The Solo – a 10W Single Ended Amp

For some time I have been keen to develop a single ended guitar amp design. All of my Lamington amps use two output valves in push-pull which is a more efficient way to operate an output stage. However there is some thought that a single output valve although inefficient produces a unique sound and I was keen to try a suitable design. Well after a considerable amount of time in prototyping and overcoming quite a few challenges, I can now present the “SOLO” – a 10W single-ended amplifier design that I am particularly pleased with.

It is my first ever single ended design to be developed, as all of my previous Lamington designs are push-pull. It is difficult to describe the tone produced by this amplifier, but it parallels my experience with a single ended hi-fi amp built numbers of years ago – it just sounds more musical for want of a better word.

First, here is an image of the final amp:

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Some background on the design:

While a single-ended (i.e. a single output valve) amplifier is quite simple in design, there are some hurdles to be overcome in developing a practical design, especially here in Australia using cheaper readily available parts.

The first hurdle was in finding a suitable output transformer. These are available from overseas but fairly costly compared to the PA line transformers that I use in other designs. However, I was really pleased to discover that a 12V halogen transformer was a suitable substitute and sounded great in a prototype.

With the output transformer situation sorted, I looked at the second hurdle of a suitable simple power supply for a SE amp. While I could have used two transformers and a quadrupler multiplier for the HT and a third transformer for the heater supplies, the power supply would become considerably more complicated than the inherent simplicity of a two valve SE amplifier. So I spent quite some time to see if I could develop a one power transformer power supply. As I did some research, it occurred to me that I already had a one transformer design – the power supply for my Valve Heaven valve tester. It used one hefty Altronics M6674 30V 2A multi tapped transformer with a “shunt” fed voltage multiplier that developed 250V of DC and 6.3V AC for heaters. The “shunt” arrangement of capacitors considerably improved the regulation of the supply, and I reasoned that a couple of extra stages of multiplication should produce well over 300V of DC for the amp. So I sourced a Soanar M6674 from Jaycar as Altronics have unfortunately discontinued the 6674. A new prototype was then constructed with the single Soanar M6674 power transformer and an 8 stage multiplier. The amp performed well with over 320V of DC produced by this power supply. However, I had concerns about the Soanar M6674 – it was physically quite small for a 60VA unit – around the same size and weight as the 30VA MM2008 30V 1A transformer. My concerns were confirmed when I soak tested the amp – the Soanar transformer became very hot in operation.
So where to from here? The physically larger Altronics unit which would have been ideal for this amp is no longer available. While other 30V higher power transformers are available, they lack the 6V tap for supplying the valve heaters.  
So it came to this – I needed an extra 6V heater transformer. M2156 (15V 2A) transformers are available, but I remembered that the common M2155 15V 1A transformers can supply more than 1A when supplying only 6.3V. I then connected a 2155 into circuit, and was very pleased to see it comfortably supply the 1.8A required by the two 6.3V valve heaters and run only mildly warm after long soak testing.

In the schematic below you can see how we develop over 300V DC from a 30V AC transformer. Eight stages of voltage multiplication are used to achieve this. Voltage multipliers have a reputation for poor voltage regulation and it is true that as the number of multiplication stages rise, regulation can suffer. However in the SOLO amp, there are several factors that deal with this. Firstly, the multiplier capacitors are connected in a “shunt” configuration as opposed to “series”. This reduces the internal impedance of the multiplier significantly improving regulation. In addition, reasonably large values of capacitor are used which also aid regulation. Lastly, the fact that the SOLO is a single ended design helps us here. Typical class AB push-pull amplifiers (read: most of them) vary quite a bit in the current drawn from the power supply. There can be up to 2-3 times variation in current drawn from no signal to full signal drive. As a result, a fairly “stiff” power supply with good regulation is needed. A class A SE amp however draws substantially the same current from idle to full signal. This means that regulation is not so important. As long as the required amount of HT is produced by the multiplier, we are good to go.

One additional benefit of this multiplier arrangement is that hum filtering is inherently very good due to the “shunt” capacitors. One liability of single ended amplifiers is that they are particularly sensitive to power supply hum unlike push-pull amplifiers which cancel hum due to their balanced nature. However, due to the large number of 100uf capacitors in the multiplier, the final power supply produced a hum free supply for the amp.


Some comments on the schematic:

First off, the power supply now reflects the change to two transformers. One bonus arising from the necessity of using two transformers is that we can use the cheaper 30V 1A MM2008 (M6672) as the high voltage multiplier transformer. As this transformer is the same size and weight of the M2165 transformer, it can be substituted in this application.
You may notice that I have used the 15V and 8.5V taps of the 2155 transformer to supply the valve heaters. This was done to tweak the voltage applied to the heaters and in the prototype supplied very close to 6.3V under load.
Beyond the power supply, the amplifier is a pretty simple affair with a 12AX7 preamp and a lone EL34 as a single ended output valve.
There are a couple of added features that increase the versatility of this amplifier. First, I have added a switch in the cathode of the first triode to provide “normal/plexi” switching for want of a better description. With the 47uf cap switched out, the preamp bass response is reduced giving the effect of a mid boost. This is similar to the arrangement in several Marshall circuits. It is particularly useful when the amp is overdriven.

The second addition is a switch to remove the earth return for the TMB tone stack. This provides a significant boost to the gain of the amp and changes the tone contour when the bottom of the tone stack is lifted. In the prototype I used a switch pot on the bass control for the switch, but a panel switch could also be used. I decided to include a full TMB tone stack as I found that this amp really benefits from tone shaping and small changes to the controls provide significant tonal changes – well worth the extra pots.

Here is an image of the underside of the amp chassis:
You can see that there is not much to it – the multiplier power supply, the two valve sockets and the panel mounted controls.

Some comments on construction:

I constructed the SOLO amp on a baking tray for economy, but it works well. The layout that I have used provides plenty of room for placement of each section. At the rear of the chassis you can see the 8 times voltage multiplier. I used my normal method of fixing the capacitors to the chassis with silicone sealant. I reason that the capacitors will last the life of the amp however they can be removed if need be with a knife edge. The multiplier diodes are soldered directly to the power supply capacitors.

The halogen transformer and the two power transformers are mounted on the top of the chassis with grommeted holes allowing the leads to pass through the chassis. The two valve socket holes (octal for the EL34 and noval for the 12AX7) are cut with a step drill or chassis punch and the front panel controls are mounted through holes drilled in the edge of the chassis which becomes the front panel. The front panel layout left to right is input socket, volume, bass, middle and treble controls and the bass cut (“plexi”) switch. The prototype uses a switch pot for the bass pot which provides the tone stack lift boost function. An extra panel switch could replace the switch pot.

The incoming mains lead is wired to the fuse holder and power switch with heatshrink tubing insulating all mains wiring for safety. The mains leads (blue and brown) from the power transformers run under the chassis and terminate to the incoming mains leads with heatshrink to insulate them. I used a cable clamp on the incoming mains lead to ensure that the mains lead is secure.

I wired most of the amplifier components in a cluster around the valve sockets as there is not much to this amplifier with only two valves. You could add tag strips around the valve sockets if you wish.