Another amp on the bench. This one was sold as needing a replacement power transformer, which was perhaps a bit of an understatement, although I suspect the seller had no idea.
I'll add some more photos once I've put it back together - it is a cool looking amp and it sounds great.
MOODY GA40 GUITAR AMPLIFIER - CIRCA 1965
The Moody GA40 is a mid-1960s Australian-made guitar amplifier from Sydney, from a time when local manufacturers were producing some interesting alternatives to the now better-known American and British amps, which were often prohibitively expensive in Australia.
The GA40 runs a pair of cathode-biased
EL34 output valves and has two channels - Normal and Vibrato/Reverb. The valve complement also includes three
12AX7s, a
6BL8 pentode/triode for the phase inverter, and a
6DX8 for the spring reverb. The matching 2 × 12 cabinet is loaded with Australian-made
Rola 12PEG Alnico speakers - it's a very well-made plywood cab.
Normal Channel
The Normal channel is fairly conventional, with volume, bass and treble controls. The tone stack is very similar to that of a
Fender 6G7 Bandmaster.
Both inputs are identical, with no high- and low-sensitivity options, and neither is grounded when a plug is removed. Not necessarily the quietest arrangement, but apparently that's how Moody designed it, so I think I'll leave it that way.
Vibrato Channel
The Vibrato/Reverb channel follows much the same arrangement, although the second gain stage has an unusual cathode-bias arrangement to accommodate the tremolo. There's also some additional filtering after the volume control, apparently intended to remove any unwanted low-frequency signal from the tremolo oscillator.
Trem
The tremolo is particularly interesting, using a 12AX7 oscillator to modulate the cathode of a preamp stage. This has quite an effect on the stage's biasing, giving the channel a somewhat gritty character, which I actually quite like.
Reverb
The reverb circuit is also unusual compared with most guitar amps. Apart from being capacitor-coupled rather than transformer-driven, the reverb send is taken directly after the first gain stage, ahead of the tone and volume controls.
The return feeds directly into the phase inverter, bypassing the channel's volume control entirely. This means the reverb level is independent of the dry signal - turn the channel volume right down, and the reverb is still there. Due to the weird set-up, channel jumpering adds reverb to the normal channel, which is a nice bonus.
Gibbs tank, DCR 170Ω on both the input and output. 2 x springs.
The phase inverter is a fairly conventional cathodyne design, but rather than using a 12AX7 or similar, Moody chose a 6BL8 pentode/triode combination. The pentode provides the voltage gain, while the triode handles the phase inversion. An interesting choice, and not one you see particularly often in guitar amps.
Output Transformer – OP452
The output transformer is marked OP452, with a date code of 6/65 (June 1965). It's designed for the pair of EL34 output valves, with a primary impedance of approximately 3.4 kΩ plate-to-plate.
I measured the DC resistance of the primary at 154.5 Ω end-to-end, with 81.9 Ω and 72.8 Ω from the centre tap to each plate connection. The slight difference between the two halves is normal, due to differences in winding length.
To determine the impedance ratios, I applied 9.70 VAC across the full primary winding and measured the voltage on each secondary tap.
| Measurement | Orange (Speaker) | Green (Extension) |
|---|
| Secondary voltage | 0.458 VAC | 0.666 VAC |
| Turns ratio | 21.18:1 | 14.56:1 |
| Nominal speaker impedance | 8 Ω | 16 Ω |
| Reflected primary impedance | 3.59 kΩ | 3.39 kΩ |
Both taps reflect close enough to 3.4 kΩ plate-to-plate when connected to their intended speaker loads.
The matching cabinet contains two Rola 12PEG Alnico speakers, each nominally 15 Ω and wired in parallel, giving a combined load of approximately 7.5 Ω. Connected to the orange speaker output, this reflects around 3.36 kΩ plate-to-plate - a pretty good match for the original EL34 output stage.
Winding
Winding info poached from AGGH - hopefully I'll never need it. https://www.aggh.net/discussion/index.php?topic=49351.60
Power Supply
The GA40 has a slightly unusual power supply, using a 170 VAC transformer secondary feeding a two-diode voltage doubler, rather than a conventional high-voltage secondary and valve rectifier. The doubler uses two 100u / 350 V electrolytic capacitors, followed by a smoothing choke and additional filtering. Voltage doublers seem common on Australian amps for some reason.
The original choke, marked FL225 on the schematic, measures approximately 1.08 H with a DC resistance of 47 Ω. After the choke, the HT supply is progressively reduced through a 4.7 kΩ / 5 W resistor and an 82 kΩ / 1 W resistor, with 20u / 450 V filter capacitors at each of the three HT supply nodes.
The 5W 4.7 kΩ is probably a little under-rated - it gets fairly hot, and I can see where it has started to melt the insulator on a nearby wire.
The power transformer also has separate 6.3 VAC heater windings for the EL34 output valves and the smaller valves. Unfortunately, the original transformer has developed a fault and will need rewinding - and as you can see, the filter caps are not in a good way, along with the rest of the power supply. I've been using a substitute transformer for testing purposes.
Power Transformer - PF1866
After disassembly, ready for a rewind. Can I just say - getting the first couple of laminations out was a nightmare.
What grounding system? There is not a single ground wire; everything is soldered directly to the chassis, wherever that part may be. And of course, despite this being the "wrong" way to do it, it does not have a problem with hum...
Coming soon... There are some slight variations to those already published, but nothing major.
