6SN7s and Microphonics

My SP-14 rings like a bell. I tap the case and it rings through the speakers. It comes from all the tubes it seems. I can tap each tube lightly and it rings through the speakers. Should I look for another set of tubes? Do the devices I've seen like silicon rings, a springy loop with wheels that ride on the tube etc work to any degree? Is this common for 6SN7s? I have Sylvania 6SN7 GTBs from the late 1940s

Yeah, I would recommend new tubes.

Mr C wrote:My SP-14 rings like a bell.  I tap the case and it rings through the speakers.  It comes from all the tubes it seems.  I can tap each tube lightly and it rings through the speakers.  Should I look for another set of tubes?  Do the devices I've seen like silicon rings, a springy loop with wheels that ride on the tube etc work to any degree?  Is this common for 6SN7s?  I have Sylvania 6SN7 GTBs from the late 1940s

Tubes might acquire microphonics as time pass. One source is repeated heating and
cooling that might move internals through the micas. Also vibrations can cause
slipping agains micas. In both cases the tubes inner elements will get loose and can
move by even the smallest vibrations.
These is no cure but new tubes.

I find that microphonic tubes are usually only noticeable in the gain stages, so I suggest that before you replace the gain stage tubes you first simply swap the gain stage tubes with the buffer stage ones and see if it makes an improvement. They may be less microphonic.

BTW, looking at the SP14 PCB, the top tubes are one channel and the ones below are the other channel. The pair on the left are the gain stage and the pair to the right of them are the buffer stage and the single tube on the right is the rectifier.

My ST-70 Series II sits close to my left speaker. When the music was really pumping, it would make my tubes ring. Putting soft, stretchy, silicone tube rings on all my tubes completely stopped the ringing. If you look around you can find these o-rings for just a few bucks on eBay. Just measure your tube's diameter and buy them a few millimeters smaller. Might be worth a try if your tubes are still in good shape and costly.

Per Mijohn's advise I swapped the tubes left to right on the same channel and the microphonic was almost totally eliminated. I was hearing it out of the right channel. I swapped the left and right channel tubes and it followed the tubes, now coming out of the right channel. So I identified the offending tube. As long as that tube is in the buffer stage socket it's much better. Boy, when its in the gain position no matter where you tap on the amp it rings. Any of the tubes, on the PCB, on the chassis, it rings. Then it will feedback and fade away. Thats too bad. these where the selected nearly NOS Sylvania chrome dome tubes upgrade from Roy. Not a whole lot of hours on them either. So is it okay to run this tube till I get replacements? What are your favorite 6SN7s?

If that microphonic tube is behaving itself in the buffer position then leave it there, apart from being microphonic in a high gain application it's probably fine in a buffer stage and not electrically faulty.

Mr C wrote:Per Mijohn's advise I swapped the tubes left to right on the same channel and the microphonic was almost totally eliminated.  I was hearing it out of the right channel.  I swapped the left and right channel tubes and it followed the tubes, now coming out of the right channel.  So I identified the offending tube.  As long as that tube is in the buffer stage socket it's much better.  Boy, when its in the gain position no matter where you tap on the amp it rings. Any of the tubes, on the PCB, on the chassis, it rings.  Then it will feedback and fade away.  Thats too bad.  these where the selected nearly NOS Sylvania chrome dome tubes upgrade from Roy.  Not a whole lot of hours on them either.  So is it okay to run this tube till I get replacements?  What are your favorite 6SN7s?

Send the bad one back for a refund or replacement. I've had A LOT of trouble dealing with that guy. Not trustworthy.

I have a pair of those Sylvania tubes in a tube buffer. Very nice, warm tubes but I'd say Raytheon are a bit nicer.

Send the bad one back for a refund or replacement. I've had A LOT of trouble dealing with that guy. Not trustworthy.


Whoa, careful, Roy will have these posts removed.

I have a set of Tung-Sols ordered. So if I under stand correctly, no need for matching between the gain and buffer tubes. As long as both gain tubes are matched and both buffer tubes are matched.