Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JFlo on January 12, 2022, 06:29:17 pm
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I am building a Sunn Model T (first gen) standalone preamp and have some questions about the power supply. How important are the chokes if there is no power section? Any reason to use the chokes over a pi filter? I am using a 50VA 280V toroidal transformer from Antek (AS-05T280) and a bridge rectifier.
This is my first post here and I appreciate the wisdom of this forum. I have previously built a Champ and Orange OR120 from kits, then Rob Robinette's SLO-Nakid Micro. I thoroughly enjoyed building these and was able to troubleshoot my mistakes but this is my first attempt at building from just a schematic.
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I would not use any chokes. 3 caps and 2 resistors will give a very smooth dcv for that very light load.
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Chokes are for high current with low voltage drop. Here you actually want some voltage drop at low current. About 6.2mA at 330V. 330V/6.2mA is like 54k of resistance. You could fool with numbers but this is the late 20th century (or beyond?).
... caps and 2 resistors will give a very smooth dcv for that very light load.
Oddly, SUNN's values are really conservative. Smaller caps will still get B+ buzz far below tube self-hiss. Since we are starting well below 500V we want to reduce the resistors, and the filtering is still excellent. Any commercial amp factory would cut the caps in half again and still have a 'perfectly' quiet B+ rail. In DIY we can spend the dimes to be generous, 47uFd.
(More stages of small parts is a lot better than one or two soup-cans.)
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Thanks, guys! That's a big help.