VALVE GUITAR AMPLIFIERS

In a valve amp the large valves (known as the output valves) are responsible for providing power to the speakers. They are in need of replacement much more often than the smaller preamp valves and....wouldn't you know it, they cost a fair bit too. Some common misconceptions abound with output valves and I would like to discuss some of them here:

A blue glow around the valve is normal, different valve types are more prone to it than others but there is nothing wrong. 

 

If the large metal plates plates start glowing a bright orange/red colour, the valve or valves are drawing to large a current and will soon cause a fuse to blow. If you see this glow turn your amp off, sometimes amps do not have fuses, or have the wrong fuse fitted and this excessive current draw can kill a power transformer (expensive.)

 

A small red/orange glow at the valve base and at the top is quite normal. This is the filament which heats up the valve cathode for normal operation. Although this glow is normal it does not neccessarily guarantee normal operation.

 

The regular replacement of the output valves is normal in valve amps with regular use. Just when to change them can vary with the type of use the amp gets and how often it is used. Most players should change their output valves once a year if they play loud and fairly often. As the output valves wear out, both the bass and treble responses of the amp will begin to suffer. This power loss from worn-out valves is not always noticeable because it occurs gradually over time and because power level differences are not easy to pick up. Regular power valve replacement will guarantee consistent and reliable performance from your Amp

 

When output valves are replaced it is advisable to check all the voltages that are related to the output valves (Anode, Screen grid and Grid (BIAS).

Never use guitar cable as speaker cable 

This is a problem that is not really an obvious one. Everybody does this at some time or another without realizing the potential for damage. Another way of describing guitar cable is by its more appropriate name, "Shielded Cable."

Typical shielded cable has a single insulated wire in its centre completely surrounded by a flexible metal braid. This braid is the cable's "ground" and shields the internal wire (called the "hot" lead) from external interference (usually 50-60 cycle hum.) While this is ideal for reducing noise for guitar inputs, this construction gives rise to problems when used in an output circuit. Owing to the large amount of conductive surface area provided by the shield, it follows that this type of cable has a large amount of internal capacitance. Given this fact, the longer the cable, the larger the capacitance. A typical 20 foot long guitar lead can add sufficient unwanted capacitance to the output circuit to interfere with the performance of the amplifier.

Since this capacitance reacts with the impedance of the speaker, an impedance mismatch occurs and the potential for circuit damage is present. An additional problems exists. In some shielded cable the "hot" lead is a very narrow gauge of wire. Speakers operate at high current levels and since this small wire is designed to operate with mic level signals (0.025 volts is typical) it is not capable of handling speaker level ( around 30-50 volts in a typical 100 watt amplifier.) In some extreme cases, the insulation on the "hot" lead in the cable literally gets hot (heat), the insulation melts and the "hot" lead shorts to the shield, effectively shorting the output of the amplifier. Amplifiers were not designed to run into a short, and damage will most likely occur. Use quality speaker cable that is at least 2-3mm thick and keep the length as short as possible, just long enough to reach from the amp to the speaker box.

Never use speaker cable as guitar cable

Fortunately, if you do this, no damage will occur. Speaker cables are not "shielded" and so are not constructed to block out the external interference. This is just a performance tip. If you plug your guitar into your amplifier using a speaker cable instead of a shielded cable you will hear all kinds of interference.

Do not exceed fuse ratings

When a fuse blows, your amp (or the fuse) is trying to tell you something. A blown fuse means you have a bad amp, not a bad fuse. A fuse is a protection circuit designed to interrupt the flow of current when the fuses current rating has been exceeded.

Biasing your Amp

The bias control on an amp is much like the idle control on an engine. That is to say, there is an optimum point of bias for an amplifier that allows for good sound and maximum valve life. This optimum bias point will be different with a new set of output valves no matter which valves you buy, so the bias should always be checked when changing output valves. A correctly biased amp will sound clean and tight at low to medium levels. When pushed to maximum, the amp will produce an even harmonic distortion - musical distortion if you will. Expect an overall tonal improvement along with better balance and sustain.

The Driver Valve 

This operates in conjunction with the output valves to form the power-amplifier section of your valve amplifier. Even the best power valves will sound bad with a weak driver valve, as this is the valve that controls the output valves. If it cannot control the output valves, then the amp cannot sound its best. This will show up particularly at higher power playing, or when playing the amp distorted. In most amps the driver valve is the smaller valve (12AT7, 12AU7, 7025 or 12AX7), which is next to the large output valves.