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Mrpong 14th March 2006 03:18 PM

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 :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?

Burnedfingers 15th March 2006 12:59 AM

What is the voltage across those resistors?

Brian Donaldson 15th March 2006 03:11 AM

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.


lndm 15th March 2006 03:15 AM

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.

richwalters 15th March 2006 08:52 PM

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.


Brian Donaldson 15th March 2006 10:31 PM

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.

dr._sleep 20th March 2006 03:07 PM

Additionally, the 5687 heater filaments consume .45 amps in series and .9 amps in parallel.


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