Tube Performance.
Tube amps tend to have a warm, round sound that will break-up into a pleasing distortion when played loudly or pushed to their limits and that's why guitar players almost universally favor vacuum tube amplifiers.
Unfortunately tube-performance slowly decreases over time.
So we need to monitor the quality of our tubes. This can be done by a amp tech or a decent tube tester.
The MaxiMatcher II and MaxiPreamp II are very good tube-testers used by LampenBakkenServies. They test the preamp tubes for gain, transconductance and noise. The power tubes are tested for plate current and transconductance.
Malfunctions:
Sometimes a tube-malfunction happens suddenly, and therefore its hardly to go unnoticed, so no need for a tube-tester here.
Below a list of relative easy to notice/find malfunctions.
You should see a orange/red glow from the filament when the amp is powered on, even in standby.
Pink or purple neon effects inside the tube due to small vacuum failure.
The condition of the getter. This is the greyish coating at the top or aside of a tube. When this color will change to pure white you have a big degassing inside or an external air-leak.
The tube becomes microphonic and very noisy. You will hear a rattle when tapping on the tube.
Total lost or low output power with lots of distortion, flabby sound.
Tone controls seem to have little effect. A very narrow frequency range, therefore bass and treble will be greatly reduced.
But over a longer period of time, a vacuum tube can degrade significantly and Its less obvious to notice that.
A tube can become gassy due to small air-leaks or internal degassing.
Also the coated surface of the cathode can wear-out (sputtering) resulting in a lower emission current to the plate.
When this emission current drops below 70% of his initial value, it will reduce the dynamics of the tube, the so called "transconductance" (Gm) and it will also reduce the Voltage Gain (mu), since Mu = Gm x Rp.
What is (Gm) Transconductance and (Mu) Gain of a vacuum tube?
Transconductance is the relationship between the output plate current and the input grid voltage .
You can find a basic explanation how a tube works in this video to help you understand this relationship.
Conductance is the opposite of resistance. Gm has the units of mhos, (amps per volt). Rp (plate resistance excl the plate load resistance) has the units of ohms, (volts per amp).
The value of tube transconductance for signals is the change in plate emission current (output) divide by the change in grid voltage that produced it (input).
This means that tubes with high Gm will reach saturation and cut-off (clipping) much more easily than tubes with a low transconductance.
Blues/jazz or clean players will probably prefer a tube with a lower transconductance.
Voltage Gain (Mu) is the product of Gm and Rp.
Its a ratio of how much bigger is the output voltage on the plate than the input voltage on the grid, and so has no units.
A Mu of 30 means that what goes in comes out 30 times bigger.
Also keep in mind that there is a difference in relevant measurements for triodes and pentodes?
Triodes are most widely used for voltage amplification in pre-amp stages and the important value here is voltage gain Mu.
Pentodes are mostley used in power-amp output stages. The value Gm is the most important here.
Keep your amps and their tubes in good shape.
Maybe it have/has lost its dynamics and sparkle.
Ik know i only scratched the surface here. there is a lot more to say about this topic.
I hope some tube amp gurus around the world will give their two cents :-).(or more).
Thx for reading.
Grtz,
Stefan from LBS.
