Bare Knuckle Pickups Forum
Forum Ringside => Tech => Topic started by: MrBump on September 19, 2013, 11:32:48 PM
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Hey, guys - last couple of times rehearsing, I've been getting some microphonic feedback, particularly on gainier settings.
Does that mean that one of the valves is likely to be on the way out? And is there an easy way of knowing if it's pre or power valves? I replaced the power valves a couple of years ago, but have never touched the preamp valves.
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Could be a bad valve when the problem occurs on both channels. You can check if the powervalves are glowing all in an equal colour. I would check the preampvalves too. Usually they last longer than the powervalves.
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Yeah, it's across both channels, independent of anything being plugged in (guitar, I mean - it's exacerbated by an overdrive or distortion being turned on.
I'll have a look round the back tonight.
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What amp is it?
One way to tell if a preamp valve is microphonic, is to turn your amp on, no guitar plugged in, and with a pencil or something tap lightly on the glass envelope of the valve. If it makes a loud ringing sound then the valve is probably microphonic and you can have a go swapping it.
high gains amps (or having higher gain settings) will be more prone to picking up the noise of you tapping a valve and V1 is usually the most sensitive but in my experience the result is normally more of a dull thud if there is no issue and you can usually tell if there is an obvious problem.
have a go at that if you can reach them while the amp is running. I've played amps in practice rooms that just sit squealing with no guitar plugged in at all. Or ones where if you tap the headshell you get a loud PING noise ringing out the cab. Horribly microphonic.
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Probably a good idea to get one preamp valve that you know is good (e.g., from another amp or a new one) and test it in the various spots. It will be one of the valves that is on all channels, e.g., V1 or the last one in line before the power amps (i.e., the phase inverter).