ROSE MUSIC: CSA. Model 35 PA Amplifier (Goldentone).

This one’s a rare find – a Communication Systems of Australia valve PA head, ~35 watts from around 1967 / 68.  It's super clean, fully functional, with all parts within spec.

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 
• 12AX7s for the preamp and phase inverter.
• A&R power transformer
• A&R type 2766 Output transformer, marked 100v, 70v and 50v.
• Silicon bridge rectifier - ITT MB4 
• Mic input is grid leak biased, low impedance.
• Cathodyne phase inverter, with NFB from transformer winding
• Additional input straight into the phase inverter so the amp can be used as a slave (octal plug)
• Simple treble roll-off tone control
• Pots 500k volume, 250k tone

COMMUNICATION SYSTEMS OF AUSTRALIA, MODEL 35

CSA 35 OUTPUT TRANSFORMER

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).  Note that I have not included the negative feedback winding yet.

16 Ω on 50 – 70 is about 3.8 kΩ p-p reflected.   Datasheet spec is 3.5 kΩ for 2 × 6CM5 at ~300 V B+

Winding / Tap	DC Resistance (Ω)	Measured VAC	Turns Ratio
Primary (p-p)	85	14.49	—
COM – 50	7.1	2.355	6.15 : 1
COM – 70	9.7	3.265	4.44 : 1
COM – 100	14.5	4.816	3.01 : 1
50 – 70	—	0.900	16.1 : 1

There's a spare noval socket on the far right, and the octal behind it is used as an input for the phase inverter.  I have seen another of these that had 2 x Mic inputs, using the additional socket for another preamp.

SOLID STATE RECTIFIER

The black square is the solid-state bridge rectifier marked as MB4 ITT - nice and compact.  The network sitting next to it is the negative bias voltage supply.

Preamp and phase inverter:  Blue, yellow and white hookup wires are colour-coded for plate, grid and cathode.  Green for ground.

CSA 35 VALVE AMPLIFIER SCHEMATIC

C8 is unusual in that it seems to be on a separate winding, compared to the rest of the output transformer.  It may be some form of shield.  Looking at the phase inverter and negative feedback network, they seem to have gone to some effort to reduce high-frequency oscillations / interference.  R12 / C9 is another example.

Voltages

B+ 380

Screens 188

Plates 375

Bias -32.5

CSA 35 CONVERSION NOTES

Despite being a little reluctant to touch the beautiful work done by the person who built it - it needs to function as a guitar amp.   

First things first - at 100k, the input impedance was too low.  Swapped the 100k for a 1meg resistor to bring it up a bit.   An input jack was added.

While the tone control was OK, a simple low-pass filter is a bit boring, and there was a spare knob...   Goldentones often have a James tone controls for bass and treble (sometimes called passive Baxandall), so I installed one to keep to the theme.  I used mustard caps and a low-value tropical fish capacitor to be consistent with the parts used in the amp.  

A bypass cap was added to the gain stage of the cathodyne phase inverter to make up for the loss of gain introduced by the tone controls, and a grid leak resistor was added to where the old tone control was in the circuit.

It sounds great - it's a solid little amp.

FURTHER READING

Oz Valve Amps

Aussie Guitar Gear Heads

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