FARRINGTON: Hybrid Valve PA head

This four-channel beast is a hybrid valve PA amp, built by a bloke named Jim Farrington in NSW sometime in the late sixties or maybe seventies.  From what I can tell, they mostly stayed local to where he lived, and you don’t see many pop up anywhere.   I picked this up on eBay, and it was sent from his hometown.

Looking at the few photos on Ozvalve Amps, it seems he used the same head and chassis for most builds, swapping out the control panel and designing amps to suit whatever job it was for.

There’s no schematics around, and I’m not about to trace the whole thing — there’s just too much going on.

VINTAGE FARRINGTON PA AMPLIFIER

Some photos from the eBay listing below.

SPECS

It's going to be a little vague at this stage, and might stay this way.

• 4 x solid-state preamps:  bass, treble, level, reverb.  Hi and lo impedance inputs 
• Master volume, reverb and tone controls (bass, treble)
• The modulation indicator lamp is not related to signal modulation or trem of any kind
• Reverb:  valve driven, Hammond / Gibbs 1122 tank.  6BM8 to drive it
• 4 x EL34 fixed bias output stage
• I'm guessing some kind of 12AX7 based phase inverter 
• A&R transformers
• Solid-state rectifier
• Line output from the speaker terminals 

A&R 2842 OUTPUT TRANSFORMER

According to OzValveamps   it's rated to 65 watts, 2k6 p-p, which is close to my measurements.  

Secondaries quoted as being:  3.7 Ω,  8 Ω  and 15 Ω

It also notes this transformer being used in the Playmaster 117

PRIMARY

Blue - plate 

Red - B+ 

Green - plate

SECONDARIES

Black - common to ground

Brown - wired to speaker jack.  

Yellow - terminated to tag strip.   

White - terminated to tag strip

• Vp(p-p) = 15.05 VAC

• Vs: Brown 0.614, Yellow 0.888, White 1.175 VAC

Turns ratios (Vp/Vs):

• Brown: 15.05 / 0.614 = 24.511

• Yellow: 15.05 / 0.888 = 16.948

• White: 15.05 / 1.175 = 12.809

Reflected primary: Zp = (Vp/Vs)^2 * Zs

If 3.7 / 8 / 15 Ω:

• Brown @ 3.7Ω → 2,223 Ω p-p

• Yellow @ 8Ω → 2,298 Ω p-p

• White @ 15Ω → 2,461 Ω p-p

If 4 / 8 / 16 Ω:

• Brown @ 4Ω → 2,403 Ω p-p

• Yellow @ 8Ω → 2,298 Ω p-p

• White @ 16Ω → 2,625 Ω p-p

A&R 1938-200MA POWER TRANSFORMER