The limited-edition plugin is available for free in celebration of Guitar Month. I do have another part about using fixed bias datasheet spec to determine your cathode bias but I don't know if it's entirely correct.Positive Grid offers the Bias FX 2 LE ($40 value) guitar effects processor as a FREE download until April 30th, 2022.īias FX 2 LE is an extensive guitar FX emulator based on Positive Grid’s flagship Bias FX 2 software. So that's why we have to push our bias that bit hotter. But in guitar amp land our version of clean actually has a touch more distortion and we love to crank our amplifiers into overdrive. They only expect you to drive the power amp hard enough to produce the highest clean output (with a tiny bit of distortion, 3-5%). I don't know about the max dissipation spec but my theory on the colder bias settings is that they are catering for nice clean amplifiers (Think hifi clean, not guitar amp clean). Now, when researching this I did look at a lot of datasheets, some cathode biased class AB amps are specced at 80%ish idle bias, some even biased cold at 55% and some at max dissipation. I just tried to minimize the bias shift as much as possible while making sure the valve was happy and not melting itself while sounding good at the same time. When biasing my amp I checked it at different drive levels using a scope and meter while also doing A/B tests (no pun intended).

At clean levels the bias shift isn't too bad but if you monitor the bias in overdrive state you'll find it shifts A LOT. If you use the fixed bias 70% rule then under load your amp will actually be biased colder. When you start to drive a cathode biased class AB amp the bias voltage increases pushing the bias colder. I was actually going to write an article on this as I did a fair bit of research but a house move got in the way, so I'll try and sum it up now. I had it at 70% and it just sounded meh, now biased at 95% it is worlds better. I went through a similar thing with my last amp which was Class AB Cathode Biased.

Bias guitar amp software