Tonight, while listening, I heard a couple of loud hums, like someone plucking a big fat bass guitar string. I thought I was imagining something in the mix, but after a while it occured again. I stopped the CD and waited for a while - and it happened again. I turned the amps off.
This noise just happened on one side. I have a feeling that it's related to a degrading tube, possibly the rectifier, because over the last month or so, I notice a noise through the speaker when I turn this particular amp off. Up until this time my Laurels have been dead quiet, and I have absolutely no hum. Over the last few weeks, when I flick the switch off, I hear a bit of a loudish hum which dies quickly away. This doen't always happen though. The other amp shows no sign of this.
I changed the 6SL7s about 6 months after I got the amps, and at the same time switched over from KR300Bs to Full Music mesh anode 300Bs. The rectifiers are the original Mullard GZ37s as supplied with the amps. I'd say I listen around 50 hours a week. So I'd estimate I have approx 4000 hrs on the GZ37s and 2500 on the TJs and Sylvania VT-229s.
I did some research here and there have been a couple of similar posts, which I did read, but I'd like to ask for help.
Which of the valves is likely to be suspect? I have tons of different 6SL7s which I can try. I still have the low-mileage KR300 tubes, and I have some NOS GZ37s. I can swap the tubes but would like to know from inmates which tubes would be most likely to cause this noise?
I must say it was pretty disconcerting to hear this noise. It first happened in the middle of Pink Floyd and I thought I was hearing something on the CD!
As you can imagine, I'm mortified to have to stop listening, and I'd be delighted to get some tips on how best to debug my problem.
Regards,
Metralla
