Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plumcrazyfx on June 22, 2018, 07:37:48 am
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So I'm reading the TUT books and he seems to like putting a 10ohm resistor directly after the rectifier diodes, followed by a .1uf to ground, followed by another 10ohm all before the first filter. Can someone enlighten me as to the purpose and whether they are a good addition to all amps or what? I don't see anything like it on vintage amps.
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Which book? What page?
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In Vol 3. He uses it or something similar a lot of his improved versions. Page 8-8 he has a 10ohm off each red tap with a .1uf from each tap to ground. Then after the rectifier diodes he has a 10ohm before the first filter. He has a very similar setup with his Bulldog update on page 9-9. Same setup with slightly different values with his Bassmaster on page13-11.
I'm most interested in his updated B15 on page 11-7. That has the setup I described in the first post with the 10ohm (I misspoke about the 100ohm), the .1uf to ground, and then a second 10ohm before the first filter.
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Does he say what the wattage is on those? It'd have to be pretty beefy, I'd think.
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I'm pretty sure those 10Ω/10W resistors simply limit the high initial inrush current that charges the filter caps at turn on. You can put them before or after the diodes.
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a .1uf from each tap to ground
In the solidstate world you will find these everywhere, typically used to suppress transient spikes on DC power taps.
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Yes, they are rated at 10w. Ok. Makes sense to limit in rush. Just a little easier on those first caps, I guess.
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Splitting the resistors and using the 0.1uf cap makes it sound like an RF filter. Would suppress switching noise from diodes and oscillation or radio signals that got picked up for any other reason.
The 0.1uf caps are seen everywhere in SS gear because electrolytics don't perform well at those frequencies. This is also why some people parallel those caps with large electrolytics, even if the +0.1uf addition doesn't look significant compared to the main cap.
Of course, tons of people don't bother with this, and never have issues.
I think if it was just an inrush/peak current limiter (which might improve the lifespan of power supply parts), there wouldn't be a need for a second resistor and cap in the middle. A single 22 ohm resistor would work fine.
I'm surprised they do need to be rated that high, as most resistors tolerate brief overloads. The peak current in that position (and RMS power as a result) is higher than the amp's DC current draw, but not that crazy. Maybe the inductance of a wire-wound resistor is an added bonus in this use case.
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Yes. Haven't read TUT 3, but one advertised point of that book is the goal of complete elimination of all noise. The hi W rating is probably to insure that the R's stay cool, so that their R rating remains intact. If so, the inrush current protection would be an added benefit. Theoretically, a .1 cap across the reservoir cap would yield noise reduction, without the R-C-R T filter. I suspect that KOC must explain the circuit somewhere in TUT 3.