Your amp might need one or more new tubes if its volume, tone, or punch has gotten
You might also need a new tube if you occasionally hear feedback
from your amp for no reason. In this case, access the
tubes and tap on them one at a time. You might
quickly find the problem tube.
If scratchy noises accompany the wiggling of a tube, its pins or the tube socket
may need cleaning or tightening.
Replacing all the tubes can be costly
and even invite new problems. Most often, only a few tubes
might need replacing.
Power & Rectifier Tubes
Power and rectifier tubes are the bigger ones.
They run hot, do all the heavy lifting, and wear out faster. An overall muddiness might be
due to weak power tubes or a weak rectifier tube.
If there are two power tubes, you could install a new, matched pair.
If there are four, you could install a matched quad. However, there is
nothing wrong with replacing just two of the four, or even just one.
Power tubes include types 6L6, EL34, 6V6, 5881, and 6BQ5.
A rectifier tube is used in some amps and it can also become weak and
degrade the power of your amp over time.
Rectifier tubes include the 5AR4/GZ34, the 5U4, and the 5Y3.
Preamp tubes are the smaller ones. They run cooler and last much longer.
The preamp tubes that are most likely to cause problems are:
The INPUT or GAIN STAGE tube:
The input tube is usually the farthest one from the bigger tubes and is
the most likely to cause problems. Tap on the tube and listen for
any strange noises.
The input tube is usually a 12AX7 type (a.k.a. 7025 or ECC83).
The PHASE INVERTER (P. I.) or a REVERB DRIVER tube:
The phase-inverter tube is usually the tube closest to the bigger tubes.
An amp with spring reverb usually has a reverb-driver tube near the reverb cables.
Phase-inverter and driver tubes are usually a 12AT7 type (ECC81) but they could also be a
Preamp tubes can become microphonic, causing howl or squeal. A
microphonic preamp tube can often be singled
out by tapping on each tube, one by one.
You should also wiggle all the preamp tubes. If there's a
crackling noise when you wiggle a tube, the tube pins and
socket probably need cleaning. You can use a spray contact
cleaner to clean them.
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 have the bias
setting checked when replacing power tubes. But it's not
dangerous to install same-type replacement tubes without re-biasing.
Newly purchased used amps are good candidates for
inspection at every level. And if you install a different, but
compatible tube type, the bias will likely need readjustment.