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Old 26th September 2005, 02:42 PM   #1
mr mojo is offline mr mojo  United States
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Default Push pull 300B and more than a few questions...

I'm in the market for a new project and recently came across some iron that may make it possible for a reasonable amount of money, but I've got some questions. If you folks would be willing to help out I'd sure appreciate it!

First off, I'm looking to make a dual mono PP 300B-anybody point me in a direction for schematics? Yes I've searched and googled but didn't come up with much. I'm hoping to use something like a 6SN7 if possible since I've got some extra octal ceramic sockets gathering dust.

Second, rectifiers: Any reason I couldn't use 5AR4s? I've got more than a few lying around and I've found some good iron with 800vct HV and 5v 3amp windings. With the 1.9 draw of a 5AR4 and the 1.2 of a 300B I think I'd be OK at 3.1 amps?

Third, output iron: I've found some 50 watt 5k Z PP outputs. Would these work for PP 300Bs?

Any help would be most appreciated-I'm jonesing to start a new project and if the iron I've found will work, I may even be able to afford it!

Thanks,
mr mojo
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Old 26th September 2005, 03:49 PM   #2
amperex is offline amperex  United States
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I am sure you understand the 300B needs a seperate filament supply. Go to http://www.tubebuilder.com//schematic1.html & look at the PP300B schematic
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Old 26th September 2005, 04:07 PM   #3
mr mojo is offline mr mojo  United States
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amperex,


I appreciate the help! So I need a separate filament supply for the 300Bs because of the center-tap? Or is it a hum issue?

Will a 5k PP output xfrmr work? The schematic shows 5k and I'm guessing it's PP output, not SE?

Also, where would I find the power supply for that PP 300B at tubebuilder? Would I have to design the power supply myself?

Best,
mr mojo
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Old 26th September 2005, 04:55 PM   #4
amperex is offline amperex  United States
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The design requires a cathode resistor that floats the 300B cathodes up at +77 volts to provide cathode bias. The filament requires no center tap, seperate winding & a 200-ohm hum balance pot across the two filament wires with the selectable 'wiper' to ground.

The 5AR4, of course, requires a seperate 5.0 volt winding. Hammond makes low cost 5.0 volt filament transformers.

(edit)- Forgot, the 5K transformers are fine business for your application. A 2K to 5K resistor can drop the B+ from the 300Bs to the other tubes. Use a 10uF polyproplyene cap after that resistor to ground. Sollen 600 volt as great.
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Old 26th September 2005, 06:30 PM   #5
SY is offline SY  United States
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There was a very nice p-p 300B project in Audio Anthology Volume 1. It was originally published about 50-55 years ago, but there's no reason not to update it a bit around the edges and use it.
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Old 27th September 2005, 02:27 AM   #6
amperex is offline amperex  United States
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Oh, it looks like PS must be designed seperate.
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