I have a Jet City 120 and it suddenly started acting weird. It sounds fine for a few minutes but then starts sounding like theres some weird effects pedal in the signal, like an octaver or a pitch shifter or something, even though I'm plugged straight into the amp. I turned it on and off a few times and switched cables but thats all, I didn't want to leave it on too much longer in case it blows up or something. Has anybody ever had this happen? If there is an easy answer I'd like to avoid the hassle of taking it to a tech over something simple. Thanks.
Last edited by PKYINK at May 22, 2011,
You may be experiencing tube failure, but there's no real way to tell exactly what it is.

You should take it in.
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That could quite easily be a microphonic preamp tube. The microphonics it picks up are the same signal that is being fed through it. That sound being picked up from the speakers is delayed slightly to the initial signal - ie, it is out of phase. A microphonic tube can sound very phaser-like indeed.
Try gently tapping each valve with a chopstick or the eraser end of a pencil and see if any sound comes through the speakers. If it does, replace that valve.
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Thanks guys. Thats kinda what I figured was happening, I just couldn't think of the word.
How do I get the built in amp controls to work.
No instructions, but looks lime battery case plugs in, but batteries wont fit it.