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jhewes

Scott 299C ringing when tapped

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Hello all,

 

I have a restored Scott 299C tube amp.

 

I have noticed that, when phono is selected as the input, there is a ringing sound when I tap on the transformers, or on the chassis. It's a low toned, short-lived ringing sound coming from the speakers. The unit is reacting to the vibrations when I tap it. 

 

This does not happen on any of the high-level inputs (tuner, extra).

 

I'm concerned that, when the music gets a little loud, that vibrations can make their way in and influence the sound.

 

Is this normal?

 

Any insights appreciated. 

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I'd say you have a microphonic tube in your phono section just guessing.  Someone correct me if I'm wrong but I think those are the pair of 12ax7's on the far left looking from the front.

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Muel is most likely correct. What brand are the two far left 12AX7's and is this noise in both channels? Try tapping the tops of the two far left 12AX7's directly you might find its just one tube.

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If tapping the chassis makes the noise then tapping the tubes will most likely also make the noise (even if it's not them) and might not be the best method of diagnosis.

 

 

I suggest swapping the two phono 12AX7's with two other 12AX7's and see if the noise changes or goes away.

 

 

Ceramic capacitors have piezo electric properties, if changing the tubes around doesn't help you or tell you anything you may need to poke some of the ceramic caps with a chopstick.

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Joe, noticing some microphonics when tapping the chassis or tubes themselves is not necessarily a problem.  The question is whether you notice anything not sounding right when listening.  Given the weight of the Scott you would need an awful lot of acoustic coupling between the cabinet and the sound field of the speakers (whether through the air, floor, rack, etc.) to make the microphonic tubes "sing."  And, the shields on the tubes also offer some degree of mechanical stability which would reduce any vibrational effects.  So, if all sounds well when you spin records, I would leave it alone.

 

Maynard

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Hi all,

 

I haven't forgotten about you - just back from vacation.

 

Maynard - why would I hear the ringing just on the phono input? Are the 12AX7's part of the phono stage?

 

I'll do a bit more tapping over the next few days and report back. Many thanks for all the replies. 

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The two far left 12ax7's are for the phono stage.  One of those might be causing it.  Do you have any spares?

 I wouldn't keep tapping too much.

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12 hours ago, jhewes said:

Maynard - why would I hear the ringing just on the phono input? Are the 12AX7's part of the phono stage?

 

I'm not Maynard, but will add that in general the phono input should be higher gain than the others, so more prone to amplification of vibration.   

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The phono stage feeds into the line stage which goes into the phase invertor driver stage which feeds the power tubes. 

 Probably a slightly microphonic tube, could be a capacitor. I have even encountered microphonic wire. 

Put some heat shrink tubing on the preamp tubes. It will damping the ringing a bit. Most tube phono stages have a bit of microphonics. 

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