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Having felt the heat given off by the 7591A, 7868 tubes over years of working on amps last night I measured the bulb temperatures with my Fluke 87 Thermo-couple.
After about 15 minutes powered-on with music at a average level the A700 Harman Kardon Award Series amp Electro Harkonix 7591A Tubes measured:
310 Deg F on the sides.
227 Deg F on the top.
Little wonder that almost all 7591A/7868 amps always have perforations around the base of the tubes where the sockets install, likely to aid in convective cooling.
Are these typical readings?
Why does the 7591A get so hot?
I always liked the 7591A/7868 tubes for their musicality.
The bitterness of poor quality remains long after the sweetness of low price is forgotten
Follow Ups:
I don't know what actual temp readings should be.If there is not a hint of cherry-red glow on the plates, then I would say it's normal readings.
Still, that heat is not doing your capacitors and other components any good. A small quiet fan will work wonders.
It's just a matter of significant amounts of heat being dissipated over a relatively small surface area.
Let's say the tube is running at 400 volts and the cathode current is 35 ma. That computes to 14 watts of heat. As well, the tube heater adds about 5 watts of additional heat. That means you have (in round numbers) 20 watts of heat to dissipate over a small area - and in an area where other heat generators make it more difficult for the tube to rid itself of the heat.
Yes, those are fairly typical temps - keep in mind the heat generated by the tube is highly dependent on the plate voltage and cathode current so there will be variance there. As well, better ventilation will reduce the temps.
Thanks Jim so much for the response to my question.
I was curious about whether these 7591A temps were within normal range.
The owner of the A700, a good friend of mine, uses it with the cover off.
Last night, I had to re-tension the Octal sockets and clean the Headphone jack.
She is now playing well and musical.
The bitterness of poor quality remains long after the sweetness of low price is forgotten
mt
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