Plate to Plate - AR-15 Amp

The Preamp

Under construction. Click on the picture to see it full sized:

It's based on the Studio .22 and Caliber .50 MESA amps. That is, a Fender blackface preamp with an extra (switchable) gain stage at the input. This is the circuit that Randall Smith patented. I added some tricks and three independent gain controls.

The preamp tubes have to be selected to be noise and microphonic free. Be careful with ground loops and proximity with the power transformer and the filament wires. The "extra" gain stage is a double triode with both sections in parallel. This is done to reduce noise. Use metal film resistors for the plate and grid resistors. Use good quality plastic caps to avoid noise, microphonics and harsh sound.

When the extra gain stage is not "on", the signal is attenuated with a resistive divider before going to the first tube stage. This increases the difference in gain when the extra gain stage is activated. This is Randall Smith's main contribution to prior art as stated in his patent.

I used 12AX7's in all stages of the preamp, but I'm considering using a 12AT7 for the "extra" gain stage to have a more usable gain range in the Yellow and Red channels.

I used a on/off/on switch that gives me three options for the "treble" cap (in the clean Green mode). One of them is 220pF, which gives a Fender sound. Other is 110pF (two 220pF in series) which gives a "mid cut" sound that is great for soul and funk clean playing. The third option gives some mid boost. When one of the dirt channels (Yellow or Red) is selected, the third option is always activated.

There's a gain boost switch that ground lifts the mid pot for an increase both in distortion and midrange content.

There are three independent gain controls, one for each channel. The Green channel features a bright switch. The Red and the Yellow channel are almost identical. The difference is that the Red channel uses a higher valued gain pot and that their Master Volumes are located in different places on the circuit. This gives the Yellow channel a sound that is a little sweeter than the Red channel, but both have basically the same amount of gain available. What I do to get three different sounds is just use the Yellow channel at lower gain settings than that used for the Red.

I used relays for the channel switching, instead of optoisolators like in the MESA amps. I'm getting an annoying click when the dirt channels are activated, so I may try the optoisolators when I can get enough of them. More details on the switching scheme page.

The gain stage after the tone stack has a switchable bass boost that can make the distortion fatter for single notes, although (more) muddy for chords when high gain settings are used.

Normally I set it as follows:

Set the desired amount of gain and the tone pots for the Green mode (it can go from really clean to TS-9 dirt, and responds well to playing dynamics);
If I'm using any FX in the loop, the previous step becomes set the amount of gain for the Green mode such that the FX is fed with adequate signal level.
Set the Master Volume for the desired level;
Set the Yellow and Red gain pots for the desired amount of crunch in each one;
Set the Yellow and Red Master volumes for the desired balance to the Green loudness.

In summary, the Green channel is very Fender-like with a sweet, bright sound that can get nasty with hot pickups and playing dynamics. The Yellow channel can go from classic Rock'n'Roll to midrangey high gain ("brown") sound. The Red channel has the same gain range than the Yellow, but the sound is harsher, and can make (any) notes sustain forever (at any volume). The loop works nice with my Boss SE-50 multifx, it can be handy to add some reverb and do some post distortion EQ. The amp can be loud enough to play clean at rehearsal and small gigs, but the way I built the power section and power supply allows me to configure it to really low power (low power tubes, tube rectifier, bias in class A) and use it cranked for more vintage sounds. Of course in this case I would only use the Green channel and not use the FX loop.

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