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tubes going microphonic always same channel???

I have been having problems with mullard 12AU7 57 sqaure getters going microphonic on me. It's always the same channel, the outside tube close to the chassis. Is it because the pseaker is close (2.5') to that tube (see my system to see how speakers are placed).

Never really noticed this problem until I put in the square getter pre 58 mullards. I love the sound of these tubes, they are in a Cary CD 303/300 CD player. I'll put the 58 halo's in or some amperex to see what they do, or what is causing this. Hate to waste good tubes.

The tubes aren't cheap and this is the thrird one to do this in the same socket. I have done some tube rolling, hope the socket isn't bad. I'm a electronic tech, and my dad is a old tube tech from way back. He says it's because they are old junk tubes..LOL Just odd it's always the same socket??

Is this just a problem with old used square plates?? Are they more sensitive?? You can really hear it when the CD drawer opens. Then they will start making noise. I pull them out when I hear them, as it means the tube is bad.

Any opinions or help would be great.

Scott
sgsoundnut
reb1208
518 posts
 
Here is your answer as to why. Check the filament and plate voltage supply to the problematic channel. The voltages are probably out of spec, ie too high. Secondly, based upon my ownership of the Cary 303/300, the circuit design allows microphinic tubes to be audible.

Good luck
sgsoundnut OP
17 posts
 
Thanks I'll have to check that. I wonder if I can find prints for the Cary.

Does each cannel have it's own supply then?

Do you still own the CD 303? If not what did you end up with? I'm real happy with sound of it.

Thanks
fredtroy
8 posts
 
Is the tube microphonic? Put it in another socket or component and problem followed the tube?
sgsoundnut OP
17 posts
 
Yes they are microphonic. After talking with my dad (an experienced tube electronic tech), he thinks it’s because the speakers are to close. He didn’t feel plate or filament voltage would cause this. It’s always the tube closest to the speaker. I did some research and it seems the long plate Mullards go microphonic in guitar amps real quick, especially the square getters.

I may try some tube dampeners. I agree with Reb1208’s statement on the Cary CD player. I’m going to line it with dyna-mat.

Right now I have some Amperex 7316’s in there. I must admit the midrange is unbelievable from them. I was surprised when I swapped in some mid 80’s CV4003 Mullards. The bass was awesome… The midrange sounds very nice too… I was impressed for a tube made in 1985.
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