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Author Topic: 2x 4cx800 on 6 meters  (Read 5861 times)
W6IZT
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Posts: 12




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« on: June 30, 2006, 12:59:09 PM »

I am working on designing an amp using 2 4cx800s on six meters, grid fed, pi-L output. I am looking for feedback from anyone who has experience with this configuration on 6.

Any and all feedback is welcome

Gregg
W6IZT
Marietta GA
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K3GM
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Posts: 2549




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« Reply #1 on: June 30, 2006, 01:46:26 PM »

Check out GM3SEK's site.  He has a lot of info on tetrode applications. http://www.ifwtech.co.uk/g3sek/

Back in the late 80's, I;m pretty sure it was CQ that published a construction article on building a legal limit 6 meter amp, but it used an 8877, a cheap tube still easily found I think. While I use the 4CX800 in my HF amp, it's only a single tube.  I think if I was scratch building, I'd lean towards using a single tube like the 8877.  Cheaper, less complicated, no need to balance the 2 tubes.
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K0BG
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« Reply #2 on: June 30, 2006, 02:14:00 PM »

I too lean toward a triode. Tetrode designs as more prone to high levels of IMD than triode are, and this is an especially important consideration on 6 meters. Further, while two tubes will certainly make you king of the hill, the fact remains when the band is open even 100 watts is a blast! And let's not forget all of the other first-class accessories which will become a prerequisite for this band and power level.

Alan, KØBG
www.k0bg.com
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WB2WIK
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« Reply #3 on: June 30, 2006, 02:58:27 PM »

The single 8877 amp for 50 MHz that was in CQ Magazine was an article I wrote, published in June 1987.  

The CQ archives are on line, but their server has problems right now, I think.  I just tried accessing it and couldn't, and the parent server (buck.com) doesn't seem to be up either.  Maybe a temporary outage.

Anyway, if they come back on line, the 8877 six meter amp article is at:

http://www.hamcall.net/cqcgi/?res=1&yr=1987&mo=06&pg=030&nam=WB2WIK

It's a good article (if I say so, myself) mostly because it's well documented with a lot of clear photographs showing how it goes together, what it looks like when it's done, close-ups of wiring, etc.  Also full schematics and parts lists.

It's easy to build and works very smoothly.  Still on the air 20 years later...

WB2WIK/6
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K3GM
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« Reply #4 on: June 30, 2006, 04:10:23 PM »

"The single 8877 amp for 50 MHz that was in CQ Magazine was an article I wrote, published in June 1987."


How 'bout that! Smiley  When a reader still has recollections nearly 20 years later, it was a good one Steve!
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WA9SVD
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« Reply #5 on: July 01, 2006, 11:06:11 PM »

I know I've probably asked this before, BUT:

    A triode amp, (typically grounded grid) requires a fair amount of drive.  WHAT would you suggest as  a boost from the 10 Watt level to the over 100-200 Watt level; not necessarily legal limit, but a significant increase from 10 Watts?  (E.g., a 13-17 dB increase?)  
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WB2WIK
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« Reply #6 on: July 03, 2006, 11:02:31 AM »

Single 4X150A or 4CX250B will take 2W up to 250W PEP output power in one shot.  10W is a bit much for this tube, but the input can be swamped with a 6dB 10W resistive attenuator (common solution).  Screen voltage can be derived from B+ through a resistor and zener string, so the only "complication" of using the tetrode is the grid bias supply.  That only has to provide a few mA total, mostly for the bleeder, so it's not complicated...

73

Steve WB2WIK/6
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K8ND
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« Reply #7 on: July 03, 2006, 12:15:29 PM »

I don't think I've ever heard of the 8877 referred to as being a "cheap" tube! $650 to $800 seems typical for new, while the 4CX800A Svetlana tubes are readily available for between $70 and $180 each (typically a bit over $120 each).
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K3GM
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« Reply #8 on: July 03, 2006, 04:27:18 PM »

I've seen 8877 pulls from medical equipment at local hamfests for under $200.  New, I'd have to agree. They're approaching $1K.  But you can find them, that's the key.  Finding something like a used 3CX1200 tube is getting near impossible, and the guys selling them know it.
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W6IZT
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« Reply #9 on: July 05, 2006, 09:34:43 AM »

I am now looking at a single 4cx800. $130 bucks for a tube that wil deliver 1kw out. Seems like a good value. I plan on using the G3SEK boards. The HV supply will be outboard. Looks like the amp will fit into a 16x12x8 cabinet, maybe a little smaller
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WB2WIK
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« Reply #10 on: July 05, 2006, 03:21:06 PM »

Does anybody else besides Svetlana make the 4CX800?

I'm not very trusting of them...since they've discontinued many tubes over the years, including ones used in what were once popular amateur amplifiers.

I also like the IMD specifications for the 8877 over the 4CX800 (which greatly favor the 8877) and the fact that the tube's been in continuous production for 30+ years, making it a staple on the used/pull market.  I bought the one I used in my 50 MHz amplifier for $100 as a broadcast pull back in 1984 and it's still working fine.

WB2WIK/6
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K8ND
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« Reply #11 on: July 13, 2006, 11:54:45 AM »

I don't know of anyone other than Svetlana making the 4CX800A tubes, but there are thousands of them out in the military and commercial surplus market, on shelves, and seemingly littering the roads in Russia and the former Soviet states.

Svetlana really screwed up with the 4CX1600B, after several amp manufacturers showed faith in them. That was a new tube, with none in military or commercial service - it was aimed only at the Amateur market, and so given low manufacturing priority.

We have a Ten Tec Titan II using that tube at the Signal Point PJ2T station, and were able to get through the drought without any loss of functionality. Now that Svetlana started making that tube again, we have  a good one in the amp, two on our spares shelf and a GU-91B and chimney for last-ditch spare as well.

We have eight amplifiers at PJ2T, and they use five different types of tubes (8874, 3CX800A7, 3CX1200A7, 4CX1600B, and 3-500Z). If I could have eight amps all using 4CX800A tubes in their place, I'd much prefer them! Fewer tube spares to stock, let cost per tube complement.
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