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

[Captain Kilowatt]

Well I finally decided to build an amp for a winter project. I have had a used pair of Eimac 4CX250B tubes for several years as well as a new pair of Eimacs, still in the boxes and figured it was about time to do something with them.I have plans in the future for an HF amp so I decided that this one would be built for 6m. Besides the tubes I also have a pair of Eimac SK-620 air system sockets and SK-626 ceramic chimneys I picked up at a hamfest years ago for the ridiculous price of $10.00 for the two sockets and two chimneys. Yesterday I decided to venture into the deep dark recesses of the basement and start digging out the stuff to build the power supply for the amp. The power supply will be built on a separate chassis and will be a floor model power supply.I didn't get any work done on it today other than parts placement and photos. The fridge has started acting up so the day was spent shopping for a new one. There goes a bunch more of my tower and amp money. I got a hell of a deal on it, too good to pass up really, but anything right now is too much. Enough of the depressing stuff and on to the fun stuff.


Below is the chassis with the major components in place. The four large gray transformers will be mounted upside down from what is seen in the photo. That will actually make them right side up and the terminals will protrude down through holes in the chassis and the transformers will be bolted from the underside. There are two identical Hammond plate transformers to be wired in series. They are the largest of the transformers seen here and weigh in at 30 pounds EACH. The two smaller gray transformers on the right are identical Hammond filament transformers at 10 pounds EACH. I only need one of them but I may as well mount the other as this supply may be used in the future with other projects.The lower left is occupied by the Sprague 2500 volt 100uF filter capacitor for the high voltage. The two transformers in the middle are for the screen supply if I use it (with red plate) and bias (smallest of the two).I figure I may as well build a supply with the ability to run tetrodes and then I'll figure what I will do with the amp later that way all the work is already done.The chassis has two breakers already installed as seen below the two pilot lamps. I may use them or just leave them in for looks. I need to recheck their ratings first.Either way they look cool.

Empty chassis.

Proposed mounting configuration of major components.

A little better oblique side view of the power supply chassis.

One of two plate transformers. BTW these are oil filled and no they do not leak.

The filter capacitor. This should work well and of course I will need a step start to keep the rectifiers from blowing.2000 volts into 100 uF is a LOT of inrush current.

Speaking of rectifiers, here are the plate rectifiers. They will be tapped in the middle to make a full wave bridge circuit. They have ceramic mounting standoffs and were in the plate supply of the old RCA transmitter I scrapped.

Here is the rectifier for the screen supply if used. It was in the screen supply of the old RCA transmitter. It also has ceramic standoffs for mounting. Sorry for the fuzzy image. The auto focus must have locked onto the desktop instead.

Front view of the chassis. This is all the original stuff on the front panel from when it was serving as airforce communications gear. I figure the two pilot lamps can be used for filaments on (green) and plate enabled (red). Lots of fuse holders to use and a nice looking pair of breakers. I plan to leave everything in place on the front panel whether it is used or not. It just looks cooler that way.The silver toggle switch in the lower right is for power ON while the switch under the red lamp will be used for switching the plate transformer's primaries in series or parallel in order to halve the plate voltage for tuning testing etc.

Hopefully I can get some holes drilled in the next day or two. I have a LOT to do outside in preparation for winter but it has been raining the past 4-5 days here and sure as hell now that I want to get this started the sun will shine.

The amp will be housed in this cabinet BTW.

I will build a sub chassis inside the cabinet to house the tubes. It will look something like this below with the tube sockets mounted down in the aluminum chassis and pressurized from underneath with a blower.

That's all for now. More updates as work progresses but don't be impatient. I have a LOT of things to do before winter arrives. Other components like the step start relay and the plate voltage delay timer will be mounted either on the underside of the chassis or on top wherever there is room.

Oh yeah, did I mention the thing weighs in at 115 pounds as you see it now?

 

[paws264]

Well I finally decided to build an amp for a winter project. I have had a used pair of Eimac 4CX250B tubes for several years as well as a new pair of Eimacs, still in the boxes and figured it was about time to do something with them.I have plans in the future for an HF amp so I decided that this one would be built for 6m. Besides the tubes I also have a pair of Eimac SK-620 air system sockets and SK-626 ceramic chimneys I picked up at a hamfest years ago for the ridiculous price of $10.00 for the two sockets and two chimneys. Yesterday I decided to venture into the deep dark recesses of the basement and start digging out the stuff to build the power supply for the amp. The power supply will be built on a separate chassis and will be a floor model power supply.I didn't get any work done on it today other than parts placement and photos. The fridge has started acting up so the day was spent shopping for a new one. There goes a bunch more of my tower and amp money. I got a hell of a deal on it, too good to pass up really, but anything right now is too much. Enough of the depressing stuff and on to the fun stuff.


Below is the chassis with the major components in place. The four large gray transformers will be mounted upside down from what is seen in the photo. That will actually make them right side up and the terminals will protrude down through holes in the chassis and the transformers will be bolted from the underside. There are two identical Hammond plate transformers to be wired in series. They are the largest of the transformers seen here and weigh in at 30 pounds EACH. The two smaller gray transformers on the right are identical Hammond filament transformers at 10 pounds EACH. I only need one of them but I may as well mount the other as this supply may be used in the future with other projects.The lower left is occupied by the Sprague 2500 volt 100uF filter capacitor for the high voltage. The two transformers in the middle are for the screen supply if I use it (with red plate) and bias (smallest of the two).I figure I may as well build a supply with the ability to run tetrodes and then I'll figure what I will do with the amp later that way all the work is already done.The chassis has two breakers already installed as seen below the two pilot lamps. I may use them or just leave them in for looks. I need to recheck their ratings first.Either way they look cool.

Empty chassis.

Proposed mounting configuration of major components.

A little better oblique side view of the power supply chassis.

One of two plate transformers. BTW these are oil filled and no they do not leak.

The filter capacitor. This should work well and of course I will need a step start to keep the rectifiers from blowing.2000 volts into 100 uF is a LOT of inrush current.

Speaking of rectifiers, here are the plate rectifiers. They will be tapped in the middle to make a full wave bridge circuit. They have ceramic mounting standoffs and were in the plate supply of the old RCA transmitter I scrapped.

Here is the rectifier for the screen supply if used. It was in the screen supply of the old RCA transmitter. It also has ceramic standoffs for mounting. Sorry for the fuzzy image. The auto focus must have locked onto the desktop instead.

Front view of the chassis. This is all the original stuff on the front panel from when it was serving as airforce communications gear. I figure the two pilot lamps can be used for filaments on (green) and plate enabled (red). Lots of fuse holders to use and a nice looking pair of breakers. I plan to leave everything in place on the front panel whether it is used or not. It just looks cooler that way.The silver toggle switch in the lower right is for power ON while the switch under the red lamp will be used for switching the plate transformer's primaries in series or parallel in order to halve the plate voltage for tuning testing etc.

Hopefully I can get some holes drilled in the next day or two. I have a LOT to do outside in preparation for winter but it has been raining the past 4-5 days here and sure as hell now that I want to get this started the sun will shine.

The amp will be housed in this cabinet BTW.

I will build a sub chassis inside the cabinet to house the tubes. It will look something like this below with the tube sockets mounted down in the aluminum chassis and pressurized from underneath with a blower.

That's all for now. More updates as work progresses but don't be impatient. I have a LOT of things to do before winter arrives. Other components like the step start relay and the plate voltage delay timer will be mounted either on the underside of the chassis or on top wherever there is room.

Oh yeah, did I mention the thing weighs in at 115 pounds as you see it now?

Any fire in the wire yet?

Bump

.

 

[Captain Kilowatt]

Sadly no. Not much has changed other than drilling some large holes for the transformer terminals but I should get some work done soon. November was busy getting crap done around the house/yard before winter and December was a right off due to work. It seems everyone in my department wanted time off in Dec. and I was the only guy qualified to replace any one of the dozen others. I hardly knew from one day to the next if I was working and whether it was a day shift or a night shift. Unfortunately just like everything I try to get done for myself, this looks like it will take a lot more time than planned. The project has not been forgotten about however.

 

[Captain Kilowatt]

Well the project is finally going to go ahead. Unfortunately I was not able to start it when I had hoped due to some things going on in my life that required my attention to family business but things are on the upswing now. The last year has been rough dealing with some issues my father had after contracting a nasty infection following surgery that necessitated a couple dozen trips to the city hospital over the last year to see various specialists. At one point he was on a ventalator in the ICU so my mind nor heart was into anything radio related. He is pretty much back to normal now and things are going well so with that behind me I want to let you all know that this project will be starting right away.

I have decided to go with the pair of 4CX250B's in a grounded grid circuit for a couple reasons. One, it is a simpler design not requiring a regulated screen supply. Two, I do not need low drive requirements as a grid driven amp would have as I have 100 watts available to me from the exciter. Three, not including a regulated screen supply will allow me to install everything including the power supply into the one single case depicted above allowing for a complete self contained amplifier.

I first have to build a sub-chassis that will be attached to the front panel and be able to be slid out of the cabinet. This will have to be on two levels with one side being lower to accommodate the tall 100 uF 2500 volt filter capacitor and the other side raised enough to allow a fan to be mounted underneath the tube enclosure. The height of the filter capacitor will not allow everything to be at one level.

With any luck tomorrow I will be doing a little layout design and maybe even getting a start on the chassis made from some 1/8 inch aluminum sheet I have. Maybe I can get it finished before the end of this years Es season.

 

[Shockwave]

Glad to hear things are going better with your father and you have the time to invest in the project again. Everything you say makes sense in your choices of design and layout. The only thing I might mention is to not forget this is VHF and reasonable care must be given as to where certain key components are placed. You'll need short, low inductance paths from the tube anodes through the tank circuit. Silver plated strap may begin to make an improvement. The Pi-L tank circuit could work well here because it will reduce harmonics and allow a smaller loading cap.

Looking at the layouts of similar power commercially produced equipment is not a bad idea if this is your first VHF amp. Since you're using these tubes in triode configuration it should be easy to find many good examples of mono band 6 meter amps in the ballpark of 500 watts anode dissipation. Your input will probably have a slightly lower impedance with two tubes in parallel. The output values would be close if the tubes ran similar plate voltage. Good luck and keep us posted.

 

[kd-5-bgt]

Just wondering CK...how much power are you looking to get out of that amp?


That should be a nice looking amp when you are finally done with it

 

[Captain Kilowatt]

My target is around 600 watts. By all accounts a pair of 4CX250B's should have no problem providing 500-600 watts with 2000 volts on the plates. That should go well with my homebrew six element yagi at 40 feet. I have seen amps made where the builder claimed 750 watts and that may well be possible just running SSB service. Some info I gleaned from Steve,WB2WIK, who has built several 4CX250B amps says that a pair of them should produce about 600 watts on HF 700-750 on 6m and nearly 1 Kw on 2m. I can make a good 2000 volts unloaded and with the 100 uF the sag should not be too bad. I have lots of silver plated copper strap that is about 1/16 inch thick and 3/8 inch wide. It was originally a large antenna tuning coil in an AM station's antenna tuning hut.The plate transformer I am going to use is different from what I originally showed above since I am now going to put everything into one package. this means I need a smaller transformer and since it will be an all-in-one package I do not need to have flexibility to run anything else in the future. Here is the new plate transformer below. The amp will be run on the dedicated 220 volt line I have in the shack. Using the 1150 volt tap and the Hi tap I can get 2000 volts DC out of the supply.

 

[kd-5-bgt]

Very nice CK !! Maybe someday I will be able to hear it inline

 

[Shockwave]

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.

 

[Captain Kilowatt]

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.

 

[KM3F]

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

 

[tubegearcollector]

Screen Regulator

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.

 

[Captain Kilowatt]

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.

 

[ElectronTubesRule]

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!!!!

 

[Captain Kilowatt]

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.