4CX1500A x 4CX1000 - worth a shot?

[KA1ZGC]

Hola!

I was stumbling and staggering through Tom Vu's thread on his 4x1x4x1 rig, and had a hair-brained idea.

I've been sitting on this 4CX1000 that I picked up at Dayton a few years back. I've been wanting to make some kind of AM transmitter out of it in spite of it's 1W grid dissipation. I know Collins ran them in Class C for an FM kilowatt of theirs, but I never pursued it very far.

Since then, I got my hands on a 4CX1500A. That changes everything, since the 1500A has 25W of grid dissipation.

So then I got to wondering if a single 4CX1000 (perhaps triode-connected) would have enough snot to modulate a 4CX1500A, given the right iron. Obviously not with the 4CX1500A running B.A.s-to-the-wall, but something a bit more in the amateur power level ballpark.

Possible? Plausible? Smoke another one, ZGC?

Just curious.

--Thom
Killer Agony One Zipper Got Caught

[W9GT]

Those are killer tubes, Thom!  I have an amplifier that I built back in the late 70's that uses a 4CX1000A, or interchangeably a 4CX1500B.  I actually am able to get more power out of a 4CX1000A, even though its dissipation is less.  The 4CX1500B has better IMD ratings.  The only thing that you have to be aware of, is that those tubes can actually run negative screen current in normal operation.  It works well to have a zero center-reading meter to monitor the screen current as I have in my amp.  As for the grid current, I just used a big 50 ohm swamping resistor on the input.  Very little drive is required, as those tubes have lots of gain.  You can still put a tuned input circuit in there, if that is what you want.

Were you thinking about some sort of single-ended modulator, such as a Heising arrangement?  That would certainly work....you might also be able to use a dynamic bias shift arrangement that would allow low idle current and switch the bias at a syllabic rate when audio is applied.  There is info in the old Bill Orr Radio Handbooks on that arrangement utililizing a 304TL (or a pair in parallel) in a bias-shift modulator.  All you need is a hefty choke for the mod reactor, no mod xfmr required.  I don't know how the 4CX1000A/4CX1500B tube would like that circuit, but worth considering.

73,  Jack, W9GT

[KD6VXI]

Hola!

I was stumbling and staggering through Tom Vu's thread on his 4x1x4x1 rig, and had a hair-brained idea.

I've been sitting on this 4CX1000 that I picked up at Dayton a few years back. I've been wanting to make some kind of AM transmitter out of it in spite of it's 1W grid dissipation. I know Collins ran them in Class C for an FM kilowatt of theirs, but I never pursued it very far.

Since then, I got my hands on a 4CX1500A. That changes everything, since the 1500A has 25W of grid dissipation.

So then I got to wondering if a single 4CX1000 (perhaps triode-connected) would have enough snot to modulate a 4CX1500A, given the right iron. Obviously not with the 4CX1500A running B.A.s-to-the-wall, but something a bit more in the amateur power level ballpark.

Possible? Plausible? Smoke another one, ZGC?

Just curious.

--Thom
Killer Agony One Zipper Got Caught

IME, they both operate about the same as the 4CX250 series of tubes, with LOTS more dissipation.  As someone else pointed out, they can draw - screen current.  They both have better IMD figures than the 250s, though.

I have a 4CX1000 here that the socket was parted out to build a socket for a 3CX1000.  I like the triode better, I've been told by more than one it's basically an 8877 with a bigger grid.  Yeah!  However, if you blow your 1000, I have a spare

--Shane

[KA1ZGC]

Yep, I'm aware of the negative screen current situation. I'll probably employ a shunt-regulated supply for the screen to give the negative current a low-impedance path to ground.

I haven't really landed on a design for the modulator yet. I'm kind of trolling for ideas in that department. My gut tells me that if I run both tubes in the 2000-2500V range with just enough RF drive to get a good solid class C, I should have gobs of headroom in both tubes and a signal from hell out the other end.

I know Chuck K1KW ran a 4CX1500A by a pair of 3CX1200s (if I remember right) by way of some broadcast iron and it worked great, but Chuck has never been one to under-build anything.

This is part of why I'm wondering if I'll get somewhere close with a single 4CX1000, and if I'm better off triode-connecting it.

Wish I had some Bill Orr Handbooks handy (no pun intended). I'll have to sniff around for the modulator you're talking about. Sounds very interesting indeed, particularly the "no mod iron" part.

Shane: yes, the 4CX1000 and 4CX250 are electrically almost-identical. I was originally thinking of modulating the 4CX1000 with a pair of 4CX300Ys in shove-yank configuration through 1.5:1 iron, but the RF drive situation is a bit daunting given the 1W limit on the grid. This has me thinking the 1500Ax1000 arrangement may be much simpler by comparison.

[flintstone mop]

Hi Thom,
I'm ashamed to say that I went through great lengths to grab a beautifully built A.M. TX using a 4CX1500B. It was capable of 2500W OUT. It needed 3500VDC @ 5 AMPS.
It had regulated screen supply, self modulated screens.The modulators were 3CX1200A7's and Gates 5KW iron and reactor.
About 20W of drive from an external source. The speech amp was a 100W stereo amp configured to give a balanced output to drive the massive mod tubes.
OVERKILL but sounded so nice, until I got it. It was starved for that 3500VDC and was designed to run happily at 2500Watts out all day long.

I foollishly sold it to a Ham, who operates once in a while, and has never once put it back on the air.

It sits wasting away, off and cold.

There's a link below that links to other pics of this once famous TX

http://www.amwindow.org/pix/htm/mop981.htm


FRED

[steve_qix]

I've used a single tube as a modulator many times - using Pulse Width Modulation.  This works very well - will perform just about as well (audio-wise) as a class A series modulator and with 90+ percent efficiency, but definitely goes against the current trend of transformers, TRANSFORMERS, oh and did I forget to mention T R A N S F O R M E R S !!!.    ( I couldn't resist!).

Regards,

Steve

[kc6mcw]

Nice pics of the big transmitter Fred!

[W9GT]

Hi Thom,
I'm ashamed to say that I went through great lengths to grab a beautifully built A.M. TX using a 4CX1500B. It was capable of 2500W OUT. It needed 3500VDC @ 5 AMPS.
It had regulated screen supply, self modulated screens.The modulators were 3CX1200A7's and Gates 5KW iron and reactor.
About 20W of drive from an external source. The speech amp was a 100W stereo amp configured to give a balanced output to drive the massive mod tubes.
OVERKILL but sounded so nice, until I got it. It was starved for that 3500VDC and was designed to run happily at 2500Watts out all day long.

I foollishly sold it to a Ham, who operates once in a while, and has never once put it back on the air.

It sits wasting away, off and cold.

There's a link below that links to other pics of this once famous TX

http://www.amwindow.org/pix/htm/mop981.htm


FRED

That is a beautiful transmitter Fred, but I don't understand the 5 Amps??  That's lousy efficiency.  2 Amps would be plenty, it seems.

73, Jack, W9GT

[W2VW]

Tom, go back and carefully read the specifications for this tOOb. You have some numbers incorrect.

http://pdf1.alldatasheet.com/datasheet-pdf/view/105083/ETC/4CX1500B.html

I'd use it as a leanyar. Cathode drive it tetrode connected. Parallel 2 for extra fun. Call it a day.

[WA1GFZ]

save both tubes for the final so you have a spare. Take a pair of say 813s triode connected as modularors. It is a waste to use one as a modulator.

[ke7trp]

Fred.. Awesome transmitter..  I feel your pain.   I had a wonderfull 4x1 Amp with a 2.5 amp 7KW Dahl supply.  Sold it in a weak moment. Think about it weekly..

C

[KD6VXI]

Tom, go back and carefully read the specifications for this tOOb. You have some numbers incorrect.

http://pdf1.alldatasheet.com/datasheet-pdf/view/105083/ETC/4CX1500B.html

I'd use it as a leanyar. Cathode drive it tetrode connected. Parallel 2 for extra fun. Call it a day.

No, you're looking at the wrong datasheet, I believe.

He has the A model.  The numbers are WAY different for the A and B models, 1000 and 1500 watt dissipation tubes.

They can be interchanged one way, but not the other.... 

--Shane

[W2VW]

Tom, go back and carefully read the specifications for this tOOb. You have some numbers incorrect.

http://pdf1.alldatasheet.com/datasheet-pdf/view/105083/ETC/4CX1500B.html

I'd use it as a leanyar. Cathode drive it tetrode connected. Parallel 2 for extra fun. Call it a day.

No, you're looking at the wrong datasheet, I believe.

He has the A model.  The numbers are WAY different for the A and B models, 1000 and 1500 watt dissipation tubes.

They can be interchanged one way, but not the other.... 

--Shane

You are right. I ASSumed he was talking about a 4CX1500B because it uses the breechlock socket like a 4CXC1000. Never mind.

[WA1GFZ]

I like the next size up the 8169 myself since I bought 6 of them for $100 a bunch of years ago. Drilling and blasting a rebuild with vacuum caps. I found an oil filter wrench is great for removing the tube from the socket.

[KD6VXI]

Tom, go back and carefully read the specifications for this tOOb. You have some numbers incorrect.

http://pdf1.alldatasheet.com/datasheet-pdf/view/105083/ETC/4CX1500B.html

I'd use it as a leanyar. Cathode drive it tetrode connected. Parallel 2 for extra fun. Call it a day.

No, you're looking at the wrong datasheet, I believe.

He has the A model.  The numbers are WAY different for the A and B models, 1000 and 1500 watt dissipation tubes.

They can be interchanged one way, but not the other.... 

--Shane

You are right. I ASSumed he was talking about a 4CX1500B because it uses the breechlock socket like a 4CXC1000. Never mind.

Only reason I caught it, I made the same assumption about 10-15 years ago, and it was pointed out to me.

Wierd, how the A of one tube pretty much corresponds to the B version of another, but hey, who am I to second guess Eimac?

Still, good tubes.  The 30S1 uses one, IIRC.

--Shane

[flintstone mop]

It was Chuck's "Big Rig" K1KW. It was a masterpiece and overbuilt, hence the 5A P.S. He did not inlcude that in the sale. So, I made my own, as much as I could afford Dahl transformer, 3000vdc 2A. Running the poor thing at "reduced power" never produced the audio everyone remembered. Big tubes like the 4CX1500B, are begging to be operated at their design volts. Reduced voltages, different matching impedances for the modulator added up to less than desired characteristics. 3CX1200A7's are massive tubes for modulators!!!
Plus it was my JN days for A.M. And not very smart decisions.

fred

[KD6VXI]

It was Chuck's "Big Rig" K1KW. It was a masterpiece and overbuilt, hence the 5A P.S. He did not inlcude that in the sale. So, I made my own, as much as I could afford Dahl transformer, 3000vdc 2A. Running the poor thing at "reduced power" never produced the audio everyone remembered. Big tubes like the 4CX1500B, are begging to be operated at their design volts. Reduced voltages, different matching impedances for the modulator added up to less than desired characteristics. 3CX1200A7's are massive tubes for modulators!!!
Plus it was my JN days for A.M. And not very smart decisions.

fred

After replacing close to 10 8877s in the 80s in my 3 tube amplifier, Reid sent me 3 1200A7s, sockets and the filament transformer.  I also got a note that not another 8877 would be replaced under warranty.  They made good on all the bad 8877s I got (you could knock them back into service with fils power, and a long broomhandle.  A carefully applied "knock" to the top of the tubes would bring them back in line for another week or so), so I had no complaints.

The only other modification to the station after that was taking the attenuator out of line to allow the viking to fully tickle the 1200s.  They take twice the drive the 8877s do in RF service.  I've never experienced them as modulators, but man....  As tough as they are for RF, they ought to be about indestructable in AF.

--Shane
 

[KA1ZGC]

Thanks for all the input, guys.

Just to be clear, I didn't pick that tube combination because I thought it was some magical combo that nobody else thought of. I picked those tubes because they're two high-(for-a-ham-)power tubes that I actually have in my possession.

Yes, it's the 4CX1500A that I'm talking about: 25W grid dissipation vs. 1W for the rest of their kilowatt-ish tetrodes.

I have always been in love with these tubes, and have always tried to conjure up some magic scheme by which one could be used in both Class C and linear services with equal effectiveness. That would mean figuring out how to modulate one in a grounded-screen configuration, and in the case of the 4CX1000, how to drive to a good hard class C without blowing the grid.

That being many rigs away for me (I'm not up to building anything that caliber yet) coupled with the recent acquisition of the 4CX1500A, it dawned on me that a 4CX1000 in a single-ended audio application should give me all the modulating power I need for a 4CX1500A in a grounded-cathode configuration, which can be driven in class C without needing to get tricky with the drive.

Steve: I would love to PDM the 1500A. Isn't pulse service a bit out of the 4CX1000's league, though? Would a string of power FETs be better suited to that?

[WA1GFZ]

During the summer I read an app note on a 4000 volt IGBT. I checked Digi Key and the price was around $60. It was rated for many amps if I remember.

[KC2IFR]

Thom,
Tim was asking me if I had some 4CX1500B's...........I wonder if they were for u???

Bill

[KA1ZGC]

Tim was asking me if I had some 4CX1500B's...........I wonder if they were for u???

I doubt it. I know one of the bible-beater FMs uses a 4CX1000/1500B for a driver, and WKTJ has a 4CX1000 in the final. There are one or two other non-broadcast pieces kicking around the property that were built around that series, he may have been looking to populate one of them. I know he wanted to perform some experiments based on what I wanted to build, so there may be that connection.

Besides, iIt's not more tubes that I need, it's a rig to put the existing ones in. 

[KC2IFR]

OK Thom,
Thanks.....

Bill

BTW......I have 6 new old stock 4CX1500B's still in the the shipping bags.

[WA1GFZ]

There is a cool mil amp that used a pair of them. It was very well built.

[Opcom]

The one near the bottom in the rack?  - that has two of them tubes supposedly. another basket case project around here. but it looks cool.

[WA1GFZ]

That is a kick A$$ machine