Checking for Microphonic Tubes

Checking for Microphonic Tubes

This How-To article covers all Rivera models

As tubes wear, some problems can come up. One of the most common symptoms is a ringing sound. This is usually due to the tube becoming microphonic. Like its name suggests, a microphonic tube is picking up sound and amplifying it.

With the amp unplugged and cooled off, examine the tubes to make sure they’re in tight and straight. Never twist them! Gently grasp the tube and wiggle it into place. Because some of the tubes are inside of metal sleeves, you will have to remove the sleeves to check them for microphonics. Grasp the sleeve with your fingers and depress it (it’s spring-loaded) and turn to the left (counterclockwise). Now pull it off; this may require a little wiggling action.

Preamp tubes

If you hear ringing (a feedback-like high-pitched sound) in your amp, it’s probably coming from a preamp tube. Here’s a procedure to find which tube is giving you trouble.

With nothing plugged into either the High Gain or Low Gain inputs, and the Master controls turned down to 5 or below, turn the amp on.

Turn up the Volume on Channel 1 and Channel 2, as well as the Reverb knob. Now use the tip of a pencil to gently tap the end of each of the small tubes (preamp tubes) and listen for sustained ringing. If trouble isn’t apparent right away, turn up the Volume and Master knobs and keep tapping until you find the tube that rings (or squeals).

Turn off the amp, and allow the tubes to cool. Now pull out the troublesome tube and replace it with one of the same value. That is, if you’re pulling out a 12AX7, replace it with a 12AX7.

Make sure that the tube is oriented correctly when pulling it out or putting it back in. If you look at the end of the tube and the socket, you’ll notice that the nine pins are arranged in an incomplete circle. Always make sure the pins are aligned correctly. Never force a tube into its socket!

Remember to put the sleeve back on after you check or replace a tube.

Power Amp Tubes

Like preamp tubes, power amp tubes can go bad or wear out. Your amp likely has two or even four power amp tubes, and if one goes bad, they should probably all be replaced. This assures optimum output and tone.

If a power tube shorts out, the Mains Fuse may be blown. Remove power from the amp and replace the fuse. If this doesn’t remedy the problem, an internal fuse may be blown. Refer this to a qualified repairperson. If there’s something wrong with the sound and it can’t be traced to the preamp tubes, it may be a defective power tube. Try the following:

  1. Remove the power tubes. Remember the way the eight pins are arranged, and note that the center hole on the socket has a keyway that matches the center post on the tube.
  2. Replace one tube. Turn the amp on again. If the fuse blows (or the tube glows cherry red, indicating an internal short), you’ve found the bad tube. Turn off the amp immediately. If the fuse doesn’t blow, replace the second tube and turn the amp on again. If the fuse blows, or if the tube glows cherry red, the tube is bad. Turn off the amp. Continue this procedure until you have found the bad tube. Before changing any tubes, let them cool off by shutting down the amp and waiting a while.
  3. When the tubes have cooled, remove them. Replace the power amp tubes with new ones. Throw away the bad one, but don’t throw away the good tubes–save them as spares!

It’s a good idea to write down the date and which tubes you change. The best place, of course, is in your manual, but you may want to write it on a piece of white tape and stick it on the back of your amp or in the bottom of its enclosure. Just make sure you can read your own writing and remember where you put the note.