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Old 30th November 2012, 10:37 AM   #101
kimbal is offline kimbal  Australia
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Quote:
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.

thanks
Building Big Tube amps has been a pet fetish for me since the 1970's when I was working on medium sized monsters (100 watt-300 watts) in my job. The problem with even a small 300 watt amp was the size of the transformers ( cost and availability is a whole other issue ).
I worked on a Phillips 300 watt PA amp in the late 70s which used two QB3/300 TX tubes in the OP stage. It ran on 2.8 KV and the power and output transformers were around 12 inches cubed. It was made on 3 chassis in a large rack cabinet ( that took 4 guys to lift - or two men and a big trolley ) and it ran 100 volt speaker lines into large metal horns throughout a large factory. I recall when I repaired it, the very next day, while on soak test, the output transformer caught on fire and it was site to see. It had spark gaps and large Oil filled capacitors and two large mercury rectifiers. It ran off a 15 amp mains at 240 volts ac that is 3.5KW available at the input, here is Australia.

So as no one will build big transformers for Audio – Mind you I have tried and tried and the legal liabilities are what scares some winding manufacturers off - A simple way to get the Kilowatts out of off the shelf parts is to use the following process. Again cost is an issue, but its a more practical construction process and the lesser of two evils –unless you keen to design and wind your own iron.

Make a number of power output stages, say 4 in all.
Drive all the output tubes from a common driver phase splitter combination - and with each output stage, drive a separate output transformer.
In this case you will have 4 output transformers. You then apply the secondaries in series parallel, or just parallel if you desire. Feed back can easily be applied if required and the tubes are common as grass to buy. For me, I favour 807’s.

So four 100 watt output stages will deliver 400 watts in all combined into a single speaker. If you run them as 4 separate outputs into 4 speakers, you can set it up so if one stage dies the other three still operate and you don’t go out of operation altogether. You might use 8 tubes, or 16, - it’s your choice. Tube matching is ideal, but not essential.
This concept was used in the early days of Tx modulators - paralleled output stages driving one massive transformer or two smaller output transformers.
RCA and Westinghouse used this approach with good results.

If you use Class AB1 which has just drive voltage swing on the input grids, but no grid current flowing - your driver stage will be much easier to manage and design. You may use more tubes to get the big watts, but as they are cheap so what. If properly designed they will last longer as well. Keep the plate dissipation to around 60% and get rid of the heat.

My own concept amp is looking at maybe 32 x 807 tubes running in the 56 watt AB1 mode per pair. That is nearly 900 watts output, or 224 watts per amp, if I have 4 output stages. I can do it with 4 smaller 250 watt transformers or do the overkill with 4 Sowter 400 watt Hifi Output Trannies.

Also too with the power supply - use stacked low voltage supplies where possible. Easier to source smaller trannies in bulk buys.

If you apply this approach say with 807 tubes you can get the above power figures from 4 power stages with 32 tubes on a B+ of 600 volts, and a half supply of 300 for the screens; and almost no grid drive power.

To do it with two big tubes you might require 2.5 to 3 kv for the plate volts and a very, very expensive non-hifi set of transformers to boot. Most BIG tubes also need driver transformers as well, since many are set up for class B. Smaller tubes not so much. Big tubes also need a decent driver stage which maybe another power amp of 25 watts or so.

Everything being equal – the above approach is a compromise, so you have to decide what is the best for your needs.

But for those that insist of BIG Iron and BIG Tubes -and some serious material on designing high power Audio Transformers, I can recommend the GE/ Naval publication from the 1960s called – High Power, High Voltage, Audio Frequency Transformer Design Manual. Almost 700 pages of material on designing audio transformers from 10kw to 350 Kw in size. A rare book indeed.
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Old 30th November 2012, 12:32 PM   #102
mike567 is offline mike567  United States
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heating elements from residential central heating units 5 kw @ 240 volts each make a nice speaker load.
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Old 30th November 2012, 04:01 PM   #103
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If you really want to build an amp that large then you are pretty much on your own. Design using parts that are available otherwise you will spend a great deal having custom transformers wound. I'm using a cnc machine power transformer reverse wired for the plate supply for 20 kt-88s. Output transformers are hammond 280 watt units. 4 of them, 2 in parallel for each channel. Also have to have something to drive all those output tubes and I'm using mosfets, one for each power tube. Those have to have their own supplies along with the filament voltage, regulated preamp filament voltage, regulated preamp high voltage, regulated output tube grid 2 voltage, 12V for cooling fans, 5V for digital control circuitry, bias supplies, etc. It's a lot and I designed custom circuit boards as opposed to point to point for the preamp, power amps, preamp controller, and some of the power supplies. I'm coming down the home stretch now, just dialing in the preamp then need to buy power tubes, test the power amps, and build cabinets. I've been working on this off and on for 3 years and have spent over 3 grand on this project and I expect it to weigh about 250 lbs. It'll be in two separate cabinets- power supply and amp. Here's a sneak peek...

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Old 30th November 2012, 04:20 PM   #104
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by kimbal View Post
Building Big Tube amps has been a pet fetish for me since the 1970's when I was working on medium sized monsters (100 watt-300 watts) in my job. The problem with even a small 300 watt amp was the size of the transformers ...
The way professionals have been handling this problem for years is simple. If you need (say) 400W then you get four 100W amps and mount them all in a big rack mount frame. Then you typically have four big 12 inch drivers in the speaker cabinet so they run an 8-pole speaker cable that connects each of the four drivers to it's own amplifier. With solid state class D amps the numbers just get bigger but you see the same set ups with 8-pole Speakon terminated cables.
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Old 30th November 2012, 04:30 PM   #105
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Wolfepunk,
I like your panels. Were they engraved or laser-cut or silk-screened or dry-transfer or what?
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Old 30th November 2012, 04:34 PM   #106
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Even one of the big old mono tube Fender 400-watt bass guitar amps had three output transformers. But they expected you to plug in three speaker cabinets, rather than parallel the outputs.
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Old 30th November 2012, 05:13 PM   #107
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My chassis were custom made of .125 aluminum, tig welded, powder coated and screen printed with the custom graphics using a two part white epoxy. I designed the graphics which took forever to get everything all lined up with the holes in the chassis. They were not cheap!. The panel that holds the output transformers is .25 aluminum u channel.
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Old 30th November 2012, 05:59 PM   #108
cotdt is offline cotdt  United States
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What about paralleling 150 6C33 output tubes and running them OTL?
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Old 30th November 2012, 06:06 PM   #109
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That's a lot of tubes! Otl would be cool, it would shave 115 sounds off my amp but would be stuck with a fixed output impedance and also stuck with funky, non production tubes.
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Old 30th November 2012, 06:25 PM   #110
cotdt is offline cotdt  United States
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Quote:
Originally Posted by wolfepunk View Post
That's a lot of tubes! Otl would be cool, it would shave 115 sounds off my amp but would be stuck with a fixed output impedance and also stuck with funky, non production tubes.
You will like having 150 output tubes per side. The turn on thump alone would shoot your speaker cone out like a bullet, into the wall.
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