The Ampeg A3 Overdrive is a distortion pedal from the A series that Ampeg produced during 1982 and 1983. It was built during MTI's ownership of the company, when everything was being made in Japan1. The A series pedals were housed in black steel boxes with brightly colored graphics in pinks, oranges, blues, yellows and other colors1.
The circuit
The A3's schematic was traced and published by Guitar FX Layouts, a technical analysis site run by mirosol and johnk, on 27 October 20131. What the trace shows:
- Op-amp: a 45581
- Transistors: used as buffers; 2N3904, 2N5088 or similar were suggested, since the schematic does not name the original transistor numbers1
- Clipping: silicon clipper diodes, with 1N4148 suggested as very close to the original1
The builders described the pedal's character from their build: it has a TON of distortion on tap, but with the overdrive cranked the dirt is a bit "messy" sounding, with a lot of low end. They also noted it is not really bass-specific1.
"this one has a TON of distortion on tap, but with the overdrive cranked, IMO, the dirt is a bit 'messy' sounding"
Output at low gain, and a fix
One practical quirk: with the gain/overdrive control set at minimum and the volume dimed, the pedal sits below unity gain, so it does not have all that much output at low settings1. On johnk's build, that was remedied by replacing the 10K resistor that goes from pins 6 and 7 on the op-amp with a 47K one, which gives a lot of output1. The same build used a second check: reversing lugs 1 and 3 on the tone pot verifies the build is correct1.
The trace also explains a detail of the volume control: level 1 goes to Vref, which acts as a virtual ground with a buffer after the volume control1.
Collectibility and the vero layout
Because the A series was only in production for 1982 and 1983, originals are scarce. From the prices he had seen, mirosol reported that these are very collectible pedals1.
For builders, Guitar FX Layouts published a stripboard (vero) layout and schematic trace of the A3 on 27 October 20131. One error was found and corrected on 8 November 2013: D3 is supposed to connect one row down, to pin 2, otherwise the pair D2 and D3 does nothing1.
If you build from the Guitar FX Layouts vero, use the corrected version: D3 must connect one row down to pin 2, or the D2 and D3 clipping pair does nothing1.
The A3 sits in a family of Ampeg dirt pedals that later grew, including the modern Ampeg Scrambler Bass Overdrive. For context on the effect type, see Overdrive Pedals and Distortion Pedals.
Questions
When was the Ampeg A3 Overdrive made?
The A3 Overdrive was in production during 1982 and 1983, according to the circuit trace published by Guitar FX Layouts. It belongs to Ampeg's A series of effects, which were built during MTI's ownership of the company, when everything was being made in Japan.
What diodes does the Ampeg A3 Overdrive use?
The clipper diodes in the A3 Overdrive are silicon. On the traced schematic, the builders at Guitar FX Layouts suggest that a 1N4148 should be very close to the original diodes, which matters for anyone building the pedal from their stripboard layout.
Is the Ampeg A3 Overdrive collectible?
Yes. Writing on Guitar FX Layouts, mirosol noted that from the prices he had seen, these are very collectible pedals. The A series was only made during 1982 and 1983, which helps explain the prices originals command today.
Does the A3 Overdrive boost volume at low gain?
No. With the gain or overdrive control set at minimum and the volume control dimed, the pedal sits below unity gain, so it does not have all that much output at low settings. A simple resistor change restores a lot of output, as described in the circuit section below.
What op-amp is in the Ampeg A3 Overdrive?
The schematic shows a 4558 as the op-amp. The transistors in the circuit are used as buffers, and 2N3904, 2N5088 or similar parts were suggested as substitutes, since the original semiconductor numbers are not given on the schematic beyond the 4558.
Is there a vero layout for the Ampeg A3?
Yes. Guitar FX Layouts published a stripboard (vero) layout and schematic trace of the A3 Overdrive on 27 October 2013. One error was found and fixed on 8 November 2013: D3 connects one row down, to pin 2, otherwise the D2 and D3 pair does nothing.
Sources (1)
- Guitar FX Layouts: Ampeg A3 Overdrive, Guitar FX Layouts (tagboardeffects.blogspot.com), 2013-10-27
