If your amplifier features cathode-biased output tubes also, then you don’t need to bias that amp – ever. You may be interested to know that all preamp valves are cathode biased which is why you never have to bias them. And that’s where the ‘fixed’ bit of ‘fixed bias’ comes in. It stays FIXED at whatever you set it at. In contrast, in fixed bias amplifiers, once set, the bias current does NOT move up and down automatically. So too little bias current and it’s turned up automatically. Too much bias current and it’s turned down automatically. So in a cathode biased amp, the bias voltage varies up and down (that’s the ‘variable’ bit) automatically to keep the correct bias. User-adjustable bias is the province of fixed bias amps (feeling confused yet?) Just because it’s ‘variable’ run away with the notion that you can vary it – you can’t. In contrast to fixed bias we also have cathode bias.Ī better name for this would be ‘variable bias’ but it’s not called that. As I mentioned earlier it’s typically around 30-50mA). (The correct bias current depends on the plate voltage and valve type. In fixed bias amps, when the output tubes are changed (always for a matched set of course), you readjust the bias until it is correct. You can bias a fixed bias amp or get a tech to do it for you. Key Point: if you have a ‘fixed bias amp’ you can adjust the bias voltage (usually with a pot, or by swapping out a fixed resistor). You’ll probably agree that ‘fixed bias’ is not the clearest term, but we’re stuck with it. It’s called ‘fixed bias’ because once you have adjusted it then it stays the same (i.e. So a ‘fixed bias amplifier’ is one on which you can adjust the bias to get the ‘tickover’ current (also called the bias current) through the output valves to be the right level (usually around 30-50mA).
‘Fixed Bias’ actually means ‘Adjustable Bias’! Work that one out! You’re going to like this first revelation. This article aims to address this very confusing topic. Both of those terms are highly confusing, but we’re stuck with them. The two sorts of bias are cathode bias and fixed bias. Have you heard of ‘biasing’ but perhaps are a little confused about what bias means and in particular what cathode bias means?ĭon’t worry – you’re not alone! Biasing is a highly confusing subject for the owners of valve guitar amplifiers. Do you own a valve guitar amplifier and wondered what ‘cathode bias’ means?
0 kommentar(er)
