a super hum

[mozarti]

All the sudden one channel of my Beepre started making a noise that is less like a hum than an alarm (very loud).  I tried to tighten up the 300 b socket and then measured the voltage on 1 to 10 and everything checked out fine. I put the preamp back with good results and then the noise came back after an hour.  There are only two things I have noticed: the led lights of the bad channel turn on a good two seconds after those of the good channel light up.  The tube in the socket of the bad channel seems much looser. If anyone has any thoughts, I would very much appreciate it. 

[Paul Birkeland]

Did you try swapping tubes between channels?

If the LED's take much longer to turn on, I would suspect an issue with the tube actually warming up (it should only take a few seconds).  This could be the tube itself, the socket, or the voltage at POS OUT not being high enough.  A loose 1085 regulator on that side might be the culprit if none of the other checks reveal anything.

[mozarti]

Thanks, Paul.  I do not think it is the tube because when I change tubes the problem remains with the same channel.  I also do not think it is lack of voltage in positive out, because when I measured it it was 9.9 volts. The only thing that has some positive effect is tightening up the nut that connects the socket to the chassis, but this effect disappears after few hours (this time the tube does not light up even though the leds are on). I cannot imagine why tightening this socket would be important.  At any rate, do you think I should install a new socket?   Thanks for any thoughts that you may have. 

[mozarti]

Paul,

There was indeed a loose regulator on that side (at least the heatsink was not adequately soldered to the board)!  The preamp is working now and am keeping my fingered crossed.  Thank you again for your help.