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High Powered 3cx300a1 Tube Amp

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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
 
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).
 
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.
 
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.
 
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...:whazzat: :smash: :dead:
regards,
Douglas
 

PRR

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Joined 2003
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> It looks like a great tube for bulding a high power high performance amplifier.

It does OK, if you like expensive inefficent no-glow un-safe tubes.
An externally hosted image should be here but it was not working when we last tested it.


Damping factor is pretty good, distortion is so-so, drive is easy relative to supply voltage but hard relative to the usual receiving tubes we like to use as drivers.

The lower line may work with standard transformer, but will need some fan. The upper line will need near-max fan blow: these tubes are unforgiving of even short-term undercooling. The roar will require you to put it in another room. And it will electro-cook a hot-dog (or finger) on the plate in short order.

It would be a corker in push-pull, but I guess nobody cares.

> a great tube for Class B RF service

It actually claims to be an audio tube, and it isn't as bent as most Class B RF tubes would be. However it isn't as straight as a purpose-made audio triode, so I agree it is probably a mod of an RF tube.
 
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.

It's probably just me, but I wouldn't consider that setup as having reasonable efficiency or linearity. With 1.6K ohms, I can get nearly 200 watts out of a pair of 6LF6s (or two pairs of 6550s), with a heck of a lot less heat, noise, and distortion.
 
Hey-Hey SY, I am a PP beam tube man myself. I'll bet this triode will do power far more ccomparable to your 6LF6 amp if we run something more like class B too. PP is where the power is, and you can't get much of it class A w/o huge plate dissipation.

With 1k6 PP and B+ of say 600V, there are far better tetrodes than the 6LF6, try 13E1, 200V screen, 600 anode with a tertiary winding to give U-L like NFB to the screen. You wouldn't even need much nfb. a little plate-grid will be fine and 200W of nice AB1. Nah, forget it, up the load a spot, decrease B+ to 500 and get 80 watts in class A, or a little more just outside of it.

Your 200W beamer amp will need a lot of NFB, likely enough to sound quite 'off the rack', I'll pass on that sort of stuff, jsut like most of the vintage bits, too into a 'for the masses' consumption, which wanted max power from minimum tubes. Might as well get my MC75's back if I want that.
regards,
Douglas
are there any tetrodes, direct radiating, bigger than the 13E1?
 
Ok, so the consensus is that the 3cx300 is not very suitable for class a designs and might be suitable in class ab->b p-p, but with the prolbem of high plate voltages and high impedance OT's. What about the 211 tube? I havent been able to find curves for this one but have seen it has a plate diss. of 100 watts. Is it a good tube for high power p-p? Where can I find curves? I've tried google, but get nothing. Thanks for the replies...
 
I'm still trying to get my $#@!ing scanner to work. In the meantime, here's a verbal description:

The plates are connected to an OPT in the normal way. The control grids are grounded, hardwired to the cathodes. Each screen is driven directly from a MOSFET source follower with the source resistor (47K) tied to a -200V negative rail. The MOSFET's drain is tied to a +250V rail. The gate of the source follower is tied (through a 270K resistor) to the bias voltage pots (very close to zero). The bias is adjusted to get an idle current of 3 ma per tube. This is similar to the output configuration of the old Audionics BA150 and the Berning EA230, except that the Audionics used an emitter follower and the EA230 used a cathode follower to drive the screens.

There's considerable screen current drawn, so the MOSFET needs to be a power device with some heatsinking. I'm using IR hexfets- I think the number is IRF820, but I'm not sure. In any case, they're the 500V 1.5 ohm TO220. And you need a good, healthy amount of drive voltage (say 100V peak or so).
 
Hey SY,
Thanks, I have a small supply of the IRF820. Used in enhancement mode cascode CCS, before I was told about HV depletion mode. What sort of B+ doe this amp require?
I had to giggle in a knowing sort of way with regard to the plate cap ruling. I have a little one myself. Note the 13E1 does not have an external connection except in the usual base.
regards,
Douglas
 

PRR

Member
Joined 2003
Paid Member
> the prolbem of high plate voltages and high impedance OT's

Plate current is a function of heater power. In commercial design, the best way to go is fairly low current and high voltage so you get the most output power spec for your power-supply dollar. Essentially all commercial tubes are designed this way, especially as you get toward the kilowatt.

Hmmm.... I see Svetlana has suggested plans using "only" 500V B+. 40 watts P-P for 150W heat and damping factor not much better than 1 isn't real impressive.

For lots of power at reasonable or low plate impedances and good damping, you want a BIG cathode. Bigger than any rational tube-designer would try to sell. It means huge heater power compared to total power demand and power output. So parallel a bunch of big-cathode tubes.

Just off the tip of my head: a dozen 6L6GC or 6550 in triode parallel single-ended would make about 40 watts with good damping in a 500 ohm load, 300 watts total (plate and heater) dissipation. That's a very custom 600mA plate transformer; the same bucket of tubes working push-pull could use one of the standard transformers from Hammond or a boutique winder and make even more power.

You might also look at Russian 6c33 monster pass-regulator triode. $29 each.
 
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