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Old 7th November 2003, 06:42 PM   #1
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Default High Powered 3cx300a1 Tube Amp

Is anyone familiar with this tube? It looks like a great tube for bulding a high power high performance amplifier. The only problem is finding a suitalbe output transformer. I have a spec sheet for this tube at my website: www.missouri.edu/~lnra54

Anyone willing to help?
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Old 7th November 2003, 08:33 PM   #2
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Two years ago I designed a simple two stage Single Ended amp for a group project. Prototype was never built. If 25W RMS (quoting from memory) per channel is ok with you, then it should be a good amp.

The power supply will cost you an arm and a leg. Besides, the tube does not glow. Metal casing i.e. the anode is exposed. Lethal voltages. Fan is a must to keep the glue (ceramic-metal contact) from melting.

Are you game or do you dare?

Mohan
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Old 7th November 2003, 11:25 PM   #3
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I'm looking at building something along the lines of Bob Danielak's "The Beast" which can be found here: http://www.geocities.com/TimesSquare/1965/pp3cx300.html

Its a very compact design, yet effective. One interesting feature is the dual differntial amp topology and the choke loading of the driver stage. I figured it would cost about $1000 in parts including a custom power tranny form Peter Dahl ($250) and the tubes ($120 each). I was lucky and found 24-680uF caps at Ebay for cheap. Instead of using 750V for the plates I'm lookin more at 1000-1200 volts and a high impedance OT. I want POWER. The only problem would be the OT. I was looking at a Lundahl OT LL1620 (I believe) that can handle 125 watts and has high impedance, the spec sheet mentions that 250watts is possible, albeit at a higher low frequency cottoff (.8 dB).
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Old 8th November 2003, 03:25 AM   #4
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Hi,
You will need a fair amount of fan induced airflow to get the rated anode dissipation from that tube. If you want POWER, there are direct ratiating triodes that will do a better job for you. Not that I'd try to talk you out of it...
regards,
Douglas
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Old 8th November 2003, 03:28 AM   #5
SY is offline SY  United States
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It's a great tube for Class B RF service. As an audio tube, it has the virtue of being unusual... and that's it. There are a lot of more suitable output tubes for high power.
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Old 8th November 2003, 03:20 PM   #6
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Default Opt

I would check the max. operating DC voltage and peak voltage the insulation the OPT will handle. Be careful, as one manufacturer gave me the breakdown voltage, not the safe max. working voltage.

Just a thought.

Good luck on your venture.
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Old 8th November 2003, 11:40 PM   #7
GaryW is offline GaryW  United States
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Someone thinks it's a fair idea: 3cx300a1 stereo amp
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Old 9th November 2003, 07:30 PM   #8
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So, you think there are other tubes better suited for this kind of operation. 572, 812 maybe? I've looked at the data sheets for these two tubes but they seem to require much larger grid voltages to get them to "swing" and get high power. Then there's also the problem of grid current draw at positive grid voltages which means the driver tube would have to be a medium power pentode (el84, sv83, etc), increasing complexity.

Push-pull parallel (6550, kt88) designs would be the other alternative for getting high power, but those designs are usually more complex and require NFB, which I don't personally like since it makes the amp sound more constricted. BTW, the 3cx300a1 was designed for audio opeartion not RF like someone mentioned, although it IS DERIVED from an RF tube.
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Old 9th November 2003, 10:02 PM   #9
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Here's the thing: adapted to audio or not, it requires an unreasonably high plate voltage and an unreasonably high load resistance to operate with any sort of reasonably efficiency and power. That means transformers with expensive (and maybe unreliable) insulation and high turns ratios. That'll constrict ya. Or you can turn the amp into a room heater that leaks out a couple of Magic watts, but I suspect that's not what you're after. You want POWER, and I don't blame you.

There are a lot of other power tube options out there with their own advantages and disadvantages with respect to linearity, power, and drive requirements. I've done pretty well with 6550, 6528, and 6LF6 in 100+watt amps.
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Old 10th November 2003, 02:21 AM   #10
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Hey-Hey!!!,
Uhhhh, not to split hairs, but the amp linked to in a prevgious post ran 1k6 Ohm in SE, that is less than 15:1 turns ratio, which is not high. Fairly easy actually. 1k9 is just a bit higher than 15:1 referenced to 8 ohm, and that isn't bad either.

Go PP with 6-700 volts of B+ and drive it with 6BX7 or maybe two 12B4, or 7233, all in common cathode PP. With proper fan and ducting, it should be easy to relaize a 75-120 watt diddipation rating. Add a bit of plate-grid nfb on the power stage, or run the driver B+ from the UL taps and you can realize silly power with this tube.

But the fan thing, and exposed anode structure make 866's look safe around toddlers...
regards,
Douglas
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