Many valve amplifiers require you to set what's called the 'bias' which is simply another word for the voltage applied to the grid of the output valve. If the voltage is set too high, the valve will produce more power, but will run hot and fail prematurely. Too low and you’ll get less power and increased distortion and noise. In most valve amplifiers, bias is adjustable and usually able to be done by the owner. In this case, all the instructions will be provided in the standard owners’ manual. In cases where the information is not in the owners’ manual, you would first need to establish whether the bias is adjustable and, then if it is, obtain the service manual for the amplifier, and follow the instructions in the service manual. In both cases, advice will be provided as to what you should do if you cannot satisfactorily set the bias. (Usually, it involves replacing the valve. If this doesn’t work, something is seriously wrong that would require the attention of a trained technician.) You should note that some valve amplifiers have automatic bias circuits that constantly monitor the bias voltage and maintain it at the optimum position. Other amplifiers provide a fixed bias and are not readily adjustable (adjusting the bias would involve physically replacing a resistor). In both these cases, I would not recommend attempting to adjust bias of these amplifiers, although it is possible. In those cases where bias is adjustable only with the aid of a service manual, I still would not advise setting the bias yourself unless you have electrical qualifications, primarily because voltages inside a valve amplifier can reach several hundred volts and if you accidentally touch something you shouldn’t there is a very real likelihood of being electrocuted, possibly fatally. You will find plenty of literature on valve amplifier bias in your local library, and a quick search using Google pulled up hundreds of links to valve amplifier manufacturers and valve amp service sites. |