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Power tube temperature

My Sophia Electric Baby started distorting real badly from its left channel when I turned it on yesterday. It plays but at a little lower volume as compared to the right channel with the distortion superimposed on the music. It was completely fine two days ago. Its a push-pull model putting out 10 wpc using four unusual 6P1T power tubes. They look much like EL84 but not the same. It always ran very hot but I attributed it to being a class A design so didn't pay much attention. Anyway, I measured the tube temperature and was shocked to see numbers like 260-300 F across the four tubes depending where I pointed the temperature gun. After a few minutes the temperature dropped a bit and settled on around 210 F. I have isolated the issue to something in the left channel by switching driver and power tubes and tubes seem fine since the right channel sounds just fine.  The question I have is what is the ballpark temperature for power tubes in a class A tube amplifier, especially ones with the smaller tubes like EL84 or EL34. Or better yet, input from anyone has one of these amps. And is it normal for them to start high at turn on and drop after warm up? I'm thinking something got damaged by the excessive temperature and causing the distortion on the left channel. I opened the back and see no obvious damage. Appreciate input from anyone with experience with these amps. Thanks.

kalali

Showing 2 responses by kalali

kalali's avatar
kalali OP
1,977 posts
 
sfar, thank you so much for your input. I feel a lot better knowing that sort of high temperature is not too far out of wack. 
I’m going to call Richard at Sophia and see if he has any suggestions or perhaps ship the unit to him for further troubleshooting and repair. It’s a great little integrated especially after I upgraded the capacitors with Jantzens. I actually have some upgrade power supply capacitors on order and really want to get this puppy fixed.  Thanks again.
kalali's avatar
kalali OP
1,977 posts
 
rob, thanks for the tip on tubes. I’ll take a look at their spec sheet to see if they are viable substitutes.  I don’t have the circuit diagram for the amp so all I could do was poke around with the voltmeter inside the unit, with extreme care of course, and it looks like both channels measure about the same and visually saw nothing out of ordinary. What is boggling me is it was perfectly fine the day before and the distortion started all of a sudden when I powered it up. Swapping tubes right to left makes no difference implying something internal in the left channel just went south but not enough to disable the audio path since it still makes music but at a lower volume and with the added distortion. This little amp, especially after I put in the Jantzen Z-Superior capacitors, sounds real nice so it’s worth fixing. 
P.S. I dug out an old thread with some contact names for techs in New Jersey and will see if I can get it fixed locally. 
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