MOODY: GA40 Guitar Amplifier

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.   

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 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. 

Grounding system

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...     

Schematic

Coming soon...  There are some slight variations to those already published, but nothing major.

Repairs

• The power supply was cooked - all of it.  Power transformer rewound; diodes and filter caps replaced - only the dropper resistors survived.  

• A few mustards had dried out to the point that they were cracked and falling apart.  Replaced all of the 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. 

VALCO: Oahu Lap Steel 345-K

This is a fine-looking little amp from the early forties - the field-coil speaker is dated March 1941, making it pre-war from an American's perspective.

No volume or tone controls, as it is a very small amp, and these controls were of course mounted on its matching lap steel guitar.  

The brief on this one is to get it working, so it can once again be paired with the lap steel.  

Starting positive:  transformers, field-coil and valves all working.   Valves tested very well.   The not so fun: pretty sure that no original components were left, and someone had modified the circuit to the point that it did not resemble a working amplifier. 

Anyways - it’s back up and running now, after essentially a complete rebuild.  

OAHU 345-K LAP STEEL AMPLIFIER

VALVES

It's a very simple single-ended amp, so it only needs three valves:  5Y3 rectifier, 6F6 for power, and 6SF5 for the preamp.

I'd never heard of a 6SF5 before.  It turns out to be a single high-gain triode with specs very close to one half of a 12AX7 - both have an amplification factor of about 100 and very similar transconductance and plate resistance.

The 6F6 is an earlier relative of the 6V6.  It's a slightly different design with different bias and output-transformer load requirements, and has lower output.

OAHU LAP STEEL 345-K SCHEMATIC

It's a pretty minimal single-ended amp, running at about 3 watts.   The design is simple, and it gets the job done. 

REPAIRS

Initially, I wasn’t sure whether I wanted to do any repairs at all. Given the age of the amp, there was a part of me that thought it might be better left alone - not working, but at least still in its original state.

That opinion changed pretty quickly once I opened it up.  It was obvious that someone had already carried out a fair amount of work over the years, so there wasn’t really an untouched original circuit left to preserve.

The large orange Sprague filter capacitors were probably the biggest giveaway.  They’re rated at 600 V in an amp with a B+ of only about 280 V before the choke / speaker field coil drops it to around 220 V.  Subtle they are not.

Before I found a schematic online, I started tracing the circuit myself, but it didn’t take long to realise that was probably a waste of time. I doubt the amp would have worked in the state I found it.

The preamp had a 100 Ω resistor between the input jack and the grid, while the cathode was connected straight to ground. The 100 kΩ plate resistor at least made sense, but the coupling capacitor was an Orange Drop rated at just 100 V, far below the required rating.

Things weren’t much better around the 6F6. Its 150 kΩ grid leak had been made up from two resistors in series, both of which had drifted well out of spec. The cathode resistor measured about 310 Ω.

I rebuilt the circuit to the original design, and it’s now working as it should. It isn’t an entirely period-correct restoration - I haven’t gone as far as sourcing dog-bone resistors etc, but I have tried to keep the rebuild visually sympathetic to the age and character of the amp.

GOLDENTONE: Model 1958 Reverbmaster

Nice little Goldentone 1958 Reverbmaster - it's a great little amp, 12w with a single 12" speaker, reverb and trem.   It's quite a clean amp, as it's missing a gain stage that is normally part of the phase inverter.

GOLDENTONE, MODEL 1958 REVERBMASTER

circa ~1968

As you can see by the size comparison to my cat - it's quite a compact amp.

Goldentone have made use of their unusual valve socket construction technique - they connected another socket to the base, creating little trees to suspend components from.  It's compact, but not always easy to work on.

A&R reverb transformer, with 6GW8 driver.

Rola output transformer, dated 26 August 1968.

Reverb

The reverb is valve-driven using a 6GW8 & a transformer.  It's a very small, low-impedance tank mounted on the side of the combo.  The reverb send and return bypass the amp's volume control, meaning that it is completely independent of it.

Speaker

Single 12" speaker, which looks to be a MSP Hi-Flux based on the shape of the frame.  Not much of a magnet (assuming ceramic), so low wattage.  It's also quite low-profile.

Valves

3 x 6GW8 (ECL86 combinded pentode / triode), 1 x 12AX7

Inputs

2 x inputs, both identical. 

47k  grid stoppers, 470k grid leak (shared)

Preamp / tone controls

Input  >  triode  >  bass, treble, volume  > triode > phase inverter

A 12AX7 is doing the work - nothing out of the ordinary.

First gain stage:  Ra: 100k,   Rk: 1k8 / 22u bypass

Second gain stage: Ra: 100k,  Rk:  2k2 / 100n bypass

Coupling cap:  22n.  Volume pot:  1meg 

Not unlike the classic Fender bass treble set-up.  250k pots

Tremolo

The tremolo uses one of the triodes from a 6GW8 - it's a very standard oscillator, feeding the bias of the output pentodes.  The speed control is also the power on / off switch for the amp.

Phase inverter

Cathodyne phase inverter using one of the triodes from the 6GW8 valves in the power section.  It does not have a gain stage, just the inverter.  

Ra 47k, 1m on grid, Rk 1k8 / 47k.  10n coupling caps. 

Power amp

Push-pull, fixed bias.  

Power

Power transformer, silicon bridge rectifier / 50uf   >  1k dropper / 50uf  > 10k / 8uf 

B+ ~240v

Negative bias for the output pentodes.  Diode, and a 40uf cap, 100k resistor to ground (yes, I need to update details)

Repairs

The previous owner thought that it just needed new input jacks - not quite.   100k plate resistor in the preamp had split, the resistor on the tail of the tone stack had a broken connection and the caps in the trem oscillator all needed replacing (as it was not oscillating).   I also changed the coupling caps from the phase inverter, as they were the same type as those used in the phase inverter.  

Valve sockets all needed a good clean & there were the remains of a very large huntsman spider to remove from the speaker.

I also replaced the 8uf cap in the power supply, as it had a decent sized protrusion on one end.  

SILVERTONE: 1472 Guitar Amplifier

The Silvertone 1472 is a small Sears/Danelectro valve combo using 2×12AX7, 2×6V6GT, a 6AU6 tremolo oscillator and a 6X4 rectifier, driving a 12” Jensen C12R speaker. 

SILVERTONE 1472 GUITAR AMP

The 1472 has separate microphone and instrument channels, each using one half of the first 12AX7. Both are fairly unusual gain stages, with large 330 kΩ plate resistors and 2.2 kΩ unbypassed cathode resistors. The two channels are then mixed through a pair of 330 kΩ resistors and fed into another 12AX7 gain stage, followed by a cathodyne phase inverter.

The output stage is a cathode-biased pair of 6V6GTs, running at around 260 V on the plates and 250 V on the screens, with a shared 220 Ω / 5 W cathode resistor. The schematic doesn't specify the output transformer impedance, although the operating conditions suggest something in the region of 8–10 kΩ plate-to-plate. There are also some very large 330 kΩ grid stoppers on the 6V6s, and no global negative feedback.

Tremolo is provided by a 6AU6 phase-shift oscillator. Rather than modulating one of the preamp stages, it varies the bias of both 6V6 output valves together.

The power supply is similarly old-school, with fairly small filter capacitors and a large 100 kΩ dropping resistor feeding the first 12AX7. That gives the sensitive input stages a fair bit of isolation from the rest of the amplifier, including the tremolo oscillator.

Overall, it's a simple but quite distinctive circuit: relatively low voltages, unusually high resistor values, a cathodyne phase inverter, bias-vary tremolo and no negative feedback. All of that should play a part in the amp's relatively early breakup and its particular character.

SILVERTONE: 1483 Guitar Amplifier

No, unfortunately, I did not buy one - just looking at the schematic, as it's a good candidate for conversion projects, outside of the normal Fender, Marshall, Vox realm.

It's about 23 watts, running a pair of 6L6 and it came with a single 15" speaker.   Two independent channels, with Volume, Bass and Treble on each.

The Silvertone 1483 is a fairly unusual mid-1960s valve amplifier.  It uses two independent 12AX7 input stages, each with its own passive bass and treble controls and volume pot, before the two channels are mixed through 330 kΩ resistors. A second 12AX7 then provides another gain stage followed by a cathodyne phase inverter.

Rather than driving the output valves directly from the phase inverter, there's an added a 6CG7/6FQ7 push-pull driver stage (which is a lot like a 12AU7), with 100 kΩ plate resistors and a shared 2.2 kΩ cathode resistor.  This then drives a cathode-biased pair of 6L6GCs, using a shared 270 Ω / 5 W cathode resistor (unbypassed).

The schematic gives around 350 V on the 6L6 plates, 345 V on the screens and 29 V at the cathodes, with B+ around 360 V.

The tone controls are also a little different from the usual Fender bass / treble arrangement.  Each channel uses a fairly elaborate passive network built around 220 kΩ resistors, 2.2 nF, 1.5 nF and 6 nF capacitors and 1 MΩ bass and treble pots.

The 1483 has an unusual “standby” arrangement.  Rather than switching off the B+ and keeping the heaters running, the standby switch shorts together the two opposite-phase signals feeding the 6L6 output valves.  The signals cancel, removing the differential drive to the push-pull output stage and effectively muting the amp while all valves remain powered normally.  

Overall, the 1483 is essentially two preamp channels feeding a common gain stage, cathodyne phase inverter, dedicated 6CG7 driver and push-pull 6L6 output stage — an unusual and quite sophisticated layout for a Sears/Silvertone amplifier of the period.

SILVERTONE 1483 - ORIGINAL SCHEMATIC

REDRAWN SCHEM - SINGLE CHANNEL

Note: I did not include the inductor across the speaker, as I have no plans to ever do anything like this (visible on image below from Mike and Mikes Guitar bar).  

FOX: Valve PA head, Serial 360

Here are a few photos and specs of a recent acquisition.  A small Australian made PA head, branded as Fox, Sydney.  It required minor repairs; I bought it as not working.  It turned out some components were not connected, so an easy fix.  

It's a very cool little amp - and despite its size, it's pretty loud, and it sounds great.  Really nice clean and edge of break-up tones.

FOX PA AMPLIFIER.  SERIAL NUMBER 360

Controls:  Gram, Tone, Mic (all 1meg pots)

Gram (turntable) screw-on inputs on the front

Mic input missing

Multi-pin speaker output on the side (McMurdo 5pin)

No power on/off switch or fuse.

My favourite feature - the pilot light on a little lampost.  

Preamp:  6AU6 pentode (7pin)

Phase invert and power:  2 x 6GW8 / ECL86 

OUTPUT TRANSFORMER

A&R marked 2676

Push-pull with a centre-tapped primary. Primary DC resistance measured 176 Ω total, with 76 Ω and 100 Ω from the red centre tap to each end

1–2 with 8 Ω → 8.30 kΩ

2–3 with 8 Ω → 8.85 kΩ

1–3 with 32 Ω → 8.57 kΩ

Pins 4 (black) and 5 (brown) are used for the negative-feedback connection; pin 4 is grounded and pin 5 connects through the 47 kΩ feedback resistor.

Secondary: black, purple, blue, brown, white 

POWER

Power transformer: A&R Type 1885-80 MA.  

285.0.285

220ohm 5w resistors in series with AC supply to rectifier anodes

6V4  EZ80 rectifier - I think.  It may be the higher rated EZ81

B+ 290v

SCHEMATIC

I traced the amp, but did not completely finish measuring / jotting down notes on the transformers - job for another day.

note:  pots reversed compared to amp.  i.e.  pins 1, 2 & 3 the other way around

REPAIRS / MODS

Minor repairs made

• Reconnected 220ohm resistors to PT secondaries
• Reconnected a cap and resistor that had ends lifted from the board  
• Removed random heater wiring that was not connected to anything 
• Installed Mic input jack
• Cleaned pots - so dirty that they did nto work in some positions 

Mods

• Added 33k grid stopper on the preamp.  Dropped 4m7 grid leak down to 1meg 
• Reduced 820k grid leaks on 6GW8 down to 270k
• Reduced 100n cap on phase inverter down to 12n
• Replaced bypass caps
• Changed tone control to Fender Princeton / Deluxe one knob
• Added a power switch and a fuse