3cx3000

[Frank10]

I am new at designing amplifiers so this high school kid is building a high power audio amp for me. He is planning on using a 3cx3000 tube for the final stage. He is trying to get a used one from the local radio station. He is going to use the LM3886 amplifier as a driver. He figures on getting 500 - 1000 watts if he uses a 2000 volt transformer.

I was wondering where is the best place to buy the tube if the station won't pan out? He said many of the tubes out of stations have lots of life left because they are supposed to put out more than 3000 watts.

Thanks for helping a newbie.

Frank

[martinab2]

Hi Frank,

Your designer seems to be out of his depth on this one - There is no way that an LM3886 could swing enough voltage to make any sort of attempt at driving a 3cx3000. I guess he has looked at audio power and current output rather than voltage swing.

I don't know of anyone who has successfully built an audio amp with one of these tubes. The 3cx3000 series are designed to produce kilowatts of rf power for transmitter or rf heating use. Trying to get one stable enough for audio would be no easy task.

Other considerations would be cooling (designed for forced air cooling) and power supplies (huge transformers required) also, what would you use for an output transformer?

Biggest problem is safety - 2000v is lethal stuff and requires expert knowledge of materials and layout which from his other design ideas, I suspect your designer does not have.

If I were you, I would call a halt to all this before he fries himself

Start with something simple - there are loads of ideas here on diy audio and elsewhere on the web - make sure you understand how a circuit works before trying to build it.

Good luck

[Sch3mat1c]

- I'll give you $200 for one that works! I actually have a use for something like that... good luck for you finding an output transformer to handle all 2000 watts of audio though.

Tim

[SY]

The other issue is the required turns ratio for a suitable OPT. The tube wants that ratio very high, but that makes producing a decent transformer a very big challenge.

This is a tube which is really best left in transmitter use. It will make a lovely linear for the 10 meter band.

[kmtang]

The RF amp runs class C but it is class A for audio amp. I really don't think the tube can run in class A operation at all.

Johnny

[Frank Berry]

"The RF amp runs class C but it is class A for audio amp. I really don't think the tube can run in class A operation at all."

They can run in class A. The Continental 317-C transmitter uses them to modulate the screens of a pair of 4CX35000C tubes.
Continental runs them in class A.

Cooling them is another matter. They require lots of air flow.

[tubelab.com]

I am all for building monster audio amps, BUT this is too far over the edge, even for me. Assuming that you had an expert, who could deal with the required cooling and serious voltage, you could never get an output transformer made for this amp, that could cover the entire audio range. If you are still considering this amp, start searching for the output transformer first. Consult with a few transformer vendors to see if anyone could build you a transformer. Assuming a single ended amplifier, producing 500 to 1000 watts would require a tube dissipation of 1500 to 3000 watts. Using your figure of 2000 volts, gives an idle current of about 1 amp. The insulation must stand about 4000 volts. This is easilly a 50 pound 1000 dollar piece of iron! If you can get the output transformer made, then you need power and filament transformers. Hey those should be easy.

You are definitely not going to drive this tube directly with a chip amp. You might drive it with a chip and a driver transformer, but now you need another custom transformer. I would look at my PowerDrive circuit and then design a bigger one, this tube is going to suck some serious grid current.

As far as the comment that this tube can only be run class C, don't believe it. I have never met a tube that couldn't be run in class A. Many of my bigger amps were made with RF tubes. Most of them were never speced for class A operation. The 833A and 811A imediately come to mind.

Keep in mind that this amp (with the required transformers and cooling) would occupy a small room and require its own air conditioner.

If this is where you want to start out, whats next? 1.21 Gigawatts? Seriously, build a smaller amp first!

[Frank Berry]

"If this is where you want to start out, whats next? 1.21 Gigawatts? Seriously, build a smaller amp first!"

I agree.

As I recall, the 3CX3000 tubes in the Continental transmitter were driven by 807's. Driving these triodes isn't a job for an IC.
It requires lost of drive voltage and current.

[amirmk]

It's a bad idea, i worked in the radio buissnes, and those are horrible triodes for audio. I belive 845 will do.

[Frank10]

I thank you for your help. I might have given bad information, he was planning on using several LM3886 because he said you can get lots of power by using several and he was planning on using an torroid transformer to get the drive voltage up high.

The cooling of the tube was something his dad was going to do for him since his dad has a machine shop at home and he works for a company that provides cooling fans for RF equipment.

The output transformer is the biggest concern for him but his uncle was the head design engineer for a transformer company called Triad (or something like that name) and he talked about a power core double E liminate winding the primary on double bobbin design. The primary winding wire is 18 guage and the secondary was a multii-cable type design. I think he said something about square wire or something like that for the output. The whole transformer was going to be vacuum varnish impregnated and the winding method took into account the capacitance of the winding.

The weight of the transformer is of some concern but the kid feels the results will be something worth the hassle.

I want to thank you all for your advice. Does anybody know where I can get the tubes?

Frank