Positive Grid Bias Amp Modeling App

  • By Phillip Dodge @tonereport
  • March 03, 2014

How many times have you wondered what it would sound like to run a Deluxe Reverb through two Celestion Blues or maybe run an AC30 or a Tweed Deluxe into a closed-back 4x12 loaded with Greenbacks? Let’s think bigger – what would happen if you took the preamp from a Top Boost AC30, paired it with the four EL34s and solid state rectifier of a 100-watt Marshall, and then ran it into the 4x10 Tweed Bassman cab?


Thanks to the new Bias iPhone application from Positive Grid, you can try these combinations and more. Bias picks up where other amp modeling applications leave off. Rather than offering you mere models of a slew of classic amps, Bias allows you to be an amp designer (without learning how to discharge filter caps or even learning how to solder). It helps if you have a general idea of the building blocks of tube amps and what each section of the circuit does, but a little trial and error never hurt anyone (at least not in the digital realm).


I spent the better part of a week testing Bias with my iPhone 5C, an IK Multimedia iRig and a Griffin StudioConnect, a range of guitars, and even a few effect pedals. What I found is that even without digging into Bias and “modding” the heck out of the amps, it’s one of the better iOS amp simulators around.

What We Like: The amp models alone are worth the price of admission. My favorite “stock” amps were the Hiway 103 Clean, British TB30, Mini Duo Reverb, and British Plexi 50w. The ability to mix and match preamp tubes and topologies, tone stack type, power amp type and tube, transformer, and speakers and cabs is a real ear opener. I’m sure the combinations don’t sound exactly like what a real-world amp of the specific parameters might sound like, but it’s awesome to hear the differences between a Top Boost, Blackface, and Bassman preamp circuit or between EL84, EL34, 6L6, and 6V6.


And then there is the option of picking between a SM57 or a C414 and then moving said mic around in relation to the speaker cone. Quite frankly, the options are a little overwhelming. But overwhelming in a good way.


Amazingly, Bias even tolerated effect pedals placed before it in the signal chain, something most amp sims can’t handle. I had to make sure I kept the output at or below unity gain, but I was able to add the sparkly cleans of a rolled off Fulltone ’69 germanium fuzz or the sweet echoes of my Catalinbread Belle Epoch to any of the low to mid-gain models with great success.


Bias also plays nice with other apps. You can open any of your amps in JamUp XT or JamUp Pro XT with one easy click and you can record them in GarageBand via the Inter-App Audio option. For simplicities sake, I’ve been getting by with the amp sims in GarageBand for a while now. It’s safe to say I won’t be using them again anytime soon.


Concerns: At full output volume on your guitar, the amps sound and perform very realistically. Trying to control breakup from the volume knob on your guitar is less effective. But that’s the case with most amp simulators. The built-in noise gate helps to reduce background noise but it makes for very odd decay as the note fades and cuts out and then back in and out again. Adjusting the threshold and decay of the gate can help, but I preferred the sounds with the gate bypassed.


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  1. Andy C

    I’m really enjoying reading the reviews in Tone Report. Can you add links to the gear, and preferably prices? Thanks!

  2. Psysword

    Awesome review of my favorite…Bias..thanks for a top notch sound check….I’m also getting excellent sounds from my guitars…..thousands of dollars worth of equipment saved.