Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: samato on January 19, 2011, 02:16:45 pm
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I want to be able to bias using the 1ohm cathode resistor method on my adjustable, fixed bias AB763 build with (2) 6v6 tubes.
I have a couple of those big, white, 10W resistors that measure exactly 1ohm (actually they read 1.3ohms but my meter reads .3ohms with both probes shorted together). The package these resistors came in says maximum working voltage is 350V.
I know these are physically larger than what is needed for this purpose but is it okay to use them? Would there be any reason not to other than the size/space?
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Of course they are OK for that purpose but you could also use lower Wattage resistors.
The current through these resistors is low which means you don't need any power resistors there.
/Leevi
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Thanks. Yeah, I just couldn't find any other 1ohm resistors locally.
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Are the 1ohm resistors Dougsells adequate for this app?
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Did you read Doug's description?
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That's what he sells them for.
Dave
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For those that wonder, do a sanity check regarding the 1 ohm resistors.
If you output tube passes 130mA, in most cases that will be way too much.
Use ohm's law: voltage = current * resistance. voltage = 0.130A * 1 = 0.13v
Calculate power: power = voltage * current. power = 0.13v * 0.13A = 0.017 watts
Rate the resistor for at least double the expected power dissipation. So, a 1/2w resistor is more than adequate.
If you're comforatble with math, and understand the concept, you can calculate power in one step.
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I use 1 watt for 6l6 or 2 watts for KT88. Reason is if something goes wrong the resistor may open and give a troubleshooting clue. Larger wattage resistors just hang in there until the mains fuse goes.