[SJ David]

The T8 tube preamp is effectively my first piece of tube equipment.  I have noticed that the unit is particularly microphonic. If I rotate the input knob, tap on the chassis or even tap on the shelf the unit is on the sound is picked up and sent clearly through to the speakers. Same effect as tapping on a turntable. I have never heard of this to be a characteristic of tube pre amps. Is this typical?

Thanks, David.

[martyo]

The T8 tube preamp is effectively my first piece of tube equipment.  I have noticed that the unit is particularly microphonic. If I rotate the input knob, tap on the chassis or even tap on the shelf the unit is on the sound is picked up and sent clearly through to the speakers. Same effect as tapping on a turntable. I have never heard of this to be a characteristic of tube pre amps. Is this typical?

Thanks, David.

Hi David. I also have a T8 and have experienced this with a highly microphonic tube. And of course our gear should be isolated.

[avahifi]

SJDavid, I suspect you have a microphonic tube that should be replaced.

In general the EH 6CG7 tubes we supply with the T8+ (JJ12AT7 tubes with the older T8) should be very quiet, but tubes being tubes, once in a while one of them will go noisy.

This is why tubes are socketed, not hard wired into the circuits.

Regards,

Frank Van Alstine

[underdawg]

True tubes can go microphonic at any time and still test new on testers. The only real way in my experience of 30 years including being trained on tubes in the Navy, is to try them in an actual circuit. Their are threads on inexpensive ways to shield tubes for microphonic including grounding the sheilds.

[SJ David]

Understood, guys. Thanks for taking the time to respond.

David

[trebejo]

Try this: remove the chassis cover, then turn it on, then tap the chassis and see if the same thing happens.

I get "microphonic" responses with the chassis cover on, but not off... I think the tube is *just* short enough not to touch the cover, but when it shakes...

I solved this by cutting up a few small pieces of cardboard and placing them between the chassis and the cover to increase the gap by a few millimeters, which solved the problem. A more elegant solution might be some washers.

Welcome to the tube life, man, and remember the universal maxim--a beautiful companion requires more attention...

[SJ David]

Try this: remove the chassis cover, then turn it on, then tap the chassis and see if the same thing happens.

I get "microphonic" responses with the chassis cover on, but not off... I think the tube is *just* short enough not to touch the cover, but when it shakes...

I solved this by cutting up a few small pieces of cardboard and placing them between the chassis and the cover to increase the gap by a few millimeters, which solved the problem. A more elegant solution might be some washers.

Welcome to the tube life, man, and remember the universal maxim--a beautiful companion requires more attention...

Thanks, Trejebo. Some further testing to mention.

I have 5 tubes, all Electro Harmonix, two new with the T8 and three more all new. One of the originals was determined as likely failing with some ringing and spitting at low levels. Of the remaining four I found another that rang a bit per the original email. That is when I tapped the chassis or shelf, or turned the input knob. This is with the cover on or off. There is also somewhat more-than-usual low level hash with this tube (I have horns for upper drivers). I can engage the mute and the low level noise disappears at the seating position.

The other three are much quieter. There is just a bit of the ringing when tapping the chasis and the noise floor level is less. I can just hear the hiss drop out when I hit the mute at my seat. IF the house is dead quiet, that is.

So it comes down to the QC with the maker of the tubes, or tubes in general. Two rejects out of five is not so good. In the spirit of your reply at the others I will take this in stride and stock up on some (good) spares.

David

[SJ David]

Posting error.