AWA: Model PA872, 20w Valve PA Amplifier (Serial Number 94)

I posted another AWA PA872 a while back, but realised I never actually shared this one.  It had been sitting on the shelf for months - one of those projects that was nearly there, but never quite finished.  I finally got around to wrapping it up this weekend.

This looks to be an earlier unit, although schematically the same. The serial number is much lower (#94), and the general aesthetic appears earlier.

Someone had already had a go at converting it—let’s just say what I found inside is a good reminder to be cautious with amps of unknown history.  I never even turned it on - deathtrap.

This one came to me as a friendly trade. The previous owner hadn’t worked on it themselves, and was unaware of the condition—they were just clearing out a few projects they knew they were never going to get around to.

With no real baseline to work from, I stripped it right back to the sockets and started again.  It’s now rebuilt as a 5E3-style Deluxe, with a few tweaks to taste.

The stock 5E3 uses 100n coupling caps on the preamp.  I dropped the bright channel right down to 22n, and trimmed the normal channel slightly to 82n.  For the tone control, I’ve gone with stock 5n on the bass side and increased treble from 500pf to 1n, for a slightly different response.  The results are good - two distinctly different, but useful channels.

I chased a pretty stubborn hum for a while - in the end, it came down to grounding.  Moving the power transformer ground to the same point as the filter caps fixed it immediately.  We're only talking a distance of about 80mm.

There was a previous attempt to “fix” the hum, which involved adding 2 x 220uf filter caps to the power supply—so it’s no surprise the valve rectifier was dead on arrival.  Yes - 220uf, not 22...  

AWA MODEL PA 872 - 20 WATT PA AMPLIFIER

I really like the blue hammered finish, and someone in the design team really nailed the look.  

Schematic screened onto the back of the amp - a nice touch for serviceability.  Serial number 94.

The amp can run a 15 ohm speaker without any dodgy rewiring of the output transformer, so that's always handy.

I patched over some holes for a couple of can capacitors, as they were dead, and not conveniently placed for what I wanted to achieve.

Not my most amazing work to date, but it is solid, and it sounds great.

FURTHER READING

Tim Robbins PA872 conversion 

My previous post for the 872, which has full tech details 

FORTIN: Fuzz

Request - here you go & I hope this is what you were after.  Could shave off a column or two if that's a bother.

The Fortin is a pretty stock silicon fuzz face with an input blend, a 10k trimmer in place of the usual 8k2 resistor and a 100n cap on the output.  A few smallish caps on the collector and base of transistors to tame the fizz.   It's not unlike the D*A*M Drag n Fly DF-05 and no doubt a few other fuzz faces with an input blend.

Schematic can be found here

FORTIN FUZZ

CUNNINGHAM AMPS: Zonk / MKI

I came across a series of photos of this build on the PedalPCB forum, and there seemed to be a bit of confusion around the schematic.  Thought it was worth documenting some of the observed values here for reference.

The board shown below is from the wedge enclosure.  There’s also a more pedalboard friendly version, which includes an internal tone control.  It looks to be implemented with a trimmer and a capacitor in series at the output of the circuit — not unlike the approach used in a Marshall Supafuzz MKI.  I tried a 4n7 in series with a A1M pot on the breadboard - worked just fine.  There also appears to be a resistor and a small value capacitor in parallel to ground on the input of the circuit - kind of hard to see that bit, but it looks like 2M and 1300pf.

A few of the capacitor values are non-standard, and there are some variations in the resistor values as well — clearly some biasing tweaks and tuning by ear going on here.

The Zonk / MKI switch is a nice touch.  It adds a 2n7 capacitor in series with a 10n capacitor at the input. It’s a clever implementation too - mounting the 2n7 directly on the switch reduces the number of connections back to the board and keeps everything neat and tidy.

They're all impeccably built - amazing work.

CUNNINGHAM AMPS ZONK / MKI

As always with these circuits, while the passive component values are easy enough to document, the real story is in the transistors - their gain, leakage, and how they interact in-circuit. That’s the part you can’t see on a schematic, and it’s what makes or breaks a MKI style circuit.

Q1: TI 2N1307
Q2: OC45 
Q3: TI 2G308

BE: MKI Tone Bender - Shorty

Yes, it's another MKI.   I designed this layout based on a Zonk to fit a specific enclosure width.  So nothing remarkable, just one for me to use later.

MKI TONE BENDER - SHORTY

It's missing the 33k resistor that normally sits across the 50k pot, cause I like to use an A25k instead.   I sometimes make a few other taste related changes, depending on the transistors, etc. 

 

RUSH: Pep Box

One that I've probably overlooked a little, given its place in the early history of English fuzz pedals - which was likely inspired by the Maestro...  

There was a three-transistor version prior to this called "Fuzzy" which by all accounts is a Maestro.   The later versions dropped the input buffer and made the jump from 3 to 9 volts.

These are gated nasty wonders of the fuzz world.

RUSH PEP BOX - GE VERSION

The 56k resistor is sitting across the 500k volume pot, so you could just drop the pot down to 50k and lose the extra part.  

RUSH PEP BOX - SI VERSION

Later versions transitioned to silicon devices and became more widely recognised in their large red WEM enclosures.  Rush had originally manufactured these units for WEM, before the company reportedly parted ways with him while continuing to use his circuit design.  

Further reading:  

Link to Stompboxes UK

Link to FSB

Reverb

MENATONE: Red Snapper Overdrive

I'd never heard of these until this morning - and I could not believe the second-hand prices of the original...   especially considering the new price of a hand-made pedal from the builder.

It's kind of a Timmy x boutique Tubescreamer type thing - can find the schematic here.   Dual tin can opamp is required - hope this is the layout you were looking for.  

  

MENATONE RED SNAPPER OVERDRIVE