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 14th March 2006, 03:18 PM #1 Mrpong   diyAudio Member   Join Date: Aug 2004 Location: Bangkok 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?
 15th March 2006, 12:59 AM #2 Burnedfingers   diyAudio Member   Join Date: Nov 2002 What is the voltage across those resistors?
 15th March 2006, 03:11 AM #3 Brian Donaldson   diyAudio Member     Join Date: Jan 2003 Location: Texas, Love it or leave it 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
 15th March 2006, 03:15 AM #4 lndm   diyAudio Member   Join Date: Mar 2006 Location: nsw 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.
 15th March 2006, 08:52 PM #5 richwalters diyAudio Member     Join Date: Sep 2003 Location: Alps:Tube amp designs over 150W, SMPS guru. bear in mind todays resistor ratings are quoted for 70°C i.e assume 50°C rise at 25°C ambient. Personally I find this 70°C too hot so I often use a 3W resistor for a 1W circuit diss with a poorish ventilated under chassis. Design for longevity. richj
 15th March 2006, 10:31 PM #6 Brian Donaldson   diyAudio Member     Join Date: Jan 2003 Location: Texas, Love it or leave it 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.
 20th March 2006, 03:07 PM #7 dr._sleep   diyAudio Member   Join Date: Jul 2005 Location: southern us Additionally, the 5687 heater filaments consume .45 amps in series and .9 amps in parallel. dr._sleep

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