AWA: Model PA872, 20w Valve PA Amplifier

AWA Model PA872, valve PA head from the early to mid-sixties.  I have its little brother, the PA774, so I had to grab this one.  I think they look pretty cool - I love the giant knobs and of course the schematic on the back of the amp is a bonus.   

• Inputs / Controls:    Mic level (250k), phono level  (250k) and tone (1meg).  There's a bass cut switch on the back.  Phono is the old-school high-output type, so it skips a gain stage.   

• Valves:  5V4G rectifier, 2 x 12AX7 for preamp and phase inverter, 2 x KT66 for the power amp. 

• Output transformer:  5k P-P.  Various outputs for 600 ohm speakers (details below)

• Power transformer:  Marked 50483

• Voltage:  B+ is about 320v.  

• Power amp: Push-pull, cathode biased power amp stage.  20 watts

• Phase inverter:  paraphase inverter of some type.

• NFB:  Negative feedback from the 50Ω transformer winding.  Bass cut incorporated into the network.

• Preamp heater voltage:  The preamp and phase inverter valve’s filaments are wired in series, and then in parallel with the cathode resistor in the power amp stage - meaning they’re running on a DC supply to keep hum down.

Further and significantly more detailed information on the PA872 by Tim Robbins can be found here

AWA PA872, 20W VALVE AMP

Schematic screen printed on the back, just like its little brother.  Terminals from the various secondary winds on the output transformer, bass cut and the fuse.  

OUTPUT TRANSFORMER

Number of 600 ohm speakers, or one 15 ohm between 6-10 and 3-5 

Grey:        Common

Yellow:    11-20 speakers / 40 ohm

Green:      6-10 speakers / 75 ohm

Black:      3-5 speakers / 150 ohm

Orange:    2 speakers / 300 ohm

Purple:    100V

Turns ratio results:

purple - orange, 15:1  

black - green, 20:1 (the connection recommended for 1 x 15 ohm speaker).   With a 16ohm load, 6k6 p-p (which is what I ran with)

green - yellow, 30:1

5k P-P on schematic.  

NFB wind is 50 ohms

Ducon filter cap:  One of the caps needs to be reformed, one seems to be just fine.  

All of the white coupling caps are leaky / show too much resistance to remain, and apart from that, most of the values are larger than anyone wants in a guitar amp these days.  There's also a wax/paper cap that is thoroughly out of spec and leaky.

 

The large filter caps don’t belong.  The 100uf cap is about double what the rectifier can handle.   It’s a bit cramped and messy in there, so I’m going to have to really think about how to approach this.  

The 1A fuse had been replaced with a 50A automotive fuse.  That’s a good way to smoke an amp.   The mains electrical cable also has to go too - the insulation feels stiff and perhaps a bit fragile.  Of course, it needs an on/off power switch installed.

AWA PA872 ADVERTISING & SPECS

Advertising above care of Kevin Chant

image care of the Internet Archive

CONVERSION NOTES

Given it only has three front-panel controls, I went with a Deluxe Reverb-style preamp - just volume, bass and treble.  I left the phase inverter largely untouched (other than removing the negative feedback), and converted it from KT66 to 6V6 (cheaper more accessible valves).  I also replaced the unusual cathode bias/heater setup with a standard heater arrangement.

I couldn’t hear any noise advantage with the original heater configuration, and basically it left almost no control over biasing, and the preamp and phase spillter vales couldn't be hot swapped.  

Overall, it sounds good and I’m pretty happy with the result.