Valve Testing and Replacement
Amplifier valves (or tubes) age with use. Under an average gigging guitarist’s use they will last between 6 months and a year before they start to tire. Worn out valves exhibit the any of the following symptoms:
- Hissing, popping or humming noises
- Loss of volume and erratic volume level
- Muffled top end frequencies
- Muddy bottom end
- No sound from the speaker at all
- Repeatedly blowing a fuse
Sometimes worn valves can become microphonic. When you tap them with a biro pen they produce a noticeable ringing rattle from the speaker. This can produce a hum or squeal from the speaker as the sound feeds back from speaker to the microphonic valve.
Valve Amplifier Biasing
The output valve bias current is the steady current flowing through each of the output valves when there is no input signal. It’s purpose is to make the valves work in the most undistorted portion of their range (if that is what is desired). No two valves that come off the production line are ever exactly the same. Output valves are tested and matched in pairs and quads so that their bias currents and other behaviours are roughly the same. When a new set of valves is plugged into an amp, it is usually necessary to adjust the bias current setting. As your valves age, it may need to be readjusted. Some valve stage amps do not have a bias current adjuster. The manufacturers supply their own valves that are selected to work in their amps (Mesa Boogie for example). Most small practice amps under 30 Watts have automatic bias adjustment built in to the design.
Valve Amplifier Care
Valve amps usually have two power switches, a main power switch and a standby switch. The correct way to power up the amp is firstly to make sure the standby switch is not already on then turn on the power switch, wait 30 seconds then turn on the standby switch. The power switch engages the the lovely warm glow of the valve heaters and on bigger amps more importantly, the negative bias voltage. A pre-applied bias voltage makes sure that when the main high tension (HT or B+) supply comes on, the valves are not going to conduct much more current than they can cope with and be permanently damaged or at least, blow the HT fuse. During a beer break, turning off the standby switch prolongs the life of your valves and also saves electricity. To turn off the amp, Turn off the standby first, wait 30 seconds then turn off he main power.
It is always a good idea to let an amplifier cool down for 10 or twenty minutes before manhandling it back into the van.
Castors on a valve amp are not a good idea. Dragging your second favourite piece of equipment (second only to your genitals) over a rough floor shakes and rattles your tubes. Better to carry by hand or get a pallet truck with some fat bouncy tyres.