Plate to Plate - AR-15 Amp

This page is under construction. Mainly because the amp is still under construction also.

It's funny that in spite of my site's name, this is the first page with tube related content.

The main reason behind this amp was that I had not a medium sized amp to do "normal" gigs and rehearsal. I own a tiny Washburn VGA-15 that's great for practicing and that I also use to play with the choir of the company where I do my day job (they sing without mics, so I have to play really quiet). My other amp is a huge 100W Fender copy that has heavy (and bigger than usual) 4 x 12" cabinet and an even heavier all-tube head. I love its sound, but you need two persons to carry it, besides it's overkill for most rehearsing studios and small bars where I usually have a chance to play. Since my my goal was to have something practical to carry, I also decided that it would be nice to have a couple of footswitchable sounds, so that I could leave my bag of pedals at home and carry only the amp and the guitar.

After studying several schematics in the Pittman book and doing some research with fellow AMPAGERS, I chose to go with a modified MESA .50 Caliber/ .22 Studio topology. The main differences from the MESA originals are:

No graphic EQ. After I built the amp I've realized that the EQ is a must for the lead channel, but I can put an EQ pedal at the FX loop, so I can live without it. Besides, I didn't have room on the chassis for the EQ and the inductors used in the MESA design are hard to find/build.
The bias is user adjustable. I have a separate bias pot for each tube, accessible without opening the amp, and there are test points to monitor the bias current (measured with the 1 ohm resistor method).
The B+ is very low. Since I don't need lots of power, it's OK. Besides, I can bias the amp into class A and use a large variety of tubes (I have tried 6V6, 6L6, 6F6 and EL34).
I put a triode/pentode switch.
I have several taps on the OT (4, 8, 16, 32 ohms and a line out winding that is still not in use).
The power supply can use a solid state or tube rectifier. It uses a special trick suggested by Gus Smalley (thanks), which allows me to have more capacitance after the rectifier tube and to use rectifier tubes that normally wouldn't stand the present current levels.
The preamp uses shared tone controls for all modes like the originals, but the gain and master knobs are dedicated. I have three channels: Green (Fenderish super clean to SRV singing), Yellow (AC/DC crunch to high gain lead) and Red (feedback at any note at any volume level - the tone is ugly, definitely needs the EQ in the loop).

The Green channel is my favorite. You could really record an entire album just with this channel, unless it's a Sepultura album.

My initial intent was to build the thing as a combo, but after some problems with microphonic tubes (it's high gain circuitry) I decided to go with separate head and cabinet. I used 3/4" plywood for everything except the speaker baffle, which is 1/2" solid "Freijó" (Brazilian walnut). All corners are dovetailed. It's covered with black vinyl. The single speaker is a 12", rear loaded.

Here are some photos:

The front panel controls from left top right (not labled yet in the photo) are:

Green Channel Gain (below it it's the bright switch that works only on the green channel).
Orange Channel Gain.
Red Channel Gain.
The input jack.
Treble control (below it is the Fender/cut/boost switch for the mid shift).
Mid control (below it is a mid boost switch).
Bass control (below it are two bass boost switches).
Orange Channel Master.
Red Channel Master.
Red, Orange and Green indicator LED's.
Red/Green/Orange Channel selector switch.
A not connected jack that is used to cover a pre-existant hole in the chassis (the chassis was from an old transistor amp). By the way the layout of the amp controls is a little strange mainly because of this.

The back panel controls from left top right (not labled yet in the photo) are:

3-prong AC power chord jack.
The Phase Switch above the Phase Light.
Two jacks for channel switching.
ON/OFF switch.
110/220V selector.
Standby switch.
FX loop jacks.
Master and Presence controls.
MESA/Fender switch for the presence control.
Below it, the Pentode/Triode switch.
Focus control (amount of negative feedback).
Speaker Impedance selector.
Two speaker output jacks (wired in parallel).
DC output jack for connecting the fan.
ON/OFF switch for the fan.
Test points and trimpot screws for bias adjustment.

Below are:

Power Transformer.

Fan (next to a filter cap).

The solid state module in place of the tube rectifier.

The shield of the PI tube.

A pair of EL34s (next to the Output Transformer).

DISCLAIMER: This is a high gain design with lots of parts. If you never built a tube amp, try something simpler for start. I added not many info for beginners and this is on purpose. Also, tube amps are dangerous. You can get killed (serious) if you try to work on them without knowing how. So if you don't know about the basic safety precautions needed, ask for the help of a qualified tech. The information here is for people skilled in the art. If someone ignores this, it's not my responsibility. Also, if what you're looking is a cheap boogie, DIY is not the way to go. The money, time, risk and labor that you have to dispose makes it not worth moneywise. Go and buy a second-hand MESA or Acoustic amp, or add a gain stage to a silverface Deluxe. On the other hand, if you want an amp that is unique and sounds and plays exactly the way YOU want, then put your hands at work! Remember not to copy blindly other people's designs but try to understand it, tweak it, change it, make it your own.

Now let's look at it piece by piece:

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