ROSE MUSIC: Communication Systems of Australia. Model 35 PA Amplifier (Goldentone).

This one’s a rare find – a Communication Systems of Australia valve PA head, ~35 watts from 1967.  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 
• A&R power transformer
• A&R type 2766 Output transformer, marked 100v, 70v and 50v.
• ITT Silicon bridge rectifier
• Standard 12AX7 triodes for preamp / phase inverter.
• 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

COMMUNICATION SYSTEMS OF AUSTRALIA, MODEL 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).  Note that I have not included the negative feedback winding yet.

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

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+

Unused 9pin socket on the far right, and the octal behind it is used as an input for the phase inverter.

The black square is the solid-state bridge rectifier - 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 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

FURTHER READING

Oz Valve Amps

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

COLORSOUND: Treble Booster, Steve Williams (partial trace)

So there are a few bits missing, but if you are familiar with treble boosters, I'm sure you'll work it out.  Being a small and simple circuit, it's a good candidate for breadboarding.

The filter pot is your standard input blend, probably similar to the values that Steve Williams uses in his Pigdog Competition Special boosters, which is followed by a silicon treble booster with a coupling cap and volume pot attached to the end.  So a little different to a Range Master in that sense.

Updated layout:  Thanks Zollinger Analog for pointing out that there is actually a link between pins 1 & 2.   I couldn't see what was right in front of me...  the link is sitting on the other side of the pot.

COLORSOUND TREBLE BOOSTER - STEVE WILLIAMS

Important to note that this is a partial trace - you may have to test this and work a few things out yourself.

With a 5n / 100n cap and a 100k filter pot, the response on the base of the transistor looks like this.  The output looks about the same with the 100n and 100k pot.  A 47n cap looks pretty similar as well.

Being a silicon boost, they tend to be a cleaner wave than a germanium boost, although this does look to get some dirt as more bass is introduced.  It is just a simulation, so real-life results may differ - and honestly, you never want to hear a silicon treble boost that is not overdriving an amp - horrid...  

ELECTRONICS AUSTRALIA: A Fuzz-box for Electric Guitars

I stumbled across this while looking for something else - so had to drop it in here.   It's a pretty weird looking circuit, and there are some really strange ideas about fuzz mentioned - including placing clipping diodes across the speaker terminals of an amp.  I would not try that at home.  

ELECTRONICS AUSTRALIA: Guitar Speakers

Just some historic speaker info from 1967 - although I had to leave a page of advertisements in, as it has a kit for a fuzz box available from Moody.   For those not from Australia, Moody was a guitar amp brand, which was one of the big names back in the day.  I had no idea that they delved into kits.

PHILIPS: Model EV4437 PA amplifier

I picked this one up recently from a factory in an industrial area - apparently, it had been sitting there as long as anyone can remember, and it was actually used to play music at one stage.   

I know one thing for sure - they never opened it up to dust it.  At least a millimetre of dust, maybe two, was sitting on this.

PHILIPS EV4437 PA AMPLIFIER

• Likely mid to late sixties.
• 100w
• 6 x Philips 6CM5 valves (it was missing two of them)
• 12AX7 preamp & phase inverter
• Solid-state rectifier

It has the standard three knobs - Mic, Pickup and tone control.  The Mic volume has a push/pull switch for bass cut, which just selects between a small and a larger value coupling cap.

The 6CM5 valves are beam power tetrodes, which were actually designed for TV tubes - something about horizontal deflection, which I know nothing about...  

What I do know is that these appeared in quite a few Australian guitar amplifiers, likely due to price and availability.  You can still find these at very reasonable prices online.   I'm not a fan of the cap on the top of the valve - it has over 300v on it, and it's not insulated.  Ouch.  

2 x additional inputs had been added, all attached to the triode used for the mic input.

OUTPUT TRANSFORMER

• Primary impedance ~1.15k P-P
• Secondary impedances 25 Ω, 50 Ω & 100 Ω
• Use 50–100 for ~8 Ω
• Use 25–50 for ~4.7 Ω (≈4 Ω)

There is also a 5w 470 ohm resistor in series with a 22n capacitor across the speaker output, which was not on the schematic below.  This is a Zobel network, which is used to dampen highs around the 15khz mark when using the 100 Ω output.

Calculations below:

Primary: 14.89 Vrms plate-to-plate at 440 Hz

Secondary:

0–25 tap: 2.189 Vrms → Np/Ns = 6.80 → (Np/Ns)² = 46.3

0–50 tap: 3.141 Vrms → Np/Ns = 4.74 → (Np/Ns)² = 22.5

0–100 tap: 4.379 Vrms → Np/Ns = 3.40 → (Np/Ns)² = 11.6

Effective primary impedance:

With 25 Ω load on 0–25 → ≈ 1.16 kΩ p-p

With 50 Ω load on 0–50 → ≈ 1.12 kΩ p-p

With 100 Ω load on 0–100 → ≈ 1.16 kΩ p-p

50–100: Vs=1.235 → Np/Ns=12.057 → (Np/Ns)²=145.364 → Zs needed ≈ 1150/145.364 ≈ 7.9 Ω.

25–50: Vs=0.949 → Np/Ns=15.690 → (Np/Ns)²=246.182 → Zs needed ≈ 1150/246.182 ≈ 4.7 Ω.

I think they put the wrong plate on this amp, as the 4417 is a slave amp from this series.

It's a fixed bias amp, but for some reason, there are 22ohm resistors to ground from the cathodes of the 6CM5 pentodes.  People sometimes put 1ohm resistors here to measure bias, but to find this in an amp like this is unusual, cause the bias is pre-set using resistors.  

PHILIPS EV4437 SCHEMATIC

I haven't completely checked mine against this schematic yet - but there are definitely some differences.  Some I know are mods, some may be factory. 

The power supply is interesting, as it has a number of different feeds, for the B+, screen voltages and bias.

FURTHER READING

Full evaluation by Tim Robbins - Dalmura site, although this is for the 4437a, which has some additional features

Aussie Guitar Gear Heads

Amplifiers with Valves - EV44 series

Don't know why I even took this photo - this is the shed in an industrial area of Brisbane where the amp was found.