2. amps-preamps
  3. 1178450866

Have I damaged my amp?

Hi,
I have a Music Angel 300B amp which has been fine for sometime now. A friend visited to hear the new single driver speaker cabinets I've made, to compare these with my previous Kef's (the amp had been on for hours) I unplugged the Rt channel only and we had a listen for 30secs or so then I swapped back, the amp was still on. A few days later it started making loud rustling sounds 'under' the music (not volume dependent) on the right channel with some squeal when I tap the chassis, there is also quite a lot of noise when I power down from the Rt channel. I have tried swapping the valves with no change and have checked inside for loose connections 'bulging' capacitors etc and can see nothing wrong. So is this a probable cause for damage and what could it be? the problem is becoming noisier but only starts after the amp has warmed up an hour or more. My first thought is to replace the main capacitors under the transformer. I don't have a schematic for it but does anyone have any views on this.

Thanks,
Iain
irbow
marakanetz's avatar
marakanetz
5,473 posts
 
bad power tube unfortunately...
irbow OP
4 posts
 
I did try gently tapping the 300B's with a pencil and they were silent. The 6SL7's were microphonic, could it be that both of them are dodgy, they're the original Chinese ones and didn't look very new out of the box.

Iain
tarsando
578 posts
 
If it were a ss amp, I'd say, sure, you fried one channel. But not tubes. Hard to say it was a coincidence...but that's my thinking. Good luck. Without schematic you are entering deep waters indeed.
rwwear
2,867 posts
 
I disagree with Tarsando. Tube amps are never to be driven with no load on the outputs.
jax2
3,031 posts
 
Tube amps are never to be driven with no load on the outputs.

This has been my understanding too. It's always a good policy to shut it down, give it about 30 seconds to bleed off any residual charge, and then do your changes in wiring. Never run a signal through a tube amp without a connection to a load. Opposite of what Tarsando said, I've do not associate a similar limitation with SS amps (running a signal through an SS amp with no load connected will not likely result in any risk of damage - but I would NOT do any hot-swapping regardless). Since this has been my understanding, I hope anyone with contrary information might chime in. So, with SS I'd still power down before doing any wiring changes. I would not hot-swap with a live component, SS or tube. I've seen others do it with no consequence, but I think it's a bad habit to get into.

When you say you "unplugged the Rt channel only", do you mean you unplugged the interconnect from the amp/preamp, or do you mean you pulled the speaker cable connection?

It wouldn't hurt to replace the Chinese 6SL7's with some better NOS tubes from a reliable source. That would likely improve your performance, though it may not solve the problem. Not surprising that a pencil tap yields noise though...I should think that's pretty normal for an input/driver tube. That might be a good, and inexpensive start to troubleshooting the problem.

If you swapped valves, as you indicated in your post, with no change (noise is still in the right side/amp) it would seem to eliminate a bad valve. Your description of a "rustling sound" does, however, sound like a bad tube, so I still think either testing the tubes, or swapping them with tested tubes that you know are good would be a worthwhile approach.

Marco
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