HELP!!! Mark III Squealing!!!

[path09en]

I was playing my Mark III for about an hour and a half, when all of a sudden it just emanated an extremely loud high-pitched continuous 'SQUEAL'!!! I quickly switched it to 'standby' mode. When I switched it back to 'on' I can only hear about 10% of the original volume!?!? I turned it off completely for about one minute then turned it on again and it was fine!?!?! I'm not sure what is going on. Has this happened to anyone else? Oh BTW, I had just changed out the China 12AX7's in the preamp to Tung Sol 12AX7's. Could it be a bad tube(s)? Could it be the other electronics? The speaker? Over-heating issues? Any help would be grately appreciated!

 

[Shep]

Tubes...pre amp most likely...

 

[Tuna141]

Yup, I concur...most likely one of your preamp tubes has gone microphonic. Find & replace the offending tube and you'll be good to go.

 

[path09en]

Yup, I concur...most likely one of your preamp tubes has gone microphonic. Find & replace the offending tube and you'll be good to go.

How do I find the offending culprit? I do not have a tube tester, so I guess I test by elimination? Do I tap and listen the preamp tubes? Since the problem is intermittent, will I have to test hours on end? Is there a 'basic' definition of microphonic?

Thanks for all your help!

 

[path09en]

Yup, I concur...most likely one of your preamp tubes has gone microphonic. Find & replace the offending tube and you'll be good to go.

You write that one of my preamp tubes has gone microphonic. How do tubes 'just' go microphonic? and Is it reversible? or does it need to be replaced?

Thanks again!

 

[stokes]

Try tapping each preamp tube with a pencil or flick it with your finger,a microphonic tube will ring excessively.All preamp tubes are microphonic to some degree.Some will seem fine and then for no apparent reason start ringing and feeding back,it is usually due to loose elements in the tube,and the vibrations in the amp cause it to occur, sometimes quite soon after installing a new tube that tested fine by the vendor.Best bet is to replace it.Sometimes you can lessen the effect somewhat by putting some kind of shrink wrap tubing on it to dampen the effect,but it is best to replace the tube.

 

[X-1]

Try tapping each preamp tube with a pencil or flick it with your finger,a microphonic tube will ring excessively.All preamp tubes are microphonic to some degree.Some will seem fine and then for no apparent reason start ringing and feeding back,it is usually due to loose elements in the tube,and the vibrations in the amp cause it to occur, sometimes quite soon after installing a new tube that tested fine by the vendor.Best bet is to replace it.Sometimes you can lessen the effect somewhat by putting some kind of shrink wrap tubing on it to dampen the effect,but it is best to replace the tube.

That would also be my suggestion. It is an easy first start.

 

[Boogiewan Kenobi]

Most likely would be the tube in V1

Swap one tube at a time until it stops.

BWK

 

[path09en]

Thank you ALL for your support! I swapped out the preamp tube in V1 and the squealing has ceased!!! I played for more than 2 hours straight without a single problem! Now, what do I do with the microphonic tube? Chuck it? Keep it for use in other slots (V2, V3, V4, V5)? I know I should probably just throw it out, but should I try to get a refund?

Thanks!

 

[Boogiewan Kenobi]

Sell it on eBay - lots of folks who want a NOS Squealer!!! :lol:

BWK