August 25 2008

Projects Precision Electronics PA-20 Conversion

Converting an old Public Address system amplifier to a guitar amp


Old public address system tube amplifiers were often very similar to guitar amps, needing only slight tweaking to turn them into guitar amps. Or, if they’re not suitable, they can provide power and output transformers for building a new circuit in the old shell.

I took a Precision Electronics PA-20 amp and turned it first into an AX84 High Octane, then a Marshall 18-Watt circuit, then a 6BM8 circuit based on the 18-watter.

The Donor Amp

This amp started out at as a Precision Electronics PA-20 PA amplifier. I picked it up cheap on eBay. With a tube complement of 2 6EU7s, 2 EL84s, and an EZ80 rectifier, I figured it would make a good guitar amp conversion.

High Octane

The first thing I transformed it into was an AX84 High Octane preamp with self-split EL84 output. It took a long time to decide on what exactly I wanted to build. With 4 triodes and a pair of EL84s you’ve got a decent number of options. Ultimately, I decided on the High Octane after reading a discussion of tweaking it and its versatility on the AX84 BBS. My amp is based on the Hi-Octane Rev. 4, with the following differences:

  • EZ80 rectifier tube, and power supply based on the pre-existing power transformer. Being a 340-0-340 PT, it required a bit more stepping down.
  • Inter-stage attenuator values tweaked slightly
  • Baxandall tone stack instead of Fender-style. This was chosen primarily due to chassis constraints (I was limited to 4 knobs on the front in order to preserve the original faceplate and knobs).
  • Self-Split EL84 output section. Base on Chris H.’s self-split EL34.
  • Using 6EU7 tubes instead of 12AX7. The original PA circuit used 6EU7. If I ever need to replace the 6EU7s I’ll probably end up re-wiring it for 12AX7s, so I can swap variants in to play with the tone. For now, I’m using what I’ve got.

Marshal 18-Watt

The High Octane amp was fun, but wasn’t really my cup of tea - I'm not really a high gain kind of guitar player. I added a pair of preamp out and poweramp input jacks to it, so I could “mix and match” my various preamp and poweramp sections.

It sat idle for months, until one day I was inspired to turn it into a Marshall 18-Watter. I was motivated by wanting a somewhat louder output section with a preamp capable of some clean headroom to act as a power amp for a preamp I’m building separately. The 18 watter seemed like a decent candidate – if you replace the input 12AX7 with a 12AU7, it has enough clean headroom to accept a line-level input.

The rebuild took about 5 hours in total after I did a revised layout. About 2 hours to strip the existing circuit, and 3 hours to re-wire it (I reused the existing turret board). I based it on the 18 Watt Lite II schematic, with a few minor differences based on what parts I had on hand. I also reused the power supply I built for the High Octane, so it has a good deal more filtering than a normal 18 watter.

Mongrel 6BM8 Amp

Naturally, I couldn't leave well enough alone, as I really wanted to build something with some 6BM8 tubes I'd acquired. The 6BM8/ECL82 is a neat tube - it's a medium-gain triode and a power pentode in one envelope. So, you can build a simple Champ-style amp in a single tube, or a complete long-tail phase inverter and push-pull pentode power section with 2 tubes.

Schematics & Sound Samples