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Building a 6 meter amplifier with a pair of 4cx250B's

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  #9  
Old 03-25-2011, 01:08 AM
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Captain, I'm just curious if Steve gave any explanation as to why the 4CX250B makes more power on VHF Hi then HF? This tube is good all the way into UHF Lo but I can't figure out why it would have more gain or PO with an increase in frequency. Can you also tell us about your interest in the 6 meter band? I know it has unusual DX propagation sometimes but have never worked it.

As a kid I can remember the days when TV stations would sign off the air at night and back on in the morning. While this would happen close to the top of the hour, many times certain stations would keep the transmitted fired up longer with an ID test card. When WCBS channel 2 in NYC was off the air, I was amazed how many other stations from many hundreds of miles away would come in on this frequency with a "K" prefix. I never experienced it on any other TV channel.

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Old 03-25-2011, 04:29 AM
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I am just parroting what I heard from Steve as well as having read on a few other sites regarding 4CX250B amps. I also have no idea why the difference in power out versus frequency. Usually it works the other way around with power rolling off on the higher bands.

I first became interested in 6m several years ago when like you I was able to pick up TV stations on channel 2 as far away as the Midwest area including Chicago. I had no 6m gear at the time and no money either but I did scrape enough together to get a Ten-Tec 1208 20m-6m transverter kit. I put it together over two evenings. It ran about 12 watts output using my Icom IC-735 on 20m as the IF. I had an old Wilson Shooting Star beam that I rebuilt into a six element yagi with a Beta match and with that I was able to enjoy many 6m contacts in the USA as well as a few into Europe and even The Canary Islands. I also converted my Mirage B-108G 2m amp to 6m and was able to drive it to 75-80 watts using the transverter at 10 watts output. Things became better for a while after I bought an FT-857 but it later developed the well known 6m stability problem and that ended things for a while. I also need to address a broken rotator issue on the tower as the yagi is fixed at about 120 degrees and I don't hear much of anything at that heading from here which is straight down the Atlantic off the coast of West Africa and the F/B and F/S rejection is very good meaning that I hear pretty much nothing at all. I have since purchased a newer FT-857D and am looking forward to getting back on the Magic Band.
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Old 04-02-2011, 12:43 AM
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I run a single 4cx family tube on 6m at 600 watts output.
The amplifier is an old Telstar rettrofitted from a 4cx250 to a Russion GS36B.
They plug in and go after the grid bias is set for class AB1 linear operation for this tube.
A European company is now marketing a 4cx250 amp here for HF.
There are no real issues using these tube at lower frequencies as long as neutralizing is done and the cooling air requirements are met.
These tubes are "forced" air cooled not just fan cooled so this raises the noise factor somewhat.
These tetrodes are a very old design and being small lost favor to the likes of 8877 etc.
As well there might have been some 'reputation' built for the 4cx family being poor on IMD.
Many used the old FAA amplifiers on 2m and suspect were over driven causing QRM on the bands.
I have come to really like this tube and have thought of building a two tube amp for 2m.
My 6m amp is grid driven and require less than 5 watts to get 600 out.
The tube is rated for 600 out so the power is being met from the tube.
The power gain is about 20 db+.
I believe the FCC part 97 rules prevent marketing an amp with gains much over 10 db so grounded grid circuits are used to limit the gains to about 10 db.
Hope this is for some value for info.
Good luck

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Old 09-06-2011, 08:42 AM
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I'd be very interested in what you are going to use for a screen voltage regulator. I build a 2 meter amp using a single 4cx250 a few years ago. I wanted it to be all tubes so my screen regulator was a shunt type adjustable regulator using a 6L6 and a double triode (a 6sn7 I think, but don't hold me to it - if memory serves) as the diferential error amp. I cheated on the rectifier board and used a canned board from Far Circuits. My 6L6 based shunt regulator gives resonably good regulation but it could be better. I found the circuit on the web and modified it to work at 300 volts. Enjoyed the photo's. Nice job so far.

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Old 09-06-2011, 06:02 PM
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Things are at a standstill at the moment for a variety of reasons but mostly a lack of time.Time and money have been diverted to rebuilding my entire HF antenna and tower system instead. The plan was to build the amp as a grounded grid design and as such there would be no screen voltage and hence no screen regulator.
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Old 09-07-2011, 01:27 AM
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Wow that should be real nice on 6m. If I recall those have a peak out put of 410 watt's right so 820 if life is feeling short.........LOL I thought about picking up some NOS 4CX250B's but I got a good line on some HV Transformer's and well it is harder to find HV transformers at a good deal then NOS tubes.....So Transformers it is. In fact tomorrow I am making a 2 hour drive one way to go and get them......Not perfect since they where designed to power 5-6 Sweep tubes but I figure I will get 2-3 tomorrow and 2-3 next pay day and I will be good to go for a while..... I will use use 2-3 for each HV supply for the price I am getting them for 3 of them will cost me 1/2 the price of having one custom wound for me "yes I got quotes" and it will cost me 1/5 the price Peter Dahl wanted. Every single company that sent me a quote was 1/3 to 1/2 the price of Peter Dahl and they where given the exact same specifications in voltage and amp needed in CCS and the same specified core design, core material, weight, and case style. Peter Dahl will never see a dime of my money as long as other reputable companies exist in the USA that will make a transformer for me!!! I specified 2500V @ .800ma CCS Most of the quote came in at about $280 that is the average of all of them from high to low baring Peter Dahl. PD wanted $485 @.800 $585 for 1 amp I think it was and all where given the same primary voltage of 120V 60cps.

Ihave not slept in about 65 hours I suffer from terrible insomnia it comes and goes so I might be a tad off on the second rating on amperage from PD........I laughed so hard when I read it that I almost fell out of my computer chair. If I had been on the phone with them it would have been a bad scene........I mean really $280 versus $485 to $585....It is no wounder people are trying to use MOT to power their rigs.......I would try 4 of them in series and paralleled to get the voltage and current I needed and full wave rectifier action going on before I would drop $585 on a transformer!!!

For the record I only sent request's for quotes out to companies in the USA California, Illinois, New Jersey and Michigan I think it was.....So not like the price of copper and steel is going to be radically different or the price of machinery or labor........Since none of them where off the shelf parts all custom designed to order I doubt they would import the part but I hope not.

Nothing is more irritating to me then to be charged a premium for a name......That has always irritated me.....I do not mind paying a premium if I am getting a part made with better materials, to higher levels of precision etc......I make custom articles for people all the time so I understand hand made!! Everyone use's hypersil high silicone steel in the core's Nickla Tesla invented that a long long time ago and the patent ran out equaly long time ago........No one is making modern high voltage transformers with iron or plain carbon steel or solid bars of material as in non-laminated etc.....not as a norm....I am sure someone has a black smith out back beating scraps of iron together and drawing their own copper wire in the basement for some arcane reason but it is not the industry norm from what I was told by companies that only make transformers for industry from mass produced to one-of's.

So a lot of smoke is being blown.....Even though I told each company that toroidial would be ok if it made it easier for them to manufacture a custom transformer for me or if it made it cheaper for them to do so no-one in the USA sent me a quote for a toroidal wound transformer they where all E-I and C cores! I also told them that they could use non-high silicone steel and all of them told me the same thing they only use high silicone steel for custom made high voltage transformer's!!!! So it was not an option I had to pay more for as no one builds them any other way it seems!

So that is my experience shopping for 2500V transformer's @ .800ma output from 120V primary made in America. It was so frustrating and such a pain that I thought I would share it......Oh on top of that I think Ameritron MFJ sells a transformer with about the same output and all the taps for $247 so go figure??? I have no idea about their quality or where they are made. I have not called them yet because I want full specifications on the transformer if I can get them instead of trying to work backwards from their literature on their Amp's and based on their tube model and count....

What is my point??? Keep and open mind, consider all your sources,wait for quotes to roll in it took about 5 days for some but we did have a major holiday. Use the resources that come with the ARRL hand books on CD room. If one supplier is $200-$300 more for similar item I think one has to really think twice about what you are getting for that Delta V in price that is a huge difference in price point!!!!

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Old 09-07-2011, 03:26 AM
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Having come from a commercial broadcasting background as well as knowing a LOT of homebrewers, many who build their own plate modulated AM gear, I can say that about 99.9% of Peter Dahl customers (now Harbauch Electronics) are quite satisified with the products and service. Take two "identical" transformers, one PWD and the other ???, and I will guarentee the PWD unit is heavier for the same ratings. No arguments from me on the matter. Just facts and observations.
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