ROSE MUSIC: CSA 35 PA Amplifier (Goldentone).

This one’s a rare find – a Communications System Australia valve PA head, 35 watts from the late sixties.

Anyone familiar with vintage Australian guitar amps will recognise the front panel straight away: it looks just like a Goldentone.  That’s no accident.  These were built by Goldentone’s parent company, Rose Music in Melbourne, and were sold under the CSA, Goldentone and Zephyr badges.

The layout is typical for the period, with mic and pickup (turntable) volume controls and a single tone control. Like a number of Australian amps of the era, the power section uses TV valves – a quirk that seems to be unique to local designs.

• A pair of 6CM5 in the power stage 
• A&R power transformer
• A&R type 2766 Output transformer, marked 100v, 70v and 50v.
• Silicon rectifier
• Standard 12AX7 triodes for preamp / phase inverter.
• Mic input is grid leak biased
• Tagboard construction, point to point wiring
• Additional input straight into the phase inverter so the amp can be used as a slave (octal plug)

Keep an eye out for updates if this is of interest - I will trace the circuit and share the schematic.  

COMMUNICATIONS SYSTEMS AUSTRALIA 35

A&R Type 2766 output transformer with a constant-voltage secondary (50/70/100 V taps). I ran basic DC resistance and low-voltage AC ratio checks to fingerprint the winding and estimate the primary load seen by the output valves (2× 6CM5).

DC resistance

Primary, plate-to-plate: 85 Ω

Secondary:

    COM–50: 7.1 Ω

    COM–70: 9.7 Ω

    COM–100: 14.5 Ω

AC ratio check (14.49 VAC across primary, plate-to-plate)

Measured secondary voltages:

    COM–50: 2.355 VAC

    COM–70: 3.265 VAC

    COM–100: 4.816 VAC

5    0–70 segment: 0.900 VAC

Derived turns ratios (primary : secondary):

    0–50: 6.15 : 1

    0–70: 4.44 : 1

    0–100: 3.01 : 1

    50–70: 16.1 : 1

FURTHER READING

Oz Valve Amps

The Trainspotter's Guide to Goldentone Amplifiers  (page 58)