KIT FOR VALVE AUDIO ENTHUSIASTS Encouraged by the resurgence of interest in valve audio over the last couple of years, Sydney firm Valve Electronics has sourced all of the necessary components and released a new valve- type stereo kit. The KTS3OW is especially suitable for today's audio enthusiasts keen to discover why many of their valve-era counterparts had such a high regard for husky output tri- odes used in the single-ended `Class A' configuration. by JIM ROWS

Back in the 1960s, the long estab- valves in the output, and operating them suppressor as well as the screen. These lished valve-based equipment quietly in the relatively inefficient single-ended additional grids generally improved gave way to the new solid state replace- Class A configuration. valve performance in various respects ments with their lower power dissipa- Have I lost you already? Sorry, let's voltage , frequency response, tion and other advantages. In the audio explain some of that jargon. First of all, power output and so on. But they also area, it didn't take long for most people valves are those with essentially tended to increase valve output imped- to realise that were only three : a filament or indi- ance, which is one of the main reasons offering higher output, wider frequency rectly-heated , which acts as a why `golden eared' audio enthusiasts response, lower and so on, as source of free electrons; a grid or con- tended to prefer : with a lower well as smaller cases and generally trol , used to control the flow output impedance, they provided better much cooler operation. By the mid of those electrons by means of a nega- damping for the loudspeaker. 1970s virtually everyone had junked tive voltage; and a plate or anode, which Just as with bipolar and their trusty valve amplifiers, or relegat- has a positive voltage applied to it to MOSFETs, valves can be operated in ed them to the storage cupboard. attract the electrons. All of these elec- one of a number of modes, offering dif- But as these things often do, valve trodes are housed in a glass, metal or ferent compromises between perfor- technology passed through its nadir and occasionally quartz envelope (hence the mance and energy efficiency. Virtually began to rise again in popularity part- old slang term `bottle'), which in most all low power linear amplifier stages are ly due to nostalgia, perhaps, but also due cases had all of the air evacuated to form operated in `Class A', where the grid to curiosity on the part of many people a fairly hard vacuum. (Explaining the and plate voltages are arranged so that who grew up after valves had faded alternative term `' . ) plate current flows at all times mere- from the scene. In particular many of The triode was the first type of valve ly varying up and down in level to con- today's audio enthusiasts seem curious capable of reliable amplification. Many vey the being amplified. to find out why an earlier generation of of the later types of valve had addition- Class A gives the most linear amplifi- `golden eared' music lovers were so al `grid' electrodes, such as the tetrode cation, but requires that the valve must be keen on amplifiers using husky triode with its screen and the pentode with its operated at a quiescent current level high

Each channel of the amplifier uses a 6SN7 twin triode to drive the two 300B output triodes which are con- R6/R? OHM CURRENT MONITORING RESISTORS nected in parallel, and operated in Class A.

Ti RI - 220 1W One Electron UBT-1 RIA - 15 1W RIB -- 220k 1W 15 Ohre R2 -- 68k 2W R3 - 33k 2W R4 -- 33k 2W R5 -- 390k 2W R6 - 1.0 1W R7 - 1.0 1W R8-331W R9-- 331W R10 - 3.9k 1W RIO Cl - 4?0uf 16V C2 -- 47uf 160V C3 - 0.22uF 630V C4 - 47uF 160V To F/Back Resistor R (optional) HT2 BIAS 5V

20 ELECTRONICS Australia, January 1998 enough to ensure that the valve doesn't A approach with the use of triodes as and started restoring, selling and trading cut off on negative peaks. This well, to ensure the lowest output imped- valve under the `Vintage means that an output stage operating in ance. To get a decent output power this Company' banner in 1991. Since Class A tends to be quite inefficient, and generally meant the use of fairly husky then the business has gradually changed, run quite hot even when it's not handling output triodes like the 2A3, or for those and gradually focussed more and more any signals. who could afford them the famous on repairing, upgrading and trading To reduce amplifier efficiency and Western Electric 300B, developed in the valve-based audio amplifiers. reduce heating, valve amplifier design- 1930s for use in cinema sound systems. The company's kit and component ers soon came up with output stages If they wanted more output than a single operations have evolved more recently, which operated those valves in modes output valve could provide, they used in response to demand. Nowadays like Class AB or Class B, where the multiple valves in parallel... Valve Electronics is offering a range of valves are run at significantly lower qui- By tradition this type of valve amplifi- stereo and mono valve amplifier kits, escent current and power levels. But er delivered `clean' and subjectively along with an impressive choice of new because these modes result in the valves very satisfying sound especially valves, output and power transformers, `cutting off' for some of the signal when coupled with a fairly efficient valve sockets, filter chokes and other cycle, these output stages really have to loudspeaker system. Although it was not valve-era components that have become use multiple valves in a balanced `push- easy to provide much negative feedback, harder to obtain plus a range of valve pull' configuration, arranged so that one due to the limitations imposed by the data books. valve can take over when the other is cut output transformer, the low output The new KTS3OW kit provides a way off. (Most solid state amplifiers use impedance of the output triodes gave to build a full stereo version of the kind push-pull output stages for exactly the fairly good control of loudspeaker `nas- of amplifier that valve-era audio perfec- same reasons.) ties', and the single-ended Class A con- tionists would have wanted, assuming Of course push-pull operation tends to figuration gave relatively low distortion they could afford it (and few of them introduce its own complications, includ- which consisting mainly of the sec- could, in reality). On a generously-pro- ing the difficulty of achieving exactly ond harmonic, sounds relatively `harmo- portioned 80mm deep aluminium chas- balanced operation and of minimising nious' anyway. sis (or the brushed brass plate and pol- `crossover distortion' when each valve is ished wood plinth shown, which is a taking over from the other during the sig- `Valve sound' kit higher-priced option), it provides two nal swings. So at least some of the `gold- It's the renewed interest in `valve amplifier channels, each with a 6SN7 en eared' audio enthusiasts in the valve sound' among today's hifi enthusiasts twin voltage amp triode driving a pair of era preferred not to use anything other which has prompted Andrew Kay, owner 300B power triodes in parallel (see than a single-ended (i.e., not push-pull) of Sydney firm Valve Electronics, to basic circuit). output stage, and operating in Class A. develop a kit for this very kind of ampli- The first section of the 6SN7 (V1) is Needless to say the true believers pre- fier. Andrew has been working with the input stage, with the amplifier's line ferred to combine this single-ended Class valve equipment for quite a few years, level input fed directly to the grid and ELECTRONICS Australia, January 1998 21 Kit for Va/ve Audio Enthusiasts the negative feedback signal from the the output valves, with each channel output fed to the cathode (across resistor Measured performance adjusted via a preset pot. R1A). The amplified difference signal at All measurements taken with an 80 load: The rated performance for each chan- the plate of this stage (pin 2) is then nel of the KTS30W is quite impressive. directly coupled to the grid of the sec- Power Output vs Distortion Power output is 14.5W RMS into 8Q at ond half of V1, where it receives addi- Total Harmonic Distortion (THD): 1kHz, with a power bandwidth (-3 dB) of tional voltage amplification. The result- 14.5W 11.5% at 1kHz 15Hz - 17kHz at 10W output. The rated ing signal from pin 5 is then coupled via lOW 6.0% at 1kHz THD at lOW output is below 7%, with C3 (the only capacitor in each channel's 6.7% at 100Hz hum and noise 70dB below lOW with signal path) into the grids of the two 7.4% at 10kHz the input shorted. Input sensitivity is 300B output valves (V2 and V3). 1W 0.4%atlkHz 170mV RMS for lOW output (1kHz), The output valve plates are combined Distortion (IMD): and input impedance is 200kf2 (essen- via low-value resistors R6 and R7 (to (50Hz and 7kHz in 4:1 ratio) tially R 1 B ). This means that the amplifi- ensure stability, and also allow conve- 10W 27% er can easily be fed from a CD player via nient current monitoring), and then con- 5W 10% a simple passive volume control system. nected to the HT supply via the primary 1W 1.4% Thanks to the Class A output stages, winding of the output transformer T1 . the power consumption of the complete This is a carefully designed wideband Bandwidth 15Hz to 26kHz +0/-3dB KTS30W stereo amp is around 250 coupling transformer, with a tapped low Noise & Hum watts. It's not going to make much of a impedance secondary to allow optimim -75dB below 10W output contribution to conserving energy, to be matching of the amplifier into different (mostly 50Hz/100Hz artefacts) sure but on the other hand it'll help speaker impedances. Output Impedance warm your listening room in winter! As you can see the negative feedback Approximately 5.6Q is taken from the 8Q secondary tap, and (100Hz - 10kHz, 10W) Trying one out fed back to the input stage cathode cir- Input Sensitivity Andrew Kay of Valve Electronics cuit via resistor R10. The overall volt- 200mV for 10W RMS output. very kindly made a built-up KTS30W age gain of the amplifier from input to amplifier available to us for a few days, 8Q tap is therefore (R10 + R1A)/R1A, so that we could carry out some testing or about 260. The power supply uses all solid state and also hook it up for listening ses- Note that each of the voltage amplifi- components and a large power trans- sions. The sample amplifier is shown in er stages uses cathode current biasing former, with a single HT bridge the photos; it included some of the (via R 1 /C 1 and R3/C2), while the output feeding the two amplifier channels via available options, including the brass valves have adjustable fixed bias. The individual filter circuits each with its plate/timber plinth and DC powering of latter allows the user to select the best own filter choke, resistors and capaci- the 300B filaments to minimise hum. compromise between power output, dis- tors. A second small power transformer As it happens, the unit was delivered tortion and power consumption. is used to produce the bias supply for on a Friday and I was able to spend quite a few hours listening to it that weekend, before we made any measurements. For the listening session I fed its outputs to a pair of efficient speaker enclosures, and fed it with signals from my Sony CD player via a `passive control unit' essentially a high-quality ganged vol- ume control pot with a stereo input selector switch, in a shield box. I played familiar tracks from many CDs, covering a wide range of pro- gramme information (although mainly classical, which is where my tastes tend to lie). And frankly, the KTS30W sounded very clean and `sweet' indeed especially on less complex material such as solo voices or instruments, chamber groups and so on. The only times that I could detect a small amount of 'edginess' due to intermodulation distortion was on loud passages of fair- ly complex material, such as a full choir and orchestra at 'full tilt'. In short, an extended listening session with the amplifier was very pleasant Underneath the chassis, everything is wired neatly in the old-fashioned way. indeed. There was very little of the cumu- The kit includes high quality valve sockets, tagstrips and other fittings. lative `tiredness and irritability' which 22 ELECTRONICS Australia, January 1998 •

A view from the rear, showing the mains input and power switch — and the speaker terminals, just behind their respective output transformers. have occurred after listening sessions parallel 2A3 valves in each channel for a with other amplifiers (including some basic price of $940 plus postage. There using much more recent technology). are also single-channel `monoblock' After the listening session I brought kits, offering single-ended 15W (300B) the amplifier back to our lab, where EA or 8W (2A3) amplifiers for base prices technical editor Rob Evans ran the of $685 or $570 respectively, and a instruments over it, to see how it mea- push-pull 12W (6BQ5s or 6V6s) design sured up against the specification. And for a base price of only $375. as you can see from the table, it basical- Of course you can also buy valves, ly met the rated figures quite easily. transformers and other parts, to build up We'd tend to call it a 'twin 10W9 ampli- custom amplifiers of your own. So if fier rather than a 'twin 14.5W', but you're interested in exploring `valve that's largely a matter of preference sound', it's well worth contacting regarding acceptable distortion levels. Andrew to explore the possibilities. ❖ Of course by modern standards, the measured figures don't look all that . wonderful especially those for THD j SEC ON and IMD. But the reality is that in prac- KTS3OW VALVE tice, the distortion products generated in this type of amplifier are far less appar- Allows construction of < stereo amplifier using twin 300B triode ent to the ear than those produced by a valves in parallel (single ended Class modern solid state amplifier when it's A) in each channel, to give a nominal even approaching these measurable lev- 14.5W per channel. Includes a husky els (which would be right at overload). solid state power supply with the abil- The bottom line is, then, that an ampli- ity to adjust _tput valve bias levels fier like the KTS3OW represents a very for optimum t peration. good way to experience `valve sound' in : Low output imped- an almost definitive form. And because ance, flexible way to experience the KTS3OW is a kit, you can achieve authentic `valve sound'. this experience with less financial outlay Big and heavy, runs coupled with somewhat more personal quite hot. Relatively low power output satisfaction. There's also a great deal of for the price, compared with push-pull flexibility, as Valve Electronics can amplifiers. offer a choice of output valves, output From $1140 plus freight, with transformers and other parts. exact price depending on options. The price of the basic KTS3OW kit : Valve Electronics, 239 starts at $1140 plus postage, but for Australia Street (PO Box 467) those who want something a little less Newtown 2042; phone 02 draining on the bank account, Andrew 9557 2212 fax (02)9516s<:398 ~~-~:.::::;::_-_ Kay can offer a twin 8W design using WISSESINSIVISSISIESERIO