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Mains Noise

12/8/1997 8:55 PM
Mains Noise
I have a JLA Tube amp, which seems ultra sensitive to mains condition at various venues.  
This week we had a PA hum problem due to, very weak mains in the building.  
My Amp would not work - just roared, without input or vol. turned up.  
At least, I think this was the problem, as it worked when I got home.  
Are there surpressors in Tube amps?. This one has a gushing noise subject to venue, and the effects loop seems to pick up all sorts of noise through cables.  
Also when I tap the input socket, it amplifies. None of my other amps do this  
I have replaced the first preamp tube, which I thought helped at first ( could be in my mind)  
Please offer some oppinions  
12/9/1997 5:50 PM
Bill Bolton

> Are there surpressors in Tube amps?  
No. Whatever noise is on the power mains can find its way into the amp. How badly it effects the amp depends on the amp design. I'm not familiar with JLA amps so can't comment further on that.  
I use an external Noise Filter, a Ground Fault Interruptor and a Spike Supressor between the wall outlet and any amp I'm using. They don't help low mains voltage problems but are a simple way of minimising most other mains related problems (short of carrying around a full power conditioner).  
> Also when I tap the input socket,  
> it amplifies.  
Something in your amp is microphonic. It may not be the first pre-amp tube, it could be any of the pre-amp and driver tubes.  
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