Introduction, Conventions

This collection started when I realised I had a number of vintage valve amp circuits that were not on the Web.

Fenders, Marshalls, no worries.
But for an Australian Goldentone, Eminar or Strauss?
Forget it.

There were hundreds, if not thousands, of these amplifiers, coming out of small factories and backyard workshops in the 1960's and 70's.

Today they are emerging out of storage as replacement valves are once again becoming available and falling in price. Other components such as high voltage electrolytics and even output transformers are also starting to appear again, albeit sometimes at Hi-Fi prices.

Moving house I found an old folder full of some of my earliest circuit tracings, designs, and bench notes from repairs.

Searching the Web I found these amps have been largely overlooked. So I started collecting related information, concentrating on the descriptive rather than the qualitative, generally writing the service manuals these amps never had to keep them going and to stop them being written out of history. This is that collection.

I am part technician, part historian, and part musician, and these pages are compiled with those factors in mind, and in that order of priority.

So the aims are:

1. circuits and repair info to keep them going
2. catalogue and identify for restoration and preservation
3. maintain the original tonality and functionality.

A working amplifier gets looked after.

When it comes to playing electric guitar, valve amps sound great. It only needs to be said once. The rest hereafter concentrates on keeping them that way.

A secondary aim is to educate in servicing techniques and good design.

If you have one of these classics, or any other information relating to amps of this period, ratingplate name model serial date, names I have overlooked, circuits, pix, corrections, expansions or any other detail, please e-mail me and tell me all about it.

Conventions

G = giga, 10⁺⁹
M = mega, 10⁺⁶
k = kilo, 10⁺³
m = milli, 10^-3
u = micro, 10^-6
n = nano, 10^-9
p = pico, 10^-12

1000p = 1n = 0.001u

r (omega) = Ohm (resistance)
F = Farad (capacitance)
H = Henry (inductance)

V,v = volts
A = current, amps
mA = current, milliamps
W = watts, power (see definition, below)

Examples of use:

2M2 = 2.2 Megohms
4k7 = 4700 ohms
0r22 = 0.22 ohm
4u7 = 4.7 microfarad
3n9 = 3.9 nanofarad
560p = 560 picofarad

Abbreviations:

HT - High Tension
p-p - plate-to-plate (output transformers); peak-to-peak (signal levels)
UL - Ultra Linear
NFB - Negative FeedBack
anode - plate
valve - tube
pot - potentiometer, variable resistor

Control curves

The codes used for the tapers or characteristic curves for pots have changed. The codes used here are as marked on the original pot and are:
A = Linear taper - (vibrato speed, depth, some tone controls)
C = Log taper - (level controls, some tone controls)

Voltages given generally measured with a 20,000 ohm/volt moving coil multimeter. Power measurements generally into 8 ohm resistive load.

Output power

Output powers quoted here are sustained watts (often called 'watts RMS') into a resistive load at the onset of clipping.

'Watts RMS' is, strictly speaking, a technical nonsense but it is accepted shorthand among non-technical musicians to mean real heating watts, honest watts, and not some copywriters confusing puffery.

It is often overlooked that how loud an amp sounds, how much 'grunt' it has when playing, also depends on the conversion efficiency of the loudspeakers it is driving.

Disclaimer

All care but no responsibility for any of the information provided - apply commonsense caution. Circuits may contain errors. Missing component values are generally unknown. Items may be subject to copyright.

---

(nobody is going to believe this, but)
It's a fact that I cut my teeth, not on a valve, but on the 1949 edition of the ARRL Radio Amateurs Handbook, which I still have. It has a blue cover, what's left of it and marks the year of my birth. And I rather savagely left my marks on it too.

When I was still a toddler I managed to conduct an experiment with a light globe which proceeded very well until the next step produced a violent electric shock (240vac between right index and second finger) and being thrown to the floor several feet away.

Amid the panic around me as I came to, was the thought
“oooee ... I need to know more about that!”

Well this set of pages marks a different encounter with valves, starting with the same ARRL handbook in one hand and a guitar in the other.

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