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300B stereo amp project help
300B stereo amp project help
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Old 26th March 2009, 03:29 PM   #1
bartasas is offline bartasas  Lithuania
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Join Date: Jun 2008
Default 300B stereo amp project help

Hi everyone,
I`m starting a Audio Note Conqueror like 300B stereo power amp project:


I`ve never done any circuits with 300B tubes. Want to order a custom SOWTER mains transformer with such secondaries:

Secondary 1: 425-325-0-325-425@0.25A (B+ 300B/6SN7)
Secondary 2: 3.15-0-3.15@2A (2x6SN7)
Secondary 3: 5@3A (5U4G)
Secondary 4: 7.5@2A (300B)
Secondary 5: 7.5@2A (300B)
(325-0-325 in case to switch for the 2A3 tubes)

Any comments would be greatly apreciated. The transformer makes 1/3 project expenses. Would be very dissapointing to get it wrong.
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Old 26th March 2009, 09:31 PM   #2
Rod Coleman is offline Rod Coleman  United Kingdom
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Location: Warwickshire UK
300B stereo amp project help
Since you plan to spend on custom transformer, please use choke-input power supply design. 300B sounds MUCH better like this. Sowter make a good choke specially for this kind of design.

For choke input filter, need 450-0-450 or 475-0-475 for a good design to give about 400..425Vdc for the 300Bs. adjust the voltage with a small (220..680nF 1500Vdc capacitor (LCR capacitors PC/HV/S series) placed after the rectifier, before the choke.

Better still, use 0-475V and a hybrid bridge of UF4007 x 2 silicon diodes and damper rectifiers (6CJ3 etc etc) or 0-490V and uf4007 x 2 and 5R4GY valve rectifier. Hybrid bridges sound superb and make better use of the transformer winding space. Do a search on 'hybrid bridge' to find lots of design tips on these.

Also, do not put 300B filament windings on the B+ transformer. If you do, rectifier noise will couple through to the 300B filaments and injure the sound. Use a separate transformer. And, you will also find that AC heating of 300Bs sounds really bad compared to proper current source heating. for DIY current circuit, see:

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Old 27th March 2009, 04:27 AM   #3
kmtang is offline kmtang  Canada
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Location: Vancouver
425-0-425 Power transformer??

It is a bit too high for the B+ unless you want to use the High Power type 300B's other than normal ones.

I would say 375-0-375 secondary would be good for standard 300B tubes.

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