Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on January 07, 2023, 09:36:06 am
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Went over this VW article about calculating the resistor value but i cannot understand what 50/C means. Can someone explain this to me?
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going to random schematics for various amps and i always see 220K resistors used. I guess i will just use those
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that's what I use.
believe your 50|C is 50 ohms per Capacitance (in Farads) so slip some decimals and try the math, I got wood to stack.
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You want enough resistance to impede the cap discharge when the amp is on, while allowing quick discharge when the power is off. Marshall 1959 SL100 uses 5W 56k for each 100uF main filter cap (112k in total). The Fender 65 DRRI uses 470k across the 47uF 500V cap and 100k across the 220uF 100V cap. So the R:C ratio can be all over the place.
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my caps are the following. It's alotta filtering i know but this is going to be a poweramp.
330uf 315v X2
220uf 350v X2
100uf 450v x2
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If you follow Merlin's advice ...
330uf 315v X2 151,515 or 150K
220uf 350v X2 227,272 or 220K
100uf 450v X2 500,000 or 470K
I would use 3 watt metal oxide for all balance/bleeder resistors.
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> understand what 50/C means.
For every 50u throw on a Meg. ("u" is 1/Meg so the decimal points do work out.)
330uFd is seven (almost) 50u's. So use seven 1Meg resistors. Which can also be 1000K/7 or one 143K resistor.
_I_ would "always" compute the worst-case dissipation. I hate when bleeders simmer to death and don't tell me they quit. Your 330uFd caps are rated 315V and we hope they never see more (or they may burst). 315V squared is 100,000. 100,000 in 143K is 694mW or 0.7 Watts. Always round-up double, but for a safety part in a hot chassis, 3X is safer. When talking dollar safety parts, round up generously, like 4X, so 2.8W. Aside from throwing heat, 3W or 5W parts may have stronger more reliable leads.