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Post subject: hotrod deville honk
Posted: Mon Jan 06, 2014 6:30 am
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I have a hotrod deville its pretty new was playing it and all at once just start making this loud noise and ever since it does it every time i turn it on its fine until i hit just one note chord whatever or if just bump the amp i dont have the foggiest. Was wondering if its a tube that maybe got to hot or something worse.


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Post subject: Re: hotrod deville honk
Posted: Mon Jan 06, 2014 6:37 am
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Hi hotrod1963,

It sure sounds like you have a microphonic tube. A microphonic tube is one that will produce sound on its own, kind of self-oscillating, and it only takes a bit of vibration to get it started. This is opposed to a rattling tube which will make a rattling/buzzing noise when played, sometimes only when certain notes/chords are played.

If you have a spare 12AX7, try swapping it into each of the three preamp/PI positions one at a time until the suspect tube is found. V1 is typically the most sensitive position, the first one on the right when looking at the back of the amp. It could be an output tube, but more likely preamp.

You can also try grasping each tube when the amp makes this sound and see if the noise subsides, being careful of the hot output tubes, maybe use a pot-holder, oven-mitt or glove.

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Post subject: Re: hotrod deville honk
Posted: Mon Jan 06, 2014 10:06 am
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shimmilou wrote:
Hi hotrod1963,

It sure sounds like you have a microphonic tube. A microphonic tube is one that will produce sound on its own, kind of self-oscillating, and it only takes a bit of vibration to get it started. This is opposed to a rattling tube which will make a rattling/buzzing noise when played, sometimes only when certain notes/chords are played.

If you have a spare 12AX7, try swapping it into each of the three preamp/PI positions one at a time until the suspect tube is found. V1 is typically the most sensitive position, the first one on the right when looking at the back of the amp. It could be an output tube, but more likely preamp.

You can also try grasping each tube when the amp makes this sound and see if the noise subsides, being careful of the hot output tubes, maybe use a pot-holder, oven-mitt or glove.


+1

If no microphonic tubes are found, I'd start looking for a marginal solder joint -- it's a recurring problem with many newer amps.

Arjay

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