triodes or tetrodes to make a great 6m amp, but if you must acquire the power supply components, chassis, socket, chimney, blower, meters and other components, even at highly discounted cost, these will sum far greater than the cost of a used HF amp.

WB2WIK/6
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KE4CQW
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« Reply #2 on: July 22, 2003, 08:32:27 PM »

Thanks,  just wondering if you might know what the going rate for a amp that could be modified for 6 meters?  Also is one type easier to upgrade verses another?  Money to buy one is tight so hand making one I thought might be easier to do.  73 and thanks for your help and input.
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N8FVJ
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« Reply #3 on: July 22, 2003, 10:29:39 PM »

First, how many watts do you have to input the amp? If 100 watts, a Heathkit SB200 for about $250 is an inexpensive amp for conversion.
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AB5Q
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« Reply #4 on: July 22, 2003, 11:12:58 PM »

About one year ago I converted a SB-1000 (Single 3-500Z) to 6m mono.  Amplifier cost $500, conversion components ran me an additional $250-300.  Amp will do a full kW PEP with about 60-65 watts of drive.   Tank components are silver plated, Pi input has air variable caps for precise cathode match, and Pi-L output with vacuum variable plate cap ($$) for superior harmonic suppression.  You could probably save about $50 by not silver plating.  Personally, I prefer silver plating for reliability and appearance.  Plate choke is reusable with simple modification, and the filament choke can be reused without modification.  I had to fabricate a few parts out of Teflon, aluminum and copper, so you will need to be handy with tools.  

A few years back, I converted a SB-200 with 572Bs but was not happy with the performance/stability.  Ended up converting it back to HF and selling it.  The Chinese/Russian versions of 572Bs are not capable for use on 6m, end of subject.  In my experience, I would NOT recommend the 572B on 6m or even attempting the mod as a novice builder.

If you would like to home-brew from scratch, take a look as the GS-35B Russian ceramic triode.  They are cheap and one can easily produce legal limit on 6m.  Keep in mind, if you do not have access to cheap parts, home-brewing can get very expensive.   Without an adequate junk box, home-brewing would be for fun and not economics.  

Feel free to drop me an email if you decide to tackle the conversion and have any detailed questions.

John - AB5Q
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KE4CQW
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« Reply #5 on: July 23, 2003, 07:46:21 AM »

Good morning all,

I have a couple of rigs that I can use on 6 meters one being an FT-817 and the other a FT-100.  So we are talking about a day and night difference in input power.  I have to admit I would like to use the FT-100 more but I think the FT-817 has the better XCVR of them.

Having the parts around should not be a problem.  I have a friend who is a tube nut to say the least.  Him and his father are both hams and both like music.  They design and build all their own equipment which of course is all tube based.  His web site is www.resamps.com if you have a chance check it out, but with his help building one should not be a problem.

I guess the biggest thing is getting hold of some designs.  Does anyone know of some 6 meter designs online or could send me some via snail mail?  I am good in the call book if some one has the time and can spare the information.  Thanks for all the help advice it sure does come in handy!  73  Moe
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WB2WIK
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« Reply #6 on: July 23, 2003, 01:08:56 PM »

I agree the SB200 is a lousy conversion candidate for six meters, simply because the 572Bs have very little gain and produce little output at 50 MHz.  The SB220, on the other hand, with its 3-500Zs, is an excellent candidate as is the SB1000, AL80A/B, or anything else with a 3-500Z in it.  

One of my personal favorites to modify for six is the old National NCL-2000, simply because it uses VHF-UHF tetrodes by design (a pair of 8122s) and those can be driven by 40W to more than 1000W output on six meters.  And it's an easy conversion.

I have never spent anything even close to $250 to convert an HF amp to six meters, and I've done dozens.  The most expensive component is always the plate tuning vacuum variable cap, and I usually use a Jennings 2-30pF 7.5kV glass or ceramic vacuum device which I can always find on the surplus market for $25-$30 -- never paid more, have frequently paid less.  I usually re-use the original plate TUNE cap as the new six meter LOAD cap, since the tuning range is just about perfect, and I discard the original LOAD cap, the original bandswitch, the original plate tank inductor(s), the original plate RF choke, the original filament choke(s) and the original parasitic suppressors, and replace them all with ones wound specifically for six meters (by me), making sure they have no resonances anywhere near 50 MHz.  I re-use the original ferrite materials for the filament choke(s) and just use fewer turns, and often re-use the original plate RFC form (ceramic, teflon, whatever it was) and simply wind a new choke with fewer turns, etc.

I've never found that silver plating improves performance in any measurable way at 50 MHz...or pretty much at any other frequency, provided I use oxygen free copper components with soldered connections.

Last amp conversion I did was an SB220 with a pair of 3-500Zs and the entire conversion cost $60, plus my time.  Yielded an amp that runs about 1100W PEP out on six with 80W PEP drive.

It would be difficult to build a six meter amp that can be driven equally well by an FT817 or an FT100.  To make a "500W" amp that can be driven by the meager power of the FT817 would take a tetrode or pentode, to develop sufficient gain, and then that tube would have far too much gain for the FT100 application.  The power supply requirements for tetrodes or pentodes are much more complex, and most of these tubes require significant warm-up time (90 to 180 seconds) before you can operate, so I try to steer away from these devices unless I have some reason to use them.

The old Gonset model 913A uses a single 4X150A/4CX250B to develop 300W PEP output with only 5W PEP drive and would be absolutely perfect for the FT817 application.  I see these all the time at local ham radio swap meets for about $250 -- far less than the cost to build one.

As for schematics: If your friends are truly familiar with vacuum tube amplifier design, trust me, they'd have absolutely no reason to follow a schematic for a single-band amplifier; I'd be very reluctant to use anyone for a technical resource who needs a schematic in order to perform this task.  However, the ARRL Handbooks and ARRL VHF-UHF Manuals from the 1950's through the 1970's (not any longer) were absolutely loaded with 6m amp designs in the 500W-1000W output range.  You can pick up these older books from Amazon or many other sources.

WB2WIK/6

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AB5Q
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« Reply #7 on: July 24, 2003, 12:32:45 AM »

Went through the figures tonight, the conversion cost me $245.00 and it’s worth every dime. I went a lot further than your “typical” low cost conversion. My intention was to build it right the first time and not to keep trying until I got it right (like those who build dozens). Makes you wonder....

Try this web site for details on inexpensive Russian Triode and Tetrode designs. http://www.nd2x.net/base-1.html Pay particular attention to the GS-35B, socket/chimney can be easily home brewed.

Granted grounded grid designs are very simple to build at a lower gain.  Grounded cathode or grid driven would provide the gain required for your 5 watt drive limitation at a slight increase in complexity. I think that the above link has a 6M grid driven design on file. There are a few tetrode control/screen grid regulator boards available at a low cost. Check out this link http://www.tomstubes.com/products.htm

Most, if not all of the Russian ceramic tubes have indirectly heated cathodes. Indirectly heated cathodes require a short filament warm up period (2-3 minutes) without plate voltage applied. A few of the after market control boards have provisions for proper start-up sequencing. Homebrew is another option, very simple to fabricate a sequencing circuit.

Hope this helps.
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KE4CQW
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« Reply #8 on: July 24, 2003, 07:42:36 AM »

Thanks a million for all your help.  It is good to know people still will take the time to help someone out.  I am going to try and keep you updated on my adventure as I do this.  My back round is "solid state electronics".  When I was in school we had about 2 weeks of tube technology and that was almost 15 years ago.  I am the one wanting the plans to build one so I can teach myself how they work again.  The guy who is going to help me pulls stuff out of his head when it comes to tubes.  Anyway thanks again I will try to keep you guys updated and please if you have any more advice please give it, I have learned so much with this post and researching what you telling me I think I might be able to pull this off!  73  Moe
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