More biased than before

Everyone talks about it. But do we really know what we're talking about? I'll try to be clear about it.

- Biasing is done to adjust the current that flows through the valve.

- Bias is a negative voltage that acts as a brake on the main current carrying the amplified signal through the valve (in a manner of speaking).

- The more bias voltage you apply, the less current flows through the valve. Too much current destroys the valve as it can't dissipate all that heat. Too little and it doesn't work right.

Well, that's pretty dull. So why's everyone talking about it?

Apart from this proper function/valve survival issue, bias becomes a matter of taste.

High current gives you power/volume, clarity, good chimey top end. Less current gives you earlier breakup, with less volume, less clarity and top end.

Usually I bias for power and clarity - if you want less volume and a browner, distorted sound, well - some amps suit this approach better than others, but generally low currents, like low voltages, in power valves don't really deliver the distortion sound you're probably looking for.

Some amps have a self-biasing arrangement (which is the secret of those new amps you can get where you can swap output valves) - but even they are worth checking out when you put in a new valve.

One more other thing... adjustable bias and fixed bias are not opposites. In fact most amps with adjustable bias are fixed bias. The common alternative to fixed bias is cathode bias, which isn't so easily adjustable.

OK? Glad that's clear.

A side issue: some Peavey valve amps seem to have a tendency to leave the factory biased very 'cold' - that is, set up so that the current through the power valves is very low indeed.

This is particularly true of the 5150s, but I've seen it on many and various Peaveys, the exception being the EL84 amps which, like most EL84 amps including the AC30, are biased for incendiary levels of heat.

But some Peavey 6L6 amps and others in the 50-100 watt range are often biased way cold. I think they do this in pursuit of distortion. The distortion created is called 'cross-over' distortion,and in most amps it doesn't sound that good. So if you like the stock 5150 sound, good luck to you and stick with it but if you feel that your Peavey sounds a bit thin and blue-bottley this may be the reason and you might want to go with a hotter bias. Unfortunately most Peaveys don't have adjustable bias and do have PCBs, so it's not just a matter of twiddling with a trimmer, but it can be worth setting the bias properly, or installing an adjustable bias system (not very hard to do).