Re-tubing an Amplifier
TO RE- OR NOT TO RE-
You might need new tubes if your amp's volume, tone, or punch has gotten
You might also need a new tube if you occasionally hear strange noises
from your rig. In this case, gain access to the
tubes and ping them, one at a time, with your finger. You might
quickly find your problem tube.
If scratchy noises accompany the wiggling of a tube, then the tube sockets
probably need cleaning or contact tensioning.
SHOULD I REPLACE ALL THE TUBES?
Replacing all the tubes can be costly
and even invite new problems. Most often, just a portion of the tubes
POWER tubes are the big ones.
run hot and do all the heavy lifting. An overall muddiness might be
due to weak power tubes or a weak rectifier tube.
If there are two power tubes, buy a matched pair.
If there are four, buy a matched quad.
A rectifier tube is found in some amps and is also
a good candidate for replacement. They're about the same size as the
power tubes. They include the 5AR4-GZ34, the 5U4, and the 5Y3.
PREAMP tubes are the little ones. They run cooler and last much longer.
The two most likely to degrade your sound are:
The input / gain-stage tube:
This little tube is usually the farthest from the big ones and is
the most likely to cause intermittent noises. Tap on it to see
This tube is usually a 12AX7 (a.k.a. 7025 or ECC83).
The driver / phase-inverter tube:
This little tube is usually the closest to the big tubes.
It's often a 12AT7 (ECC81) but could also be a
Any little tube can be microphonic (howl), especially with the
volume turned up. This tube defect, too, can often be isolated just by
pinging the tube with your finger.
WHAT ABOUT BIASING?
Hidden inside some amplifiers is a control for adjusting the power tubes'
bias or balance. Not all amps have this control.
If your amp's bias is adjustable, it's a good idea to check the setting
when replacing power tubes. But it's not dangerous to swap tubes
without rebiasing. After all, it's the tubes that wear out, not the
Newly purchased used amps are good candidates for
inspection at every level. And whenever you install alternate tube types, the bias is
likely to need resetting.
PULLING THE TUBES
Some power tubes have an over-the-top style coiled spring retainer. You lift the retainer off the top of the tube before pulling the tube.
Some power tubes have a leaf-spring clamp with teeth that grab the
base of the tube. Push down on the wings of the clamp while
removing the tube. Tighten the wings by pulling up on them before inserting
a new tube.
Always grab large tubes by their plastic base, not by the
glass, and wiggle the tube as you pull it out of its socket.
Preamp tubes are often covered by metal shields that have a twist lock.
Push down on the shield and turn it counterclockwise to remove it.
Then rock the tube out of its socket.
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