need some help with Ohm's law-fast! - diyAudio
 need some help with Ohm's law-fast!
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 30th September 2005, 05:25 PM #1 diyAudio Member   Join Date: Jun 2005 Location: Iowa need some help with Ohm's law-fast! Howdy folks, got a question about using ohm's law: Schematic for my 7591 PP amp calls for 480v on the plates. I'm getting 520v-ouch! I've got high line voltage to my residence and I know the power xfrmr is putting out @840v CT rather than 800v CT. Right now the situation is a pair of dropping resistors for each channel. 200ohm, 10 watt. Voltage before the resistor is 540. Voltage drop after the resistor is now 520-but I need 480. If I did the math right, I need a 600ohm 10w resistor to bring the voltage down to 480-is that right? If anyone could jump in and help quick I'd appreciate it-I'm gonna try and make a break from the office this afternoon to get the resistors-if I don't get them today I'll have to wait till Monday which means no music over the weekend. Cant have that! Thanks for any and all help mr mojo
 30th September 2005, 05:41 PM #2 diyAudio Member     Join Date: May 2001 Location: NL, The Hague Ohm's mother in law Your math is correct. Currently there is a 20V drop over a 200Ohm resistor. The current throught the resistor is 100mA. For 60V drop you need indeed 600Ohm. BUT... the power dissipated in this resistor will be 6 W (600 V * 0,1 *0,1). A 10W resistor will become quite hot. You could try and put 2 1k2 10W resistors in parallel. __________________ J-amp powers Avantgarde-Acoustic Trio
 30th September 2005, 05:45 PM #3 diyAudio Member   Join Date: Apr 2002 Location: Cayman Islands You are withing 10% of the voltage so I personally would not worry about it. I have an EL34 amp thats supposed to run at 480V on the plates and its 525V measured. Been like that for about 5 years. If you really want to drop the voltage and you have a pi filter you could try a smaller uF value irst capacitor instead. Andrew __________________ None of us are leaving this world alive. Enjoy it responsibly as you may.
 30th September 2005, 06:08 PM #4 diyAudio Member   Join Date: Jun 2005 Location: Iowa Wowee!! You guys ROCK-thanks for the quick replies. Andrew, I'm not so concerned about the plates-it's the 500v electrolytic filters on the output transformers that concerns me! Yow. At least I know the Spragues will handle a bit more than their rated voltage-but for how long is anybody's guess. My guess would be not long. dutch, Thanks for backing me up on my math-I'm gonna go with either a 560ohm 25w or a 680ohm 25w. That's as close as I can get to 600 and the 25w rating should handle the power better. I appreciate the help-I really do. I know enough to be dangerous-but heck, that's half the fun. Best, mr mojo
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Re: need some help with Ohm's law-fast!

Quote:
 Originally posted by mr mojo ...calls for 480v on the plates. I'm getting 520v-ouch! I've got high line voltage to my residence and I know the power xfrmr is putting out @840v CT rather than 800v CT. ... Thanks for any and all help mr mojo

Your math is right but I don't think it matters. For a tube circuit you are practically dead on. With voltages pretty much anything within 10-15% is fine. Remember tubes were designed when 10-20% resistors, 20% capacitors and fluctuating line voltages were the norm.
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