Anybody running 4CX250B's on 6m?

[Captain Kilowatt]

I have beating my head trying to decide what to do with a pair of NOS Eimac 4CX250B's. I say they are NOS but they are only about six years or so old and never used. I was thinking about building an amp this winter but can't decide whether to put them on HF or on 6m. If I go for 6m they will see very little use as the band does not open that often this far north but they would work nice with my six element yagi. If I build the amp for HF it will see a LOT more use but I also want to build a 2x 3-500Z amp in another year or two so it would be redundant. I have scoured the net looking for designs and have gotten caught up in the cathode driven/grid driven debate. Cathode driven GG service is pretty simple but IMD performance sucks whereas grid driven has much more gain,although that is not really an issue, and has much better IMD performance but requires a screen supply and grid bias.In any event the power supply will be on a separate chassis so room for extra components is not really an issue.Right now I am leaning slightly towards HF but if anyone runs these tubes on 6m I would appreciate some input and try and convince me to build the amp for 6m instead.

 

[Moleculo]

Build it for 6m and send it to me. I'll test it out for you Better yet, build it for 2 meter and send down my way!

 

[Captain Kilowatt]

Build it for 6m and send it to me. I'll test it out for you Better yet, build it for 2 meter and send down my way!

If memory serves me correctly we had that discussion before. I also seem to remember you lost that time too.

 

[W4KVW]

PRIDE DX 300 CONVERTED TO 6 METERS

I know TWO people using PRIDE DX 300's converted to 6 meters & they BOTH sound & work GREAT with just a SINGLE tube! I would LOVE to have EITHER one since 6 meters is my FAVORITE band!

SIX-SHOOTER

 

[paws264]

I have beating my head trying to decide what to do with a pair of NOS Eimac 4CX250B's. I say they are NOS but they are only about six years or so old and never used. I was thinking about building an amp this winter but can't decide whether to put them on HF or on 6m. If I go for 6m they will see very little use as the band does not open that often this far north but they would work nice with my six element yagi. If I build the amp for HF it will see a LOT more use but I also want to build a 2x 3-500Z amp in another year or two so it would be redundant. I have scoured the net looking for designs and have gotten caught up in the cathode driven/grid driven debate. Cathode driven GG service is pretty simple but IMD performance sucks whereas grid driven has much more gain,although that is not really an issue, and has much better IMD performance but requires a screen supply and grid bias.In any event the power supply will be on a separate chassis so room for extra components is not really an issue.Right now I am leaning slightly towards HF but if anyone runs these tubes on 6m I would appreciate some input and try and convince me to build the amp for 6m instead.

Six meters is not quite VHF so, conventional HF circuits will work. Build yourself a "10 meter" amplifier and then stretch the tank coil open and tune for your 6 meters.

You could build it "Cathode Drive" just like a 6LF6 amplifier with 300 volts on the screen and, an adjustable negative going PS (-30 volts to -60 volts). Put an ammeter in the cathode return line and adjust the negative volts to set you class of operation.

Or, you could run it "Hot Grid" where, you ground the cathode and, apply the RF drive to the Control Grid. The Screen should be at +300 volts, the negative bias is applied thru a choke connected to the control grid; I would place a current meter in the cathode circuit. This circuit has a higher gain and, neutralization will be required especially if operation at 50 MHz is anticipated.

.

 

[Radio Tech]

Was gonna do the same with a Pride 300 a few years back.
But made one mistake. I posted it on qrz.com. After all the negative feed back on that, I stripped the pride down.
Second mistake. Should had fixed the amp and carried on.
Not coming here and asking, was my third mistake.

 

[Captain Kilowatt]

Six meters is not quite VHF so, conventional HF circuits will work. Build yourself a "10 meter" amplifier and then stretch the tank coil open and tune for your 6 meters.

You could build it "Cathode Drive" just like a 6LF6 amplifier with 300 volts on the screen and, an adjustable negative going PS (-30 volts to -60 volts). Put an ammeter in the cathode return line and adjust the negative volts to set you class of operation.

Or, you could run it "Hot Grid" where, you ground the cathode and, apply the RF drive to the Control Grid. The Screen should be at +300 volts, the negative bias is applied thru a choke connected to the control grid; I would place a current meter in the cathode circuit. This circuit has a higher gain and, neutralization will be required especially if operation at 50 MHz is anticipated.

.

Yeah I know HOW to build it but like I said it's a trade off between simplicity and IMD performance and since the exciter is capable of 100 watts gain is not really an issue as I have more than enough drive. If I go grid driven I will have to throw away buckets of power but have better IMD performance. It's just a balancing act although I do prefer to run a CLEAN station so grid driven is the likely result.

 

[Captain Kilowatt]

Was gonna do the same with a Pride 300 a few years back.
But made one mistake. I posted it on qrz.com. After all the negative feed back on that, I stripped the pride down.
Second mistake. Should had fixed the amp and carried on.
Not coming here and asking, was my third mistake.

I don't bother asking questions about amps on QRZ.com anymore, not after the ripping I got from the usual a-holes that assume everyone that asks about homebrewing an amp and has not flashed their "Broadcast Engineer" badge before posting is a CB'er looking to build an 11m amp.

Speaking of amps, how did you ever make out with the 4x811A amp? I dropped the 4x572B project when some other things took up my time and after I scrapped the RCA tx I found myself with all the goodies to build a bigger and better amp but that one is still on the drawing board for now. No matter how much power you have you can't do much with crappy antennas so I am putting my time and money into a new tower and antenna system next year. I've already got a NIB Explorer-14 with the 40m add on kit and will likely pick up the DB-1217 as well when I get the new tower. After that comes a pair of 3-500Z's for HF but right now I may look at a 6m amp with these Eimac 4CX250B's. They should produce a clean 6-700 watts and that will work quite well with the six element yagi I have.

 

[Radio Tech]

I don't bother asking questions about amps on QRZ.com anymore, not after the ripping I got from the usual a-holes that assume everyone that asks about homebrewing an amp and has not flashed their "Broadcast Engineer" badge before posting is a CB'er looking to build an 11m amp.

Speaking of amps, how did you ever make out with the 4x811A amp? I dropped the 4x572B project when some other things took up my time and after I scrapped the RCA tx I found myself with all the goodies to build a bigger and better amp but that one is still on the drawing board for now. No matter how much power you have you can't do much with crappy antennas so I am putting my time and money into a new tower and antenna system next year. I've already got a NIB Explorer-14 with the 40m add on kit and will likely pick up the DB-1217 as well when I get the new tower. After that comes a pair of 3-500Z's for HF but right now I may look at a 6m amp with these Eimac 4CX250B's. They should produce a clean 6-700 watts and that will work quite well with the six element yagi I have.

Hey Bro.
Yeah I have basicly gave up on that forum. Too dramatic for me.
Sounds good on the antenna system. The antenna will do more justice than the amplifier anyway. I used to love to play with the 4CX 150's and 250 in my younger days. I built a single tube amp for 2 meters way back then. A freind of mine (now SK) Doug Beckwith N4CZT was a 4CX amp guru. He built a 2 meter amp using 4 150b's. That thing was hot! Wish he was still around.

Well, I have all the parts for mine still sitting here. Bought some tube sockets at a ham fest a few months back. Problem is, I have been so swamped in the shop I just do not have time to get back on it. Untill 2 weeks ago, my ham equipment has sat idle. Not even a antenna hooked up. I have had more HF amps in the shop in the last 5 months than I have had in 2 years. Just finished a Drake L4B for a guy. Did a complete re-build. Had 2 MLA 2500 in for repair over the last month. Both had origianl tubes. That worked. go figure.

Really though between the CB and Ham repairs, just had not been able to do anything else. But that will change. Winter is coming, things will slow down. Then I can get back to playing around with my stuff.

Also looking at taking on a few more things. Transformer re-winding. Done a few small ones. Working on a plate transformer now.
Transformer re-wind - GoKarters.Com ___Hobby Forum
Just pictures no documentation yet.

Anyway, back to your thread and question.
HF or 6 meters??
Build the amp. Leave room for upgrades. HF plus 6 meters.
Only way to go. 50, 28, 24 mhz should be able to tap off the same tank.

 

[Captain Kilowatt]

Also looking at taking on a few more things. Transformer re-winding. Done a few small ones. Working on a plate transformer now.
Transformer re-wind - GoKarters.Com ___Hobby Forum
Just pictures no documentation yet. [FONT=&quot]

[/FONT]

Transformer rewinding. Oh yay. :headbang All I think of when I remember rewinding transformers is EIEI....OH NO! I'm sure you get the EIEI part.

Anyway, back to your thread and question.
HF or 6 meters??
Build the amp. Leave room for upgrades. HF plus 6 meters.
Only way to go. 50, 28, 24 mhz should be able to tap off the same tank.

If I go 6m, and I probably will, it will be monoband only. Stretching a tank circuit to operate from 12m or even 10m is really compromising the Q on 6m. Six meters is too low for strip line and too high for a good conventional tank circuit and adding a switch and tapped inductor just does not sit well with me. It can be done but a monobander will give better results in this frequency range. Besides an all band HF amp is in the future,maybe not this year or next but not too far down the road.

 

[Radio Tech]

[/FONT]

Transformer rewinding. Oh yay. :headbang All I think of when I remember rewinding transformers is EIEI....OH NO! I'm sure you get the EIEI part.




If I go 6m, and I probably will, it will be monoband only. Stretching a tank circuit to operate from 12m or even 10m is really compromising the Q on 6m. Six meters is too low for strip line and too high for a good conventional tank circuit and adding a switch and tapped inductor just does not sit well with me. It can be done but a monobander will give better results in this frequency range. Besides an all band HF amp is in the future,maybe not this year or next but not too far down the road.

EIEI OH YEAH!. all 200 of them lol. wonder how many others will catch that.

If you vould copy the SB 200 tank circuit, modify it for the 4CX style, it can be done. But even at mono band level you can always change it to fit your needs.

I think if I was gonna build an amp with steel tubes, 3CX800. But since you have the 4CX already. Great.

I have 2 tubes that I am going to build for a friend of mine. I will post up a picture of them tomorrow for ya. All I am saying is, they make 3-500's look like baby food jars

 

[Captain Kilowatt]

I am assuming they are glass tubes? I say the king of glass tubes.........the 4-1000.

 

[Toll_Free]

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[Shockwave]

If I were you I'd put the 250B's on 6 meters. Especially if you have plans for a two tube 3-500Z HF amp. They will make a very nice 6 meter amp if you like that band. Because you have 100 watts drive available I wouldn't even consider running the tubes as tetrodes. Yes the IMD will be better but then you have to deal with a screen regulator that can handle negative screen current. You need to apply negative feedback to neutralize the tube. If you over drive it once the grid will go bye bye without overload protection.

It would be easiest to ground the control grid and screen grid directly to RF ground. This should make neuturalization unnecessary. You could add some positive screen voltage to boost the gain however, don't forget about negative screen current if you do. You need a shunt regulator here because when the tube conducts heavily, secondary emissions are generated.

This causes the screen grid voltage to climb because the secondary current flow wants to drive the screen voltage towards where the plate voltage is through the secondary electron flow. Most builders ignore this issue and the result is unstable output power that wants to increase the longer you are keyed. This is similar to thermal runaway with transistors and can melt the screen grid into destruction if not controlled.

 

[Captain Kilowatt]

Well I just finished rounding up all the iron I need forthe power supply and OMG is it heavy. Pictures tomorrow. Being frugal (OK cheap) I am using what I have on hand and making a floor model power supply. Truth be known it HAS to be a floor model due to the weight. Two Hammond airforce surplus plate transformers in series and a good 2500 volt 100 uF capacitor will yield just about the 2000 Vdc required for the plates while a pair of Hammond airforce surplus filament transformers will be more than required for the filaments. I will have about 350 volts for the screens and an adjustable bias of somewhere between -30 and -70 volts to play with when all is wired up. The plan is to have step start on the plates as well as a two minute timer before the plate voltage comes on after the fils. come up. The 4CX250B's like a 90 second warm up time. Parts placement takes place tonight and with any luck I start drilling and blasting holes in the power supply chassis tomorrow.