Do you have a valve guitar amplifier?<\/p>\n
If so you have probably heard of \u2018biasing\u2019 and maybe become confused about what is fixed bias<\/strong>.<\/p>\n There are two sorts of bias: Fixed bias and cathode biased. This short article aims to demystify this highly confusing topic.<\/p>\n Let\u2019s make a start. You\u2019re going to like this first nugget. Fixed Bias actually means adjustable bias! Go figure!<\/p>\n So a fixed bias amplifier means that it is one on which you can adjust the bias to get the \u2018tickover\u2019 current through the output valves to be the correct level. The term \u2018fixed bias\u2019 comes from the fact that once you have adjusted it (set it) then it stays the same (fixed) until adjusted again. Yes, I agree, \u2018fixed bias\u2019 is not the greatest name, but that\u2019s what it\u2019s called.<\/p>\n Take home: if you have a fixed bias<\/em> circuit in your amp, it\u2019s one on which you can adjust the bias voltage. A fixed bias amp can be biased by you or a tech.<\/p>\n In contrast to fixed bias we have Cathode Biased. This could also be called \u2018variable bias\u2019 but it isn\u2019t. Now because it\u2019s \u2018variable\u2019 don\u2019t get the idea that you or a tech can vary it \u2013 you can\u2019t. That\u2019s reserved for fixed bias amps (feeling confused yet??)<\/p>\n In a cathode biased amp, the bias voltage varies up and down automatically to keep the bias correct. In fixed bias amplifiers, once set, the bias does NOT move up and down automatically. And that\u2019s where the \u2018fixed\u2019 bit of fixed bias comes in.<\/p>\n For your interest, all preamp valves are cathode biased which is why you never have to bias them. Cathode bias is a kind of self bias circuit.<\/p>\n Some amps (e.g. Vox AC15) have cathode biased output tubes (EL84) and unlike fixed bias amps, these amps don\u2019t need biasing. In fact they can\u2019t be biased.<\/p>\n The vast majority of guitar amplifiers are fixed bias tube amp though. Fixed bias on the output tubes means they are capable of being adjusted to raise or lower the bias current. This is usually done with a pot, or swapping out a fixed resistor.<\/p>\n So now you know.<\/p>\n Fixed bias = adjustable (!) but it stays the same (fixed) once adjusted.<\/p>\n Cathode biased = non adjustable and it varies up and down with conditions (so it\u2019s \u2018not fixed\u2019).<\/p>\n","protected":false},"excerpt":{"rendered":" Do you have a valve guitar amplifier? If so you have probably heard of \u2018biasing\u2019 and maybe become confused about what is fixed bias. There are two sorts of bias: Fixed bias and cathode biased. This short article aims to demystify this highly confusing topic. Let\u2019s make a start. You\u2019re going to like this first… Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-89","post","type-post","status-publish","format-standard","hentry","category-stuarts-workshop-wisdom"],"_links":{"self":[{"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/posts\/89","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/comments?post=89"}],"version-history":[{"count":1,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/posts\/89\/revisions"}],"predecessor-version":[{"id":90,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/posts\/89\/revisions\/90"}],"wp:attachment":[{"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/media?parent=89"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/categories?post=89"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.fixguitaramps.co.uk\/blog\/wp-json\/wp\/v2\/tags?post=89"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Fixed Bias \u2013v- Cathode Biased<\/h2>\n
Summary of Fixed Bias and Cathode Biased Amps<\/h3>\n