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MUDL ARK A 205 a u d i o s y s t e m s A 20-watt/channel class A2 triode amplifier Over the years, we have had lots of requests to design and publish a valve stereo amplifier. We have always resisted, partly because of the huge expense of a valve amplifier and the comparatively poor performance relative to well-designed solid-state amplifiers. But now we present a modern valve amplifier, designed by David Whitby. It uses quite a few novel techniques to keep the cost down while obtaining good performance. We’ll let David tell the story . . . Pt.1: By DAVID WHITBY V ALVE AMPLIFIERS are undergoing a resurgence today and those who would like to own one are often confused by all the technical and marketing jargon (eg, which type, brand, model, circuit, valve types, etc are best?). Many people are understandably put off by the fact that they may have to sell the family car to get the best model and the special speakers and super cables, etc that they are told you may need to make it all work properly. Take heart – the Mudlark A205 is a modern stylish design which works well with medium-efficiency speakers and comes as an affordably-priced kit or as a fully built-up amplifier. The Mudlark A205 comes in two parts, the main amplifier and the separate power supply box. The main chassis has four large output valves and two smaller driver valves. These are placed either side of a central rectangular housing which accommo30  Silicon Chip dates the filter and plate load chokes and output transformers. The chassis itself is a rugged aluminium extrusion while the front panel is made of polished black Bakelite with the labelling screen-printed in white for good legibility. The two vertical Perspex panels at the top of the amplifier are edge-lit with blue LEDs. Combined with the soft reddish glow from the filaments of the valves, this really adds to the appearance of the amplifier. The front panel is quite simple, with just a toggle switch for the power and two push-on/push-off switches for program selection and negative feedback on/off. Typical program sources which can be used include CD, tuner or tape deck. The overall styling is a combination of retro and modern which should have a very high PAF (partner acceptance factor). Why Mudlark? The Mudlark A205 is really the big brother of the Australianbuilt Mudlark A2-1 (pictured in this article), which was chosen as one of the highlights and also the “cutest thing” at the 2003 HiFi and AV Show in London. It has now been sold in Europe, the UK and the USA. The A2-1 is a small 2-channel valve circuit teamed with a solid-state subwoofer amplifier, while the A205 is a bigger full-range valve system. The A2 designation stands for class A2 operation. This refers to class-A which involves positive drive to the grids of the output valves on positive signal peaks for increased efficiency and higher power (sometimes called “extended” class-A in valve literature). Valve sound Simply put, a valve amplifier will have a particular tonal quality, desiliconchip.com.au The front panel controls are simple and include a power switch, a volume control, a negative feedback (in/out) switch and a source selector switch. pendent on the circuit configuration and valve types and even the brand of valve. This will be due to factors such as the relative levels of remnant second harmonic versus third harmonic components. Even very minute levels and differences can alter the perceived richness, sharpness, smoothness, etc. The generally lower damping factor of a valve amplifier (compared to the very high damping factor of solid-state amplifiers) also shifts the tonal spectrum, producing the “warmer” sound often attributed to valve amplifiers. All of this is hard to measure and quantify and is quite subjective but the bottom line is that valve enthusiasts find the sound attractive and enjoy experimenting with the circuits and loudspeaker combinations. The measured performance of valve amplifiers usually looks worse than for a solid-state amplifier but siliconchip.com.au The rear panel has RCA sockets for the source inputs plus the left & right channel speaker outputs. August 2005  31 Fig.1 (left): the Mudlark is a 3-stage single-ended amplifier operating in class-A2. Its two output valves are connected in parallel to give increased current drive, doubling the power available from a single valve. The circuit is unconventional because of the emitter follower used to drive the grids of the output valves and in the use of a choke as the plate load instead of a conventional output transformer. don’t let this put you off as the type of distortion and not just the total (THD) has a real influence on the final sound character. A valve stereo amplifier need not cost the earth, as you will see with this amplifier. The aim with the A205 was to get the looks and sound of a good valve amplifier with enough power to drive medium to high-sensitivity speakers. Design features Rather than going for a push-pull design, the chosen mode for the A205 is single ended triode. This early mode of valve amplifier operation is becoming quite common with audiophile designs today and although it is harder to get high power and low distortion than with a push-pull design, its particular sound character has found many followers. The residual distortion components of single-ended designs are mainly second harmonic (harmonious!) and a smaller amount of third harmonic, allowing operation with low or zero negative feedback. Most single-ended circuits of the 1940s to 1960s used pentode valves (eg, radios or “radiograms”), which had their own “tone” and often boomy bass – liked by many but hardly hifi. Very few people have experienced single-ended triode circuits with good transformers and high quality modern signal sources. I prefer triode operation for the output valves but have used a pentode in the output valve driver circuit in a configuration which has very low distortion. Modern rugged Russian valves from the 6L6 family have been chosen for the A205.These are beam power valves and are used as power triodes by connecting the screen grid to the plate (anode) in the circuit. Of all the reasonably priced power valves available 32  Silicon Chip siliconchip.com.au that I have tried, I find the modern 6L6 family of valves to be a good choice for sound, reliability and availability. Big power triodes like the 300B are good but they can be really expensive and I think most people will be happy with the 6L6 triode sound, their long life and low replacement cost. The actual types supplied with the kit are Russian-made Sovtek 6L6WXT or Electro-Harmonix 6L6GC/EH or the 5881. The latter is a ruggedised version of the 6L6 and it has a slightly different sound than the other valves. All these valves are made in the millions every year for famous name guitar and audio amplifiers around the world. The pre/driver valve is the 6GW8/ ECL86 or 14GW8/PCL86 triodepentode which combines one half of a 12AX7 type triode with a 9-watt power pentode. Both valve types are identical except for heater voltage and this is catered for by a link on the main PC board. The 6GW8 was used in huge numbers of sound systems in the 1960s, while the 14GW8/PCL86 was used as the sound section in millions of black and white TV sets in Europe, the UK and the USA. The 14GW8/PCL86 is available in large quantities and this is the one supplied in the kit. The 6GW8 is available as NOS (new old stock) and is more expensive. These valves were designed when miniature valve design was well advanced and are excellent as a combined pre/driver and output valve for small amplifiers and for driving larger power valves in higher-powered systems. In the A205, these valves are operated at far below their maximum ratings, resulting in good linearity and long life. Transformer solutions The power transformer in a typical valve amplifier is usually big and costly, with multiple windings for HT (high tension), heaters, bias, etc. By contrast, the Mudlark uses two standard toroidal transformers interconnected in a unique way to provide all the necessary voltages. This results in big cost savings without loss of performance. Just as expensive as the power transformer in a typical high performance valve amplifier is the output transformer, especially so for a single-ended design. The high plate current in the primary winding of single-ended siliconchip.com.au Fig.2: this set of plate current curves for the 6L6 family of valves shows the additional characteristic for positive grid voltage (red curve). transformers means a 20-watt unit can weigh several kilograms and cost $200+ each. To overcome this, we have used an output configuration that has become known as “parafeed”. This uses a relatively low-cost high impedance choke as the load for the plates of the output valves and a capacitor couples the audio signal to the primary of the output transformer. As a result, there is zero DC in the primary winding, so the output transformer can be much smaller and lower in cost without sacrificing quality. Circuit description Let’s have a look at the amplifier section first – see Fig.1. The audio signals are switched by a 2-way push-on/push-off switch and fed directly to a 50kW dual-ganged (log) potentiometer. From there, the signal is fed via a 22kW 1W resistor to the grid of the triode section of valve V1 (14GW8, etc). The output signal The Mudlark A205 is the big brother of the Mudlark A2-1, a small 2-channel valve amplifier teamed with a solidstate sub-woofer amplifier. The “A2” designation stands for class-A2 operation, which involves positive drive to the grids of the output valves on positive signal peaks for increased efficiency and higher power. August 2005  33 The power supply box houses two large toroid transformers, which are connected back to back. Their outputs are fed to an octal socket and connected to the amplifier board via a 1.5-metre cable. from the plate of this valve is coupled via a 22nF 400V polyester capacitor and an attenuator consisting of 470kW and 220kW resistors to the grid of the pentode section. The pentode section is supplied with a screened grid voltage of 120V. This is below the lowest voltage swing of the anode and under these conditions, the driver stage contributes very little distortion to the amplifier. The output impedance of the pentode driver is 13.5kW, as set by the two 27kW 3W paralleled plate resistors. Fig.3: want to bypass that dreadful transistor? Here’s how to do it. The result will be a small reduction in maximum power output. 34  Silicon Chip This is suitable to drive the grids of the 6L6 valves directly (class-A) but improved performance (class-A2) is provided by the use of a BF469 transistor as an emitter follower. This is important because, depending on the signal amplitude, the grid is driven positive and does draw grid current but only on the highest positive peaks. Incidentally, this is a no-no for some valves but the 6L6 is characterised for positive grid current, as can be seen in Fig.2. In practice, with average listening levels and reasonably efficient speakers, class A2 is seldom reached but is available if needed. It allows higher peak power for more headroom, which also translates into higher RMS power if measured with a sinewave input signal. Two valves are used in the output stage. They are connected in parallel and have a common 56W cathode resistor, bypassed by a 220mF capacitor. The plate current for the two valves passes through a 9H (nine Henry) choke – essentially three standard compact fluorescent ballasts connected in series – and this provides a high AC impedance but low DC resistance to supply the HT to the plates. The output signal is then coupled to the primary of the output transformer via a 10mF 400V polypropylene capacitor. As result, since there is no DC in the primary of the output transformer, it can be a small and relatively lowcost hifi 100V line transformer. This is my version of the “parafeed” mode referred to earlier and it gives substantial cost reductions while giving good performance. The bias on the output valves is a mixture of negative grid bias, provided via the emitter follower, and cathode bias provided by the common 56W cathode resistor. The latter provides a degree of automatic adjustment of the plate current to compensate for changes in plate voltage and valve aging. For those not familiar with valve operation, they can be likened to Nchannel FETs, in which the operating current is set by the negative gatesource voltage. In a valve, the plate current is set by the grid-cathode voltage, called the “bias”. By the way, for those people who don’t like the idea of a transistor being used in a valve amplifier, it is possible to keep the circuit “pure” siliconchip.com.au siliconchip.com.au August 2005  35 Fig.4: this is the circuit for the power supply. It incorporates a delay circuit to prevent HT being applied to the output valve plates until their cathodes are at the correct operating temperature. Here’s a sneak preview of the PC board inside the amplifier case. We’ll show you how to build it next month. and bypass the transistor, as shown in Fig.3. This does cause a reduction in maximum power of less than 3dB. This is noticeable only at very high output but otherwise the character of the amplifier is not changed. While we’re on the subject of bias, the pentode section of the first stage operates with cathode bias, provided by the 220W 1W resistor. This is not bypassed by a capacitor, so we have cathode current feedback, otherwise known as “cathode degeneration”. This serves to lower the stage gain and slightly improve linearity. The triode section of the first stage also employs cathode bias. However, in this case, the 680W resistor is bypassed by the 220mF and 470nF capacitors, so the full gain of the triode is obtained, although to set the required gain and reduce noise, some of it is lost in the following resistive attenuator. Note that the 680W resistor also carries the cathode current from the triode section in the other channel (V4). 36  Silicon Chip The triode grid also has a small degree of negative feedback applied from the secondary of the output transformer, via a 220kW resistor to the junction of the 100kW and 10kW grid resistors. Switch S2, on the front panel, allows the negative feedback to be cancelled, which gives a gain increase of about +6dB and a corresponding increase in harmonic distortion. The switch is included for those people who may prefer to use the amplifier without any overall feedback. The overall gain of the driver is set at 20 and the drive voltage capability is in excess of 120V peak-peak. Two output valves were used in parallel to get the required power. With both channels operating, power output is 17-20 watts per channel (3440 watts in total) over a 220-245VAC mains voltage range. Power supply Most of the power supply is housed in or above the main chassis while the two toroidal transformers are mounted in a separate box. The power supply circuit is shown in Fig.4. The power supply looks fairly complicated but that is mainly as a result of the delay circuit which stops the high voltage (HT) being applied to the valves before their cathodes have been heated by the filament current. Without this delay, the HT could initially rise to quite high values, which could damage some of the filter capacitors. It could also cause what is known as “cathode stripping” when higher than normal HT voltages are applied to the plate before the correct cathode temperature is reached and material is stripped from the cathode, thereby reducing valve life. The extra complication is well worth it for extended valve and component life. In essence, the two 160VA toroidal transformers, T1 & T2, are connected back-to-back. Transformer T1 runs conventionally, supplying 25VAC to the 30VAC centre-tapped secondsiliconchip.com.au ary winding of transformer T2. With 25VAC applied in this way and coupling losses due to reverse operation, T2 produces 150VAC at full load on what would normally be its primary winding. This feeds a voltage doubler rectifier circuit consisting of diodes D1 & D2 and four 270mF capacitors. The DC from the voltage doubler is further smoothed by the LC filters consisting of 3H (three Henry) chokes and 100mF 400V capacitors. The resulting output is about 400V DC. Incidentally, the 3H choke used here is the same compact ballast choke as used for the valve plate chokes. These chokes are of extremely high quality and low noise. Better still, they are Australian-made and are a bargain when compared to hard-to-get and costly EI lamination chokes. If you have looked closely at the valve circuit of Fig.1, you will have noted that the supply voltages to the plate chokes for the output valves are + 480V (nominal). How so? The extra 100V comes from a diode pump involving diodes D4, D5, D6 & D7 and two 1000mF 63V capacitors. The 100V DC is stored in four 1000mF 63V capacitors which are connected as two series-connected pairs, to obtain sufficient voltage rating. This diode pump circuit also supplies +60V, from the junction of diodes D5 & D6 and the associated 1000mF capacitor. The +60V is the positive supply rail for the emitter follower Q1 (and Q2 in the other channel). Another diode pump circuit, involving diodes D8-D11, provides the -27V and -120V rails for the emitter followers (Q1 & Q2). HT delay & protection circuit As noted previously, the circuit provides a delay so that HT is not applied to the plates of the output valves until their cathodes have been warmed up. This delay is provided by a 555 timer IC, connected as a conventional monostable to drive relay 1. This switches one of the 150VAC secondary feed wires from transformer T2 and thus stops the abovementioned voltage doubler rectifier from working. Transistor Q3 monitors the AC current drawn by the filaments (heaters) of the four output valves, which are connected in series across the 25VAC siliconchip.com.au Although not visible here, the two Perspex panels at the front are edge-lit with blue LEDs for a really fancy appearance. supply from the secondary winding of transformer T1. If one of the valves is removed from its socket, no filament current can flow and so Q3 turns off. This removes the negative supply to the 555 and the relay, which then switches off the HT rail. Note that there is a link provision in the circuit so that the filaments (heaters) of the driver stages can be 25VAC for two 14GW8 filaments in series or 12.5VAC if two 6GW8s are used. Normally, 14GW8 valves will be supplied in the kits. All of the DC and output chokes and the output transformers are mounted under the long central cover on top of the chassis. They are connected to the main PC board by a short cable and octal plug which makes dismantling and removing the board relatively easy. Next month’s article will provide the full assembly details and the perSC formance measurements. Where To Buy A Kit The price for the full kit (including finished metalwork, panels and all components) is $870.00 (including GST) or with a fully assembled and tested PC board $970.00 (including GST). All metalwork and panels in the kit are fully finished and no drilling, cutting or punching is required. Kits will be available from September 2005 and we strongly recommend that you order before mid-August to receive your kit in the September shipment. The A205 will also be available fully built and tested. Please contact Gless Audio for prices. Valves and other components will also be available. Kits and fully built amplifiers will be available from: Gless Audio, 26 Park St, Seaford, Vic 3198. Phone (03) 9776 8703; email sales<at>glessaudio.com August 2005  37