STEANE'S SOUND SYSTEM: Model S 311 Valve PA Amplifier

I picked up a couple of valve PA heads on the cheap, without valves, so jotting down a few notes for later.  I believe that this is almost the same as a Philips 982, based on early observations. 

STEANE'S SOUND SYSTEMS MODEL S 311 VALVE PA AMPLIFIER

VALVE LINE-UP 

Valves were removed by the previous owner and moved into his sales stock.  

2 x 12AX7 dual triodes

2 x EL34 pentodes, cathode biased

1 x GZ32 full wave rectifier

Expected output, 25W

note: The maximum reservoir capacitor was 60 µF and the minimum series resistance to limit current was 150 Ohms.

INPUTS / OUTPUTS / CONTROLS

INPUTS:          PU (phono input for crystal cartridges)  & Microphone 

CONTROLS:   PU Volume, Mic Volume, Tone (music / speech)

OUTPUT:         Speaker binding posts, fixed impedance, currently set to 500 ohms

TRANSFORMERS

POWER:   

A&R Type 1791, 2886

300-CT-300 for B+ 

6.3-CT-6.3 for 12AX& & EL34 heaters

5v for GZ32 heaters

OUTPUT:  

A&R Type 2504, 2826

0, 125, 250, 500 + FB winding

More wax paper coupling caps - at least the rest looks clean and there's a decent amount of space to work in the chassis.  Despite the pots looking very corroded, they still very smooth to turn.

Output transformer:  negative feedback, common, 125, 250 & 500

Ducon electrolytic caps seem to be common in a lot of Australian amps.  In my limited experience, they seem to be OK after they have a chance to reform.  Fingers crossed that these will be the same.

 

UNDER THE HOOD

Full credit to Tim Robbins for drawing the original schematic - as it is almost identical, I just amended it with the values that I found in this amplifier.  Hope you don't mind Tim (if you ever see this).

The main difference is R24 & C11 across the primaries of the output transformer.