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Old 30th September 2005, 05:25 PM   #1
mr mojo is offline mr mojo  United States
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Default 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
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Old 30th September 2005, 05:41 PM   #2
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Default 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.
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Old 30th September 2005, 05:45 PM   #3
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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
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Old 30th September 2005, 06:08 PM   #4
mr mojo is offline mr mojo  United States
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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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Old 30th September 2005, 10:55 PM   #5
Sherman is offline Sherman  United States
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Default 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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