Tung-Sol.com:: FAQs I have heard the word "Microphonic" used to describe a tube. What does it mean?

FAQs

Question:
I have heard the word "Microphonic" used to describe a tube. What does it mean?
Answer:
Microphonics describes the phenomenon where a tube transforms mechanical vibrations into an undesired signal (noise or feedback). Due to the mechanical construction all tubes exhibit some inherent microphonic tendencies. The most critical application regarding microphonic tubes are high gain preamp stages that are close to the input. This is especially true in combo amplifiers where the chassis is mounted in the same cabinet with the speaker. The preamp stage close to the input of the amplifier has more stages after it to amplify the signal plus any noise and microphonics. Most technicians will handpick the first preamp tube for the least noise and microphonics. A tube that is not suitable for the first preamp stage can many times be used in a later preamp stage in the signal chain. As the signal is amplified, the increasing level of the signal will override the microphonic tendencies. Since there are less stages of amplification following these stages the microphonics will be amplified less.

You can check your tubes for microphonics by performing this simple test:

1. Remove any back cover that may be on the amplifier to gain access to the tubes and any tube shields on the preamp tubes.

2. Plug a guitar (with the volume control at minimum) or a dummy plug into the input of the amplifier.

3. Turn up all controls on the amplifier.

4. Turn on the amplifier.

5. Tap each tube with the eraser end of a pencil. If you get feedback or a bell-like ringing sound, the tube is microphonic. If you hear a dull thump, the tube should be fine and perfectly usable. If you find a microphonic preamp tube, you can many times switch it with another preamp tube of the same type that is farther down the signal path in the amplifier to eliminate the problem. If this does not solve the problem, replace the microphonic tube with a new tube.

6. When the microphonics problem is cured, remove power from the amplifier then replace any tube shields and covers that were removed to gain access to the tubes.

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