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Discussion Starter · #1 ·
With no guitar plugged in, volumes turn down, and when mu all tube amp is on for about 1/2h, the transformers start to hum, phisical vibrate, you can feel it when you touch the combo or listen with your ears on the wood of the combo.

If you turn up the volume, there is that hum but also trough the speaker, and I don't mean the typical tube hum, I can live with that...

any suggestions? Is this a warranty case?

My 23 yo Marshall top does not hum and this new high quality amp does hum...
 

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Sounds like a ground problem somewhere. I had a Mesa Single Recto once that had a really nasty hum, I spent hours working on it, even replaced the output transformer, it turned out it was the footswitch jack. It was a metal jack that went through a hole in the chassis (also metal), the previous owner had tightened the jack (good maintenance) but that caused it to break through the clear coating on the chassis therefore grounding the outside of the jack to the chassis. Unfortunately the outside of the jack was not at ground, but at the bottom half of the AC heater voltage for the preamp tubes (it had the bottom half of the AC sine wave going through it at ~-3.3V), see the problem? Now you are injecting this waveform into the ground. I was pretty upset when I found this, it should have at least had a plastic flanged washer around it so it couldn't touch the chassis, if there was no other way around not having the outside conductor of the jack at ground potential. I lost alot of respect for Mesa on that one, that and I really didn't like the Recto sound, not enough harmonics...

Sorry, kinda OT, but, the moral is, sometimes hums can be really hard to diagnose. Could be a bad preamp tube, bad filter cap, bad ground somewhere (or unintentional ground), open circuit (which could be a cold solder joint on the PCB), the list goes on... Unfortunately, it could also be just bad design, but I can't say that for sure. It's hard to tell without more info.

Roger
 

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Discussion Starter · #3 ·
This is a very interesting story thank you.
I went to the shop today with my amp and plugged it in, volumes at 0.Unfortunately it was almost closing time so I could not let the amp warming up well. But I've let my amp in the shop and tomorrow they are gonna test it. my first impression is that he noise is less, but it is tested in a large musicstrore with a lot of surrounding noise and an amp not at workingtemp.
 

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Good luck, hope they find the problem. Also, a high gain amp will have more hum than a Marshall 2203, which is more a medium gain kinda amp, but a great sounding one! The fact that they physically vibrate leads me to believe that it is something more though. Can you say what kind of amp it is? It doesn't mean they all are bad, just that this one has a problem.

Roger
 

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Discussion Starter · #5 ·
I have my amp back and the conclusion is bad tubes....They are only 4 months old.....
It concerned a mesa amp just like yours but it is not a grounding problem.
I'm now at home and removed the back plate and when you tick the tube softly the hum is away for half a second...so it must be the tubes.
I'm now going to check if they're placed well in their soccet...

I definately won't use mesa power tubes again. At the store they say any pair of matched ones is okay.
 

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Is it a Dual Rectifier? I wouldn't use Mesa tubes... they come with Sovtek 6L6 WXT tubes IIRC... Get REAL Svetlana 6L6GCs or even Svetlana EL34s, but if you're switching to EL34s you need to make sure they're the correctly rated and such.
 

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Discussion Starter · #7 ·
it is an F50 combo with 6L6 tubes. At the store they say I can trow in any matched pair.
I asked what about the fixed bias? They have to be matched because of the class of amp (A/B if i'm not wrong)

But what if they are indeed matched but run too cold or too hot?? Is the only solution buying mesa tubes???
I'm reading a lot on the internet about tubes and at the store they were just telling rubbish imho.
How can I be shure of a good idle current for a good tone. matched tubes will give the only the same current but this can vary upon paired tubes if I'm not wrong...
 

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A matched set will do fine, even the Mesa sets are rated by color. The matching means that they will push and pull equally at a given bias point. Mesa's may be biased a little on the cold side, but that is for safety and to get more life out of the tubes.

IMHO, if you've got an old Marshall, what do you need with a Mesa:rolleyes: I'm definitely more into the Marshall tone...

Roger
 

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Tsorovan said:
Is it a Dual Rectifier? I wouldn't use Mesa tubes... they come with Sovtek 6L6 WXT tubes IIRC... Get REAL Svetlana 6L6GCs or even Svetlana EL34s, but if you're switching to EL34s you need to make sure they're the correctly rated and such.
Yes, the Svetlana tubes are a bit better than the Sovteks, but not quite as good as JJs or EHs. The SED Winged "C" tubes are probably the best new power tubes you can get, where the Svetlana, made by the same company is the cheap version of them.

And I would recommend the same for any tube amp not just a Dual rectifier.
 

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jemke said:
it is an F50 combo with 6L6 tubes. At the store they say I can trow in any matched pair.
I asked what about the fixed bias? They have to be matched because of the class of amp (A/B if i'm not wrong)
The bias also has to be adjusted because it's fixed bias, not cathode. I hope you just mean that you need a matched pair as opposed to you need a matched pair and the bias is fine.
 
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