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Runoff Groove Britannia (AC30 emulator) -- hot off the press!

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Published on Mar 5, 2014

The Britannia is Runoff Groove's redesign of their English Channel, a Vox AC30 with Top Boost emulator. The circuit uses a combination of FETs with mild diode clipping between each stage, and a really neat power tube emulator with LED soft clipping in the feedback loop of an op amp at the end of the circuit. It does touch sensitivity very well, cleans up with the guitar pot, and both the low-gain and the high-gain settings sound excellent to me, without fizziness and with a very natural decay.

The tone controls are as on the amp (though a couple work backwards):
Brilliance (cut): This cuts the treble after the preamp section. (This is backwards from the amp in the pedal)
Bass: This is a fairly standard bass cut control. (Also backwards from the amp -- probably because it would have had to be a rare pot value [50KC instead of 50KA])
Treble: Simultaneously cuts bass and boosts treble and upper midrange.

Overall the circuit retains treble extremely well without getting too noisy. There is a mild bass cut even at "flat" tone settings, but not severe enough to translate to the video. This is as in the top boost channel of the amp and is probably related to the size of the coupling cap after the first stage; I believe the same mod as on the English Channel could simulate the normal channel or jumpered.

Like the Azabache, this is a much more sophisticated circuit than the predicessor ROG circuit it "replaces", and there are all sorts of cool things in it, including a cabinet simulator. Although I think I prefer the Azabache for the lower gain sounds and because its EQ is more what I want to hear, I am extremely impressed with the Britannia and plan to use it on an upcoming recording.

I highly recommend building this one. Runoff Groove has a 1590BB perfboard layout available, and I've included my own perf layout for a 1590B in my perfboard library available on my website. There is also a PCB layout by Ian Birdsall (and I imagine more to come).

Huge thanks to Runoff Groove for sharing their design! :)

Gear used:
Runoff Groove Britannia
Don Quixotecaster with Reilander custom pickups
Tone King Imperial
George L's cables
El Capistan (dotten eights setting) for "I Will Follow."

Songs played:
The Waiting (Tom Petty) - low gain/jangle
I Will Follow/Where the Streets Have No Name (U2) - medium-high gain with delay
Driver 8 (REM) - high gain, high treble setting "dirty clean"
Forgive any playing mistakes, I just learned the riffs and I film the whole demo without cuts and splices!

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