200- 500 watt Tube Amp project - diyAudio
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Old 13th May 2010, 12:37 AM   #1
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Default 200- 500 watt Tube Amp project

Hi All,

I'm interested in building a dual channel tube amp that can produce up to 500 w rms per channel.

But I'm not finding any schematics for my little beast. Does any one know where I might start to make the concept a reality using parts available in today's world.

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Old 13th May 2010, 01:02 AM   #2
kevinkr is offline kevinkr  United States
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Almost no one builds tube amps this big except for a few very specialized applications, VTL builds the MB-450 which is a 450Wrms monoblock that sells at $15K a pair, and uses 8 6550 per amp to achieve this.

Given their economies of scale at VTL I suspect you would have a hard time sourcing the components to build a comparable diy amp for less than half this amount.

You can't buy off the shelf power transformers and output transformers for this sort of power. Creative power supply design might overcome the power transformer issue (several identical toroids in parallel) but the OPTS are another matter - expect to throw a lot of money at these. (Several $K)

What are you planning to drive with these that requires so much power? A smaller amp in the 100W range is probably sufficient for most home uses I can imagine.

The largest amplifiers I have ever built and sold commercial were 4 chassis 120Wrms (4 x 6550 per channel) and in the late '90s these cost about $4K per stereo pair to build.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine

Last edited by kevinkr; 13th May 2010 at 01:09 AM.
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Old 13th May 2010, 02:02 AM   #3
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Originally Posted by toddbailey View Post
Hi All,

I'm interested in building a dual channel tube amp that can produce up to 500 w rms per channel.

But I'm not finding any schematics for my little beast. Does any one know where I might start to make the concept a reality using parts available in today's world.

One way that occurs to me is to use the 1900 ohm 280W Hammond OPT but that 475 ohm load line needs a lot of cathode current (over 1A peak). Maybe 2 or more likely 3 parallel 6550s and you could get 250W+ out. Even at that low impedance you need something approaching 600V B+

I don't know of any schematics but the OPT is the place to start and there are few at the >200W power level.
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Old 13th May 2010, 02:06 AM   #4
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You can't buy off the shelf power transformers and output transformers for this sort of power.
Maybe not rated for 500 watts but 400 watt OPT's are available off the shelf from Plitron. The PAT-4141-00. It weighs 23 pounds and costs $408 EACH!

I am sure that a suitable power transformer or combination of transformers can be found too.

I plan to build a big amp. I have no set power goals other than 200 to 500 WPC. The lower limit is set by the fact that I have already squeezed 200 WPC out of Petes red board, and the upper limit is set by what I can get out of the wall outlet multiplied by a realistic efficiency number.

An amp of this size has been discussed on this forum for over two years and to date I don't know of anyone who has built one. There was a thread where a member disclosed a source of 400 watt Plitron OPT's for $134 each, of course I bought a pair, and so did a few other members.

Eight 807's in Push Pull?

This amp was most recently discussed here:

Monstrous 1kW amp!

I have seen the red board crank out a stable 150 WPC through the Plitrons with the 8 ohm load on the 4 ohm tap to reflect a 2500 ohm load. Build a big power supply, and use two red boards, one for each channel, with the two channels on each board paralleled through the Plitron set for its rated 1250 ohms. This will make 300 WPC. Of course this is the easy way out, and I only have one red board, but I might try it since I have everything.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 13th May 2010, 04:28 AM   #5
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Cannibalize an old AM ham radio transmitter. You could use the modulator; a 1000W rig would have a 500W modulator.

Only problem, the secondary wouldn't be voice coil friendly. But it could be rewound.

Use a pair of 250TH tubes. Maybe 2500 or 3000V on the plates.
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Old 13th May 2010, 06:51 AM   #6
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I find output transformers are a limiting factor- in cost, voltage rating or weight! I think the size/weight of the transformer limits winding machines.

If you run a pair of "big" transmitting tubes, the plate voltage is high (1-2kV) and the output transformer can't take it unless you have special HV windings. That's custom iron and surely expensive. Also the big bottles use forced air cooling (i.e 4CX250's).

Otherwise, you use a bunch of medium-voltage output tubes (i.e. six to eight 6550's at 550V) with a Hammond 1650W or PAT-4141. They have "low" voltage ratings intended for 600V or less plate voltages.

The Hammond 1650W is rated 280W at 1900ohms primary, weight is 28 lbs, about $315.

The Plitron PAT-4141 is rated 400W at 1250ohms primary, weight is 23.5 lbs, about $410.
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Old 13th May 2010, 07:29 AM   #7
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320w approx, 12 x 6550 per channel

Would you build one like this? I don't know, but it's what Pat Turner built.
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Old 13th May 2010, 09:25 AM   #8
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Champ 1000 Watt Tube Amp

Champ Electronics - Valve Amplifier Construction Work

Have you considered trying to go OTL?
Real tubes have top-caps
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Old 13th May 2010, 12:22 PM   #9
cnpope is offline cnpope  United States
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The circlotron OTL by Alan Kimmel claims to give 210W into 8 ohms. This is with 8 EL509 sweep tubes in the output. I've built a "lite" version, using just 4 JJ EL509S tubes, and it works beautifully.

Details and schematic are available at
Sweep Tube OTL Monoblock 2004

You could probably add a few more EL509's and beef up the power even more, if you really wanted!

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Old 13th May 2010, 07:18 PM   #10
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Toddbailey......what is the lowest frequency operation at full power you are anticipating ? I'm strongly inclined to keep design to commercial tubes that are currently manufactured. 6550,KT88,KT90 and so on.
Example:-An E&I 200W o/p tranny set at 15Hz will weigh 25Lbs. (designing for 40Hz can expect half the iron weight) and the mains probably around 35Lbs or more. With the chassis (reinforced), Can you lift this heap or need a fork truck ?

Parallelling same make o/p trannies is an idea but not a space saver, but I prefer a single lump.

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