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. After a bit of work, it's up and running again.
If you have a spare 'M' knob, please let me know... I'm missing one on the front panel. I can live without one on the reverb knob on the rear, but it would be nice to have both.
MOODY G.A.40 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. The valve complement also includes three 12AX7s, a 6BL8 pentode/triode for the phase inverter, and a 6DX8 pentode/triode for reverb.
The matching 2 × 12 cabinet is loaded with Australian-made Rola 12PEG Alnico speakers - it's a very well-made plywood cab. It's roughly the same size as a Fender 2 x 12 sealed-back enclosure, just a little smaller.
How many things are wrong in this photo? Clear to see that one of the filter caps is in a bad way - and if you look to the top right, you can see the bypass cap for the EL34s has been placed hard up against the bias resistor next to the EL34 - it had been thoroughly cooked by heat. The mustard cap bottom centre fell apart when I unsoldered it - mustard caps are great, unless they have cracks.
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 channel 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'll leave it that way.
Vibrato Channel
The Vibrato/Reverb channel follows much the same arrangement in terms of the first gain stage and tone controls, although the second gain stage has an unusual cathode-bias arrangement to accommodate the tremolo. It sounds different to the normal channel as a result of this - lower volume and a bit dirty.
There's also some additional filtering after the volume control, apparently intended to remove any unwanted low-frequency signal from the tremolo oscillator.
Vibrato
The tremolo is interesting - it uses a 12AX7 oscillator to modulate the cathode of a preamp stage. The way that it's set-up, 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 a little different compared with more common 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.
Moody put the reverb knob on the back of the amp, which I can only assume means they were using an existing amp head chassis, and decided to drill a hole in the back, rather than having a new one made with space for a reverb knob on the front.
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.
It uses a Type 4 Hammond / Gibbs tank model 1122, date code: 6429. DCR 170Ω on both the input and output. The reverb on/off switch doesn't ground the circuit as most do - it breaks the signal chain.
Phase Inverter
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 gain, while the triode handles the phase inversion. An interesting choice, and not one you see particularly often in guitar amps.
Output Transformer – Ferguson OP452
The output transformer is marked OP452, made by Ferguson 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. These appear to be a special order, and do not appear in the Ferguson catalogue (at least from what I could find).
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.
Output transformer winding
Winding info poached from AGGH - hopefully I'll never need it.
EL34's
The EL34's are running close to the datasheet specs, running at almost 100%.
note: currently running amp on 250v tap
P 355V
S 357V
K 22.1V
Rk 130
Power Supply
The GA40 power supply uses a Ferguson PF1866 transformer: 170 VAC transformer secondary feeding a two-diode voltage doubler. 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.
There's a power selector for the primary - there are only three taps on the primary, but there are four voltages to choose from. From what I can tell, 220v & 230v are both the same tap.
Primary windings
Orange 250v
Red 240v
Brown is wired to both 220v and 230v
Black
Secondary windings
White 170V HT
Yellow Preamp heater voltage (with grounded blue centre tap)
Brown EL34 heaters, no centre tap
Rewound PT - old one was pulled apart, measured and rebuilt.
HT1 360
HT2 239
HT3 123
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 not technically beefy enough for the voltage drop that it's providing, and I can see where it has started to melt the insulator on a nearby wire. With a 160v drop, it was dissipating just over 5.4W - not ideal, but it has survived all these years.
The power transformer also has separate 6.3 VAC heater windings for the EL34 output valves and the smaller valves. Unfortunately, the HT taps turned into a burnt mess, and the transformer is away for rewinding - and as you can see, the filter caps are not in a good way, along with the rest of the power supply. After replacing all the caps, I've been using a substitute transformer for testing purposes.
Grounding System
What grounding system? There is not a single ground bus/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...
Moody GA40 Schematic
There are some slight variations to those already published, but nothing major.
Fender Bandmaster 6G7 vs Moody tone controls
As mentioned above - the tone controls are pretty similar to a 6G7 Bandmaster from the blonde era (along with the Pro 6G5 and Super 6G4).
Compared to 6G7, the Moody has an extra 8k2 resistor on the tail of the stack, which reduces the mid-scoop. I tested this on the bench with a jumper lead, both sounded good; in some cases I preferred one over the other, but it was very pickup-dependent. I'm leaving it as stock, cause what's the point in trying to turn everything into a Fender.
Moody GA40
Fender Bandmaster 6G7
Repairs
- The power supply was cooked - all of it. Power transformer rewound; diodes and filter caps replaced - only the dropper resistors survived. Quite often, Ducon filter caps from this era are still in great shape.
- A few mustard caps had dried out to the point that they were cracked and falling apart. Replaced all of the electrolytic bypass caps, as a few looked awful and didn't measure well at all.
- Usual maintenance work - valve sockets and pots needed a good clean.
- Added some insulation in parts, as the spaghetti tubing had disintegrated.
- Not a repair as such, but I replaced the 5W 4k7 resistor with 2 x 5W 2k2 resistors to keep them comfortably below their rating. Only changed HT2 voltage by 10V
- 10amp fuse replaced with a 3amp fuse.
- Footswitch mod: I removed the separate reverb and trem footswitches and their messy wiring. Using a spare hole in the chassis, I installed a stereo jack so a standard two-button footswitch can now be used. I also changed the reverb switching to ground the recovery stage grid, rather than running the audio signal out through an unshielded cable and back. The tremolo switching was already conventional, so I left it unchanged. It's all reversible, and I kept the original switches with their dodgy cable.
Power transformer rewind
After disassembly, ready for a rewind. Can I just say - getting the first couple of laminations out was a nightmare. At one stage I was going to attempt this myself, with help from a very knowledgeable older chap with a winding machine. I decided to go the safer route, and it's now in some very capable hands elsewhere.
Here's some action shots from the transformer rewind. The guy doing the rewind said that the transformer seems to be running very much at the edge of capacity - I don't know the dark arts of transformer winding, but here's what he had to say "the core Bmax is being run at around 1.2 Tesla this is considered close to the max that power transformer iron should be run".
.jpg)
No comments:
Post a Comment