RNR Workshop Belfast :: Leif B, professional guitar tech :: 07811225591 :: LEIF.B@ME.COM

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RNR Workshop 100-watt hi-gain amp

hand-wired in Belfast :: inspired by Marshall and Soldano

Gutted and remodelled, this amp used to be a Sound City SC-120, but there’s almost nothing left of the original. It has been redesigned and repopulated, and now this humble shell sounds nothing like its old soul.

:: What does it sound like? ::

Think of an old Marshall JCM800 (the 2204 circuit) but with a bit more bottom end.
A little bit AC/DC, a little bit Cream

Right. And?

Here’s some sciency stuff...

There are four stages in the preamp. When you plug into ‘Pre amp distortion’ you’re going through all four triodes: 1, 2, 3, and 4.

When you plug into ‘Power amp distortion’ you’re going through three triodes: 2, 3, and 4.

Triode 1 is inspired by Soldano, in that it uses a 1uF bypass capacitor on the cathode, as opposed to Marshall’s 0.68uF. This is a high-pass filter leaving behind a bit of bass from the guitar signal – but not quite as much as Marshall leaves behind. When you crank the gain for lots of distortion, you get a clearer distortion, without muddy low end. (Marshall’s 0.68uF choice is pretty strict when it comes to keeping down the mud, but it’s why some people don’t like a Marshall clean sound.)

Triode 2 is a bit of a mixed bag. In an old Marshall circuit, there’s no bypass capacitor here, but in this amp, I’ve installed a 4.7uF capacitor to let more bass frequencies pass through to the next stage. When you’re using ‘Power amp input’ this is your first stage. As mentioned above, for first stages, Soldano tend to use 1uF, and Marshall favour 0.68uF, while Fender usually go for a 25uF capacitor here. So 4.7uF gives a little more bottom end than Marshall and Soldano, but not as much as Fender.

From triode 2, the signal goes to the gain control, and onward to triode 3.

Triode 3‘s job is to distort heavily when you use ‘Pre amp distortion’. But when you use the ‘Power amp distortion’ input, there’s not much break-up here.

Triode 4 is what’s known as a follower, or cathode follower. The signal from triode 3 comes in hot, and leaves at a reduced voltage.

Next it’s the tone network: common Marshall-style ‘stack’ comprising of bass, middle, treble – then the master volume control and…

Phase inverter: while always magical, the ‘long tail’ P.I. does pretty much the same job no matter which input you use. In this amp the phase inverter valve is a 12AT7, as opposed to Marshall’s 12AX7. The 12AT7 gives a little less gain.

WHAT?! Less gain? What if I need that extra push over the cliff?!

This amp is plenty loud. When tested with a 12AX7 in the P.I. socket, the speaker saw 100 watts and MORE. The ethos at RNR: don’t kill components. There is a point where enough is enough, and you don’t want to go stressing your output transformer, which in this amp, is a brand new 100-watt Hammond that will set you back £100+.

Next it’s the power valves

This is a push-pull output stage, and the EL34s operate as pentodes. The grid-bias voltage is adjustable for each side, in case your valves aren’t matched perfectly.

Presence control

As with many amps, the presence control filters out high frequencies from the necessary negative feedback taken from a speaker output. Negative feedback helps keep the amp from self-oscillating in a high-gain environment. That said, this amp may indeed squeal all by itself if you turn every single control all the way up. It sounds horrendous, and it should go without saying it is NOT recommended that you do this. Not only is it bad for your ears, it’s annoying as hell to anyone sharing the room with you.
Speaker sockets

There are a total of five speaker outputs: 2 x 4-ohm, 2 x 8 ohm, and 1 x 16-ohm.

Why so many?

The chassis already had the holes, and they were easily repurposed. Also it was easier/cheaper than installing an impedance selector switch. AND… each output is now hard-wired to the output transformer windings. So, with the absence of an impedance selector, there’s one less contact point to worry about.

And here’s a look at the inside. The layout wasn't ideal, but that's what the rock gods provided. As you can see, it’s all hand-wired, with almost all new components.
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