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Sunday, November 25, 2012

What to do with that extra channel

Almost all the Fender blackface and silverface amps have 2 channels, a Normal and a Vibrato (which may also have reverb). This is a historical feature. In the early days, one amp was used for the guitar, microphone, and possibly an accordian. Now with the one-amp-per-instrument model, having two similar channels is not the most flexible setup.

For one thing, the Normal channel requires a 7025/12AX7 tube, and if you're not going to use it, why buy one? You can not install the first preamp tube, but there's a subtle difference in sound. The B+ voltage goes up a hair (probably not sonically noticeable), but because the cathode resistor one the second triode in the tube is shared between the Normal and Vibrato channel, the Vibrato channel's tube is now set up differently. Instead of the cathode resistor being 1500 ohms, it drops to 820, which changes the bias and makes it distort a little easier. Try it and see if you hear a difference.

What about running the channels in parallel by plugging into #1 on the Vibrato channel and running a short patch cord from Vibrato #2 to Normal #1? On the non-reverb amps like the Band Master and Dual Showman, this works, and is an interesting sound. But on the reverb amps, the two channels are out of phase and there is some cancellation which doesn't sound good. Each time the audio signal passes through a tube stage, its phase is inverted. The tubes used in the reverb circuitry invert the phase of the signal when compared to the Normal channel signal, so this isn't a good idea for most uses. (Maybe you're one of the few that has a use for the phase cancellation sound.)

Running the preamps in parallel works really well for Marshall 4-input amps. A lot of players jumper the two channels of a SuperLead as a standard way of setting up the amp. This came about for another reason (see my June 20, 2012 post) but creates a nice thick tone.
There's a simple mod that lets you use the Normal channel for finer reverb control (on amps that have reverb). Run a shielded cable from the output of the reverb tank to the input on the Normal channel (you'll need and RCA to 1/4" cable). The Volume knob is now similar to the Reverb knob in function, but you have tone controls. Easy enough to try without a big commitment.

For more intrusive uses, you can cascade the channels, which was the idea Randall Smith had in the '70's which became Mesa Boogie. This is a bit of an art, as you can't get a good sound by just running one channel into the other (see the Marshall One-Wire mod on my website). Distortion sounds better if it's built up over several tube stages, so you'll need to attenuate the signal. Plus bass muddies the tone, so you'll also have to add a high-pass filter to get something usable.

Another thought is to re-voice the channel so it sounds different. One idea on my website is to turn it into a Tweed style preamp. Other people change the tone stack components, or add a Marshall style preamp (where the tone controls are after the 2nd triode stage). Something to give a different sound and make the amp more flexible. By using an A-B switch box you can add channel switching to these amps.

Consider it--it might be a fun way to play with your sound. But be cautious about modifying these amps in a way that can't be returned to stock (like by drilling holes). As they get older they're becoming more valuable, and sometimes it's better to mod your amp ownership rather than your amp.

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