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Old 14th March 2006, 03:18 PM   #1
Mrpong is offline Mrpong  Thailand
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Exclamation Need help: 5687 tubes and heat dissipation

The schematic below is the headphone amplifer using 5687 tubes.

5687 tube headphone amplifier

In the schematic, you will see a group of 20K resistors which are my problems. Those 20K resistors are terribly hot even though we use the 3-watts type. Is this because of poor design of this schematic or because of the characteristic of 5687 tube? Any comments please?
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Old 15th March 2006, 12:59 AM   #2
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What is the voltage across those resistors?
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Old 15th March 2006, 03:11 AM   #3
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They will get hot. Use ohms law and if your less than 1.5 W, just give them free air and let 'um burn. I biult a simular HP amp, but used a long tail pair input to keep absolute phase and It sounds quite nice.

enjoy
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Old 15th March 2006, 03:15 AM   #4
lndm is offline lndm  Australia
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Knowing that voltage would be helpful.

Lets assume for a moment that the 260v rail is half across the triodes and half across the resistors. The 100 ohm resistor doesn't see much.

7 x 22k in parallel = 3k14. Now ((130v)^2)/3k14 = 5.38W.

This is not massive. Less than one watt per 3w resistor will make them hot, but not hot enough to concern over. Make sure they are separated with some airflow around them.
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Old 15th March 2006, 08:52 PM   #5
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bear in mind todays resistor ratings are quoted for 70C i.e assume 50C rise at 25C ambient. Personally I find this 70C too hot so I often use a 3W resistor for a 1W circuit diss with a poorish ventilated under chassis.
Design for longevity.

richj
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Old 15th March 2006, 10:31 PM   #6
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Aslo, dump the electrolytic on the output and buy the biggest film cap you can find. A little bass roll off is far better than the nasty grunge from the dilectric absorption of the 'lytic. Give it a listen.
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Old 20th March 2006, 03:07 PM   #7
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Additionally, the 5687 heater filaments consume .45 amps in series and .9 amps in parallel.

dr._sleep
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