![]() For instance, you can swap out preamp tubes and also power amp valves, or you could completely swap out the preamp section of one amp model and drop in another, therefore building your own hybrid virtual amplifier. With plenty of control over each and every aspect of the various amp models, the cabinets and also the microphone placement, BIAS Amp 2 has many options at a component level, meaning that you can seriously tailor your tones. It is a simple thing, but does make a difference and means less loading up time and overheads when you just want to play or build your own tones. One thing I did enjoy was that I can run BIAS Amp 2 without loading up my DAW, as it will happily run standalone and it means I could just audition and create my own patches quickly without needing to fire up Logic Pro X on my iMac. Currently, there is a sale on and so you can shave $30 off of each of those two prices as well. However, you can buy their BIAS FX Standard from $99 or the FX Pro at $199 if you want an integrated effects solution and the two work together, I did not test the effects and so will only be writing about my experiences with the Elite version of the software. Unlike Line 6’s Helix it does not include the effects, apart from a basic reverb and a noise gate, so you need to take this into account if you are really counting the pennies. It caters for different budgets, with the Standard version coming in at $99, Pro at $199 and Elite at $299. Biasing should really based on what sound best rather than an attempt to maximize power.Positive Grid’s BIAS Amp 2 has three distinct versions, with the price rising as features are added. The tubes will last longer and the amp will sound better. Many cathode bias amp will benefit from reducing the dissipation a bit by increasing the value of the cathode resistor. Fixed bias amp are more usually biased cooler in class A/B. Cathode biased amps are often bias very hot and near Class A (tube amp rarely actually meet the standard for being true class a). One thing to keep in mind is that fixed bias and cathode biased amps are often biased differently. Many tube amps are over powered for the average users volume needs. It is important to note that the volume difference is actually quite minimal. The same amp in fixed bias would be about 40 Watts. Example: a cathode bias Tweed Bassman type amp with two 6L6’s would be about 25 to 30 Watts. Fixed biased amps require biasing to change power tubes as well as periodic bias checks to ensure they are working properly.Ĭathode biased amps are less efficient than fixed bias amps. Fixed biased amps are cleaner.Ĭathode Biased amp do not require biasing to change power tubes. With all other things equal: Cathode biased amps are more easily overdriven. One thing to note is that properly set up cathode bias can be quite firm but still singing. Fixed Biased amps have a firmer feel but less dynamics and sustain. So what is the difference?Ĭathode bias amps have more sustain and dynamics but a spongier response particularly with bass notes (more sag). This is what give fixed biased amp a crisper stiffer response. Since the voltage applied to the grid does not change the bias is “fixed”. Since the grid is negative relative to the cathode thus bias is achieved. In fixed bias a separate circuit provides negative voltage to the grid of the power tube and the cathode is simply grounded. This gives cathode biased amp their awesome sustain. As you hit a note the tube calls for more power and negative voltage on the cathode decreases briefly changing the tube's bias. In cathode bias a small value resistor is placed between the cathode and the ground so it develops negative voltage on the cathode thus achieving bias. When using the terms cathode or fixed bias we are discussing the power tubes of an amp (pre-amp tubes in instrument amps are nearly always cathode bias or grid-leak biased but that is a discussion for another time). A poorly biased tube can either be biased too cold making it sound dull and lifeless or biased too hot where the tube is destroying itself. You could think of it as a train without breaks. Without biasing a tube would just burn up. Biasing controls the amount of power a tube dissipates relative to the voltage applied to it. Technical Explanation (feel free to skip ahead) :Īll power tubes needs to be biased in some fashion to function. I will briefly provide a technical explanation and then provide some bullet points of the major differences in tone, maintenance and power. The terms cathode bias and fixed bias are often used to describe amps but lots of folks are not really sure what they mean.
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