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BRENT JESSEE RECORDING & SUPPLY, INC.

www.audiotubes.com

MICROPHONICS, NOISES, AND TUBES THAT GO TAP IN THE NIGHT......




Living with vacuum tubes can be a sublime experience. If you have never heard vacuum tubes in a good audio hi-fi system, you have a treat in store for you. You will never go back to solid state hi-fi gear. However, there can be some growing pains and break in issues with new old stock vacuum tubes. Hopefully we can help you understand and resolve the issues here on this page.


New old stock tubes, in usually rare instances, can be either noisy, microphonic, or even both. It is not always a death sentance for the tube, but can be frustrating if you hear it, and to a tube newbie, hard to understand what level of these issues may actually be normal. To start with, we need to identify what each problem is and the correct way to diagnose it.

Microphonics is the phenomenon where components in a vacuum tube are extremely sensitive to vibrations they pick up from the air, the chassis of the equipment or by tapping on them, and transforming the mechanical vibrations to electronic signals. The term comes from the microphone, a device that is designed to deliberately convert airborne mechanical vibrations (sound) into an electrical signal. In a microphonic tube, the elements sensitive to vibration can actually move with the mechanical vibration, changing the distance between two or more elements in the tube, and thus changing the electrical charge with the movement, much like a capacitor microphone. This unwanted electrical signal gets amplified in the tube and in the rest of the audio chain. Very microphonic tubes can make loud clunking noises when controls are turned on the tube equipment, and also pick up music and other sounds from the hi-fi system loudspeakers. This acoustic energy is converted to electrical energy. In the case of a microphonic tube picking up the same music it is amplifying, it causes a problem known as "sonic image smear". The acoustic energy (the music) is picked up fron the air milliseconds after it has already passed through the tube, and once again the same sound passes through the tube now as microphonic distortion. Since it is delayed a few milliseconds from the original signal, it becomes out of phase with the original signal. This can cause a collapse of the acoustic image and a distortion of the sonic sound stage. Take note that older long plate 9 pin tubes, and military type 6SN7 or 6SL7 tubes often have some degree of microphonics. These high end tubes sound so good that a small amount of microphonis does not degrade the superior sound of these rare tubes. Once in a great while a new old stock tube can be microphonic at first but will disappear after the tube has burned in for 48 hours or so.


Tube Noise is even easier to recognize. It is quite simply one or more of the following annoying noises: hissing, whooshing, humming, crackle, pops, or a rumble like distant thunder. Not what you want to hear at the same level as Beethoven's Ninth Symphony. This is usually caused by a flake of cathode material that has come loose and lodged in the grid of the tube. If it touches the plate or other internal parts of the tube it can cause the above noises. It may not be present until the tube warms up and the internal parts expand with the heat. Sometimes the tube can be very noisy when cold, and then quiet down as the parts heat up and break the intermittent short. Plate shorts and other internal leakage can also cause noise. Vintage small preamp tubes are more often subject to this problem, and vibration during shipment of these tubes can cause the problem. Sometimes noise is just a symptom of the awkward period of time in a new old stock tube life called "burn in" and may vanish after 48-100 hours of use.


How NOT to confirm there is a problem: DO NOT turn the volume all the way up and stick your head in one of the speakers! No only will you look completely stupid, but you WILL hear something! Every tube system has a noise floor, and often the tweeters of your speakers will have a soft hiss with no signal present. This is normal! DO NOT turn the volume all the way up and start beating on the tubes in your system with your head by the speaker! You will remove all doubt about your ignorance to an outside observer, and once again you WILL hear something. Any tube if treated in this manner will amplify the taps and will be heard if the volume is all the way up! If you call me and tell me you confirmed noise or microphonics by one of the above methods, I'm gonna read you the riot act!!


How to confirm there is a problem without looking like an idiot: For either problem, start by playing your favorite music source, sit in your favorite chair and adjust the volume level to where you normally listen, not any louder. This applies to headphone listening as well. Hear nothing but the music? Good! Now stop the music and sit in the chair and just listen. DON'T touch the volume! Hear nothing? Good! For noise issues you are good to go. For microphonics, do one more thing: reach out and lightly tap the front of your amp or preamp. Hear nothing through the speakers? Good! For microphonics issues you are also good to go!


Now, what if I heard something? For noise, at normal listening volume at your listening position you will clearly hear hiss, pops, crackles, rumble, whistles, or any combination of the above. For microphonics, at normal listening volume you will clearly hear tapping through the speakers as you tap on the unit. In the case of noise, you may be able to fix it yourself. First, locate which tube is noisy. You may have to substitute one tube at a time and try the unit out until the noise vanishes, indicating the last tube you took out is the one. If you don't have spare tubes, just swap the tubes one at a time from the left channel to the right channel and listen after each tube swap. The noise will follow the noisy tube to the other channel. Take that tube and tap it on its side firmly on a table top. Then, hold it upright and tap it firmly with a wooden pencil. This will nearly always knock the offending flake of material out of the grid where it will cause no more noise. Reseat the tube and give it a good 12 to 48 hours of burn in time and the noise should be gone for good. I know it sounds extreme but in at least 85% of noise cases, this resolves the problem. Some audiophiles have said tapping a tube can CAUSE it to become microphonic. I totally disagree. I have tried this trick for 25 years with mostly successful outcomes, and never once has a tube become microphonic after tapping it.


Fixing microphonics can sometimes be more difficult. The problem may just be stresses in the grid of the tube. The same tapping trick outlined above may resolve the issue, especially after 48 hours or more of burn in. Should microphonics persist, there are good tube dampers widely available that can reduce or eliminate microphonics. Our own Brass Ring and Copper Stopper dampers are two that work well on eliminating this problem. There are other brands out there that may be more suited to your individual microphonic issue. One will certainly be right for your tube and resolve the issue.


The bottom line here is that you want to hear music, not noise. If noise or microphonics are as loud as the music, try some of the fixes on this page. If you take the "non-idiot" confirmation test and hear little to nothing, relax and enjoy the music. Don't get upset over normal background noise if you happen to listen at high volume, I would be more concerned about my hearing health in that case! If you have questions about the subject of noise and microphonics not answered here, feel free to call or email us.





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