Amber Alert Tube Pedal

Author: July 3rd 2009 4:23 am

The Amber Alert is a prototype for the forthcoming Shock Hazard tube guitar pedal. Two stages of pure tube gain in a stompbox – with no starved plate or silicon clipping trickery!

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I’ve always wanted a tube booster pedal. I often marveled at John Roberson’s bass sound and lusted after his precious Soldano Supercharger GTO. The only real reason I haven’t built a "tube-pre-in-a-pedal" before is because I didn’t really grasp the full potential of such a device. I wrongfully assumed that a simple tube-like overdrive pedal would do the same thing in a pinch, without the $500 price tag or the math problems and imminent electrocution involved with building my own. Ignorance was bliss, and I kept rocking dinky silicon stompboxes.

As fate would have it, my totally hot rodded Peavey VTM-120 was stolen from my venue and I needed a replacement amp head fast. I got on eBay and found an old Bell 5630 for cheap. It’s a great amp with really weird input stages for its 3 mic level channels, but without rewiring or drastically rebiasing the whole thing, the low gain made it absolutely necessary to use some kind of booster in front of it. I tried just reversing a DI box to drive the mic transformers a little better, but still no joy. Adding a stompbox gave me that classic crappy silicon sound simply passed cleanly through the amp with a little coloration. Nope, that just wasn’t going to do.

After experimenting with solid state boosters, it became obvious that without an intricate multistage design or a supply voltage greater than 9V, I wasn’t going to get the sound I was looking for. I wanted to hit the preamp HARD and drive those esoteric little input tubes into nonlinearity, rather than just fuzzing out my guitar and driving the amp clean. Then one day, Charles from Hans Condor stopped by to change the tubes in a Marshall he needed to use. I mentioned that I thought I had a couple 12ax7’s lying around and I dug them out for him to try. He didn’t need all of them so I went to put away the leftovers when I realized that I held in my very hand the ultimate solution to all of my tone woes. If I needed to build a larger power supply anyway, might as well build one for a tube right? (Just go with it.)

I scavenged all the parts I would need from various old amps, failed projects, and scraps from the basement. I began researching similar commercial offerings hoping to glean a bit of wisdom from working designs, and was appalled to find that the vast majority of tube based guitar pedals out there use something called "starved plate" topology. Basically, that’s double speak for "not actually a tube gain stage." The tube appears to be either used as a simple diode or as a self-biased, extremely low voltage common cathode stage. It doesn’t take a rocket scientist to understand that a tube with a B+ of 15V isn’t exactly going to have much headroom (no real boost) and is going to go into grid conduction so easily that it would be no better than a silicon diode clipping stage (super fuzz with no tube-like qualities). What a rip off! I found very few other examples of decent designs, but took a lot of cues from the Supercharger and Fred Nachbaur’s Real McTube II. Fred had the brilliant idea to use two 12V transformers to generate the heater supply and the HT voltage without needing special tube-friendly transformers. Without this breakthrough I’m sure I never would have gotten around to building this thing, or I would have almost certainly used the mains directly at some point, which would have been tragic I’m sure.

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The design I settled on has the ability to really pump out some low end, so this version will ultimately be spun off as a new bass tube pedal in the future, with a new design called the Shock Hazard taking its place. The openness of the design was intentional, but I think armed with the experience from this build I can make an improved version, custom tailored specifically for guitar only.

I’ll be posting specifics regarding the design along with any improvements I make very soon. Audio samples are also on their way so check back or subscribe to my blog feed for updates as they come.

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