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Introducing: ’ ‘The Currawong By LEO SIMPSON & NICHOLAS VINEN A 21st Century, All-Australian Audio Amplifier featuring both Valve and Solid State Technology The Currawong is a paradox: a stereo amplifier design that’s as modern as tomorrow but uses a 1950s-era valve power amplifier design! It has a power output of around 10 watts per channel into 8-ohm loads and is mainly built on one PCB. Using the tried and tested ultra-linear class AB configuration with four valves in each channel, it would be a great match for our recently-described Majestic 2-way loudspeakers. W e have had this amplifier under development for about six months but it has been the subject of lively discussion in the SILICON CHIP offices for considerably more time than that, with extra stimulation from occasional correspondence from readers with a deep desire to build and own a modern valve amplifier. And while it might have seemed as though we have ignored these plaintive requests, we have had it in mind to “do something” about a definitive “modern” valve amplifier, if only to finally settle the debate about the relative merits of solid-state versus valve amplifiers. Well, this amplifier is ultra-modern in a number of respects. It uses a large double-sided PCB with plated-through holes for virtually all of the circuitry, apart from a small daughter board for the infrared remote control and the connections to the power transformers. It also uses very compact modern PC-mount electrolytic capacitors for the high voltage filter and bypass capacitors. No electrolytics are used for signal coupling. Instead, we have employed dipped polyester (polyethylene terapthalate) capacitors for the high voltage signal coupling and MKT 24  Silicon Chip We believe this to be very close to the final PCB layout, although there may be some minor component changes as we tweak performance. Apart from the power transformers, almost everything mounts on one double-sided board. siliconchip.com.au While it’s pure valve in the preamp and power amp, we will upset the purists by using semiconductor diodes in the power supply along with a few LEDs . . . polyester capacitors for low voltage signal coupling. And all the low power resistors are modern 1% tolerance metal film resistors. The valve line-up in each channel consists of two 12AX7 twin triodes and two 6L6 (or KT66) beam power tetrodes. So apart from the valves themselves, the components are modern and far superior to those available in the heyday of these amplifiers. The Currawong circuit So what does the circuit configuration look like? Since the amplifier is still in the development and testing stage, it is too early to publish the circuit. We aim to do that – and present the construction details – very soon. However, by sheer coincidence, this month’s Vintage Radio section carries an article about the restoration of a Mullard 5/10 valve power amplifier (see pages 92-97). This was a highly-regarded mono amplifier for hobbyists that was first published in 1954. Two were needed for stereo operation. A stereo version, the Mullard 10-10, was produced much later, with a very similar circuit arrangement but instead of 6L6s in the output stages, it used 6GW8 triodepentodes. It also used two large PCBs, siliconchip.com.au . . . and we probably should mention the infrared remote volume control also has a few semiconductors, too. Then again, we’ve never seen a valve-powered infrared remote! one for each channel. It just happens that the circuit arrangement of the 60-year old Mullard 5/10 is very similar to that of the Currawong, except that the Mullard valve line-up includes an EF86 pentode as a preamplifier stage rather than the 12AX7. Preamplifier stage With regard to the preamp stage, we can be much more specific because it is based on the 12AX7 valve preamplifier published in the February 2004 issue of SILICON CHIP, with only slight changes to the component values. In brief, the preamp is a two-stage design with negative feedback applied from the plate of the second triode to the cathode of the first. The heaters for all four 12AX7s are fed from a regulated 12V DC supply, to ensure low hum. Power amplifier stage Following the preamp stage, the circuit of the Currawong is conceptually very similar to that of the Mullard 5/10. The two 6L6s drive an output transformer in Class-AB mode, essentially meaning that one valve handles the positive half (or swing) of the signal while the second valve handles the negative swing, similar to a transformer-coupled Class B transistor output stage. The transformer primary is centretapped and the primary leads are connected to the respective plates (anodes) of the valves. Thus the transformer provides the high voltage DC to the plates but the magnetic fields produced by the separate valve currents are cancelled in the transformer core; otherwise it would be liable to saturate. Since the output stage works in push-pull mode, the signals to the grids of the two output valves must be out-of-phase (ie, 180° phase shift between them). These out-of-phase signals are derived by the two triodes within the second 12AX7 in what would be regarded today as a “long-tailed pair” (similar to a 2-transistor differential amplifier). An “in-phase” signal comes from the plate of the second triode, while the out-of-phase signal comes from the plate of the first triode. The input signal to the “long-tailed pair” is fed to the grid of the first triode while the grid of the second triode is effectively grounded. This will be fully explained in the circuit description (when we publish those details). The other significant feature of the output stage is the “so-called” ultraOctober 2014  25 As a concession to 21st century operating convenience, this little remote control PCB (shown at left very close to life size) is suspended underneath the main PCB (shown in situ at right). It employs a microprocessor to control a motor-driven volum potentiometer, also suspended under the main PCB. Of course, if you must have genuine 1950s operation, you could always substitute a standard pot . . . linear output connection whereby the screens of the tetrodes are connected to 80% taps on the primary of the transformer. This is (and was) a very highly regarded configuration providing much lower distortion. There is also limited negative feedback from the secondary of the output transformer to the cathode of the second valve in the 2-stage preamplifier. Again, all of this is quite similar to the configuration of the Mullard 5/10. The valves Like most valves these days, those in The Currawong are of Russian manufacture, typically branded Sovtek or electro-harmonix and available in Australia from Altronics Distributors or Jaycar Electronics. Our initial results show that the harmonic distortion will be quite low (maybe around 0.2% in the mid frequencies), about what we would expect. But the amplifier will be particularly quiet, mainly as a result of the DC fed to the filaments of the small signal triode stages. Any more detail about performance will have to wait until the final amplifier design is present, maybe in a month or so. The power supply is based on two toroidal power transformers, one to supply the AC and DC rails for the valve heaters while the other provides the DC HT (high tension) rails for the valve plates (or anodes). And while the 12AX7 heaters are fed with DC from an adjustable 3-terminal regulator, the HT rail is also a departure from 60-year old practice. 26  Silicon Chip In this case, the DC supply is derived from two 1000V silicon diodes and two 470F 400V capacitors in a voltage-doubler configuration to develop about 310V DC. This configuration is very similar to that used in valve TV sets produced in Australia in the 1960s. Our up-date of the circuit is to feed to the DC from the voltage doubler to a 3-transistor capaciThe valve line up consists of two 6L6s (left) and two 12AX7s in each channel. tance multiplier which greatly reduces the 100Hz ripple on the HT rail. This also contributes to the general quietness of the amplifier. This part of the circuit also provides a switch-on delay for the HT rail so that no DC is fed to the valves until their filaments have provided a suitable preheating. Both these features could have only been dreamed of in the 1950s. Mind you, these measures will have little effect on the overall quality of the “valve sound” apart from the overall quietness. We hope the purists won’t mind... Other refinements While there are no solid-state devices used in the signal path in the main amplifier, we have made some concessions to 21st Century operating convenience. In other words it will have an infrared motor-driven volume control that is quite similar to the volume control for the Ultra-LD Mk3 amplifier (See SILICON CHIP, November 2011). The only other “refinement” worth mentioning at this stage is that we have added blue LEDs on the PCB to illuminate the output transformers. This is purely a cosmetic feature. Again, if the purists don’t like the LEDs (“but LEDs weren’t invented in the 1950s!”), they could be omitted. Mention of the transformers brings us to the other interesting feature of this amplifier. As old-time valve amplifier enthusiasts will attest, the output transformer was always the most critical and most expensive component in a high quality circuit. It still is. In fact, high quality ultralinear output transformers typically run to several hundred dollars each! Gulp. And this is the interesting feature. We have employed standard 100V line transformers normally used in PA amplifiers. Conveniently, we are able to use the power taps on the transformer for the screen connections to the tetrodes. Intrigued? It works quite well. But you will have to wait patiently until we have fully checked the final design. Until then, stay tuned... SC siliconchip.com.au