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Old 3rd February 2011, 09:59 PM   #1
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Default push pull 813 tube in ultra linear mode

Does anyone know of a good tested schematic for a push pull 813 tube ultra linear design amp. I was thinking of 250 watts from two 813 tubes with ultra linear output transformer taps.
Also, is there any reason not to use separate transformers for each needed voltage. The power supply will be a separate unit mounted under the main chassis. Many of the components for this project will be salvaged from various items I can acquire cheaply. A multi-tapped power transformer is more complex than several individual units. It is also easier to calculate power demands on only one secondary voltage/load.
This may possible be my last amp project. The eyes and hands are not what they used to be.
Any suggestions on this project will be much appreciated.

Thanks Tad
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Old 3rd February 2011, 10:10 PM   #2
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2,500V on anodes and 750V on screen grids is a nice mode for them to get 250 Watt of output power. You will need additional windings isolated from primary for ultralinear mode.
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Old 3rd February 2011, 10:25 PM   #3
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frick! At 2.5kv on the anodes you are right this MAY well be your last project!!!

Good luck...
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Old 3rd February 2011, 10:36 PM   #4
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Wavebourn,
Is there any advantage or disadvantage to having separate transformers for each needed voltage? What would be the optimum area to tap the output for the ultra linear voltage? There is very little 813 information on the net. Most tube amps are using the KT series tubes. I see nothing wrong with the old 813 item. It has many really good characteristics. It also is one hulking monster!!!!

Tad
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Old 3rd February 2011, 10:40 PM   #5
AJT is offline AJT  Philippines
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hey tad, if you want power, you might want to try out 4D32 tubes, i bought 2 quads from Surplus Sales of Nebraska.....at 600 volts you can get 120 watts.....
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Old 3rd February 2011, 10:40 PM   #6
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Quote:
Originally Posted by tryonziess View Post
Wavebourn,
Is there any advantage or disadvantage to having separate transformers for each needed voltage? What would be the optimum area to tap the output for the ultra linear voltage? There is very little 813 information on the net. Most tube amps are using the KT series tubes. I see nothing wrong with the old 813 item. It has many really good characteristics. It also is one hulking monster!!!!
I would not use them: too dangerous, inconvenient, and PITA to get good OT. I currently plan to build 200W amp prototype based on 4xGU-50 tubes.
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Old 3rd February 2011, 11:04 PM   #7
Matt BH is offline Matt BH  United Kingdom
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Here is a good design for an 813 amp:

Champ 1000 Watt Tube Amp

Notice its for PA use not HiFi.

If you want to go with UL then it limits your supply voltage to around 1100V because the screens wont take much more. Even this is pretty high especially with top caps IMHO.

Getting some output trannys large enough with such a high primary impedence and also good bandwidth is almost impossible.

Cheers Matt.
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Old 3rd February 2011, 11:07 PM   #8
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Hi there,
With the 813 there are 2 routes you can go with, in ultralinear:
Use a B+ of 750 volts and figure out the loadlines to get your output transformer impedance. I am thinking it will be a low value, like 5-6K, but you will have to figure it out to be more exact. In this case the output transformer will not be too exotic and most likely an off the shelf item will work.

The other scenario will get you more power because of the higher B+ voltage; 2KV. You will have to have a custom built output transformer with 2 windings; one for the plates @ 2KV with an impedance of about 12-15K and another winding for the screens with the voltage at 750 volts. The 2 windings will probably have to be wound side by side so there is adequate coupling, but it still will not be as good as traditional ultra linear where the same winding is used, with taps for the screens. This will cost a lot as many parts will be custom made for your project.

The 3rd scenario would be to go traditional pentode mode with 1.5-2 KV at the plates and 750 volts for the screens. Output transformer impedance would be 10-16K. I have been thinking about going this route for a Super PA / Guitar Amplifier which I know is impractical but would like to see how well it would work. I have some UTC CVP-5 output transformers that are rated at 10 or 14K, 300 watts and hi-potted for 3.5KV. I would run them with 1.5KV B+ as I am reluctant to push them any harder. I am expecting , which is what the RCA handbook says, 250 watts for a pair of 813's.
I love the 813.... To me it looks like a 6550 on steroids.
Daniel
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Old 3rd February 2011, 11:10 PM   #9
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Maybe you find something interesting here: oestex.com/tubes/ul.html
Much info on UL and high power amplifier schematics.
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Old 3rd February 2011, 11:12 PM   #10
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This working better? ultra-linear
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