Construction Project: HIGH QUALITY STEREO USING VALVES - 1

Here's your chance to build a genuine, newly designed high quality stereo audio amplifier based on thermionic valves — the Stereo Eighty. It's not a rehash of an old project, nor a quirky and 'over the top' design that aims for perfection at any cost. The author has even organised the supply of all com- ponents that are nowadays hard to get, and is offering complete kits for less than $1000. What bet- ter opportunity could there be to find out for yourself whether the claims made for valve are true?

by TEAN Y. TAN, B.E.(Hons.) Recently, there has been a resurgence dynamics and greater ambience are United Kingdom. Many brands with dif- in the use of amplifiers among terms used by reviewers in ferent output tubes and power outputs are (more commonly referred to as `valve' describing valve amplifiers. Only the available in the market. Listen to one in a amplifiers) for domestic and studio en- most expensive solid state amplifiers better hi-fi retail outlet and you're likely vironments. Their re-emergence is large- are capable of attaining these charac- to be convinced. Of course valve ly due to the fact that many people teristics. amplifiers also tend to be expensive, believe valve amplifiers are capable of As an indication of the sonic quality of nowadays, due to the cost of the power producing music more accurately than valve amplifiers, a large number of them and output transformers which are their solid state counterparts. are now sold in both Japan and Hong sourced from overseas. Musical characteristics such as a larger Kong each year. These amplifiers are But in the project described here, all soundstage, better imaging, better also gaining acceptance in the US and of the transformers are designed and

Here's the author's prototype for the amplifier, built on a standard rack-type case as a chassis. The kits which will be available from the author will use a slightly different chassis, custom designed for the project.

34 ELECTRONICS Australia, September 1992 INPUT C7 (.450V) R22 .1.80VI C P1 H + IOk R28 R1 R2 220k 220k k F 100k 145 0V log 3 V3 I.18ov1 I•180vi T2 ALTERNATIVE I C3 R16 1. INPUT CIRCUIT R3 V2a V2b FB 0•L7jF! 22k R 1.00V R18 1k 2 7 (.90V) 1.70k 1 1.90V) 8n Cl R5 8 R6 V3 0 5 1PF 1M 941/1 1.94V1 1M BIAS POT 250V1 R7 Ln 100 100 0 BIAS POT R26 SPEAKER 1 Via V1b R19 10 INPUTC C7 470k R21 10V) CL R17 1k O 0.6811F 10 TO R28 RM I 0.14v) R10 2 2k 100k 510 4400V VL R14 1-46V) al BC546 R13

1N41L8 22k R15 R12 12k V1 : 12AX7/ ECC83 1k pF V2 12AT7 I ECC81 V3,V4 6CA7IEL34 a. 75V rvY,207sr Large / Chassis ground Small signal ground

Each channel of the stereo amp uses four valves, connected as shown In this schematic. VI and V2 are connected in a cascode long-tailed pair configuration, using Q1 as the common load. manufactured right here in Australia. In mistakes and convenience in terms of ques. All these factors have been addition to keeping the cost down and any future maintenance. Minimum taken into consideration when design- providing greater reliability of supply, hard- wiring is required between the ing this amplifier, and will now be briefly this has other benefits — it helps various components. discussed. Australia's balance of payments, and also The layout of the major components The valve amplifier has been around assists in keeping Australians employed. and PCB assembly are such that they are for a long time. Its application in the One of the aims of this project has very close to each other, with only short audio field received more prominence been to provide a high quality and an af- lengths of signal cable used throughout. after the publication in 1947 of the fordable stereo valve amplifier for audio This improves the overall sonic quality. technical paper `Design for High enthusiasts and music lovers. Imported The amplifier comes with an optional Quality Amplifier' by D.T.N. William- valve amplifiers are very expensive — volume control, so that a CD or DAT son, published in the UK magazine Wire- over $2500 — largely due to the import player can be connected to it directly if less World. duty and high mark-ups imposed by desired. The output speaker terminals can Williamson's paper highlighted the retail stores. This goal has been achieved, be configured as either four ohms or fact that he was able to improve linearity, and now there is an affordable eight ohms to suit the speaker type. The frequency response, phase shift and out- amplifier which is less than $1000 in normal position is eight ohms, to suit put resistance through prudent use of kit form, for those who enjoy building most standard voice-coil type speakers, negative feedback. their own units. A fully assembled and but four ohms would be more suitable for Since then, many more design varia- tested unit is also available. many electrostatic and ribbon speakers. tions on the `' have The amplifier is housed in a rigid case There is an on-off switch on the front been published. One of the more notable which is made of mild steel and has a panel, with a red LED to indicate the works published is that by Hafler and powder- coated black satin finish. The `power on' condition. For those who Keroes on the ultralinear amplifier. In paint is sufficiently thick for it to be ef- wish to ground their auxiliary equipment, essence, an amplifier operating in fectively scratch proof. there is also a ground terminal at the rear ultralinear mode is able to achieve high The transformers, valves and large fil- of the case. power output at low . Another ter capacitors are mounted on top of the The construction and adjustment publication by B. Hedge on the 'long- case. The smaller components such as details of the project will be explained in tailed cascode pair' talks about improv- resistors and capacitors are mounted in- the next article. ing the linearity and high frequency side the rase on a printed circuit board response of the input and driver valves. (PCB). All components are within easy Design background The topology of this present design's assess to the builder. This should enable The factors which contribute to the circuit embodies the principles estab- anyone with reasonable experience in as- sonic quality of a valve amplifier are cir- lished by Williamson, Hafler and Hedge, sembling electronic projects to build the cuit design, component quality and the but couples these with contemporary project easily, with a minimum of use of appropriate construction techni- design techniques. ELECTRONICS Australia, September 1992 35 stages is determined by the constant-cur- class AB operation results and a power Valve amplifier rent source formed by transistor Ql, output of 40W can be achieved from zener Z1 and diode Dl, and resis- each channel. Circuit description tor R15. This gives a total current of Output valves V3 and V4 have their The final circuit is one which provides 2.2mA. Because the transistor acts as a screen grids connected to taps on the out- the following advantages: current source, it therefore appears as a put transformer primary windings, in the 1. A push-pull cascode configuration is common AC cathode impedance of al- correct ultralinear configuration. Each used for the input and driver stages, most infinite value, for the two sections tap is at approximately 43% of its valve's which improves the linearity and of VI. half of the primary winding, which is the symmetry. This means that they are tightly point shown by Haller and Keroes to 2. A highly stable constant-current coupled in the so-called 'long tailed pair' produce minimum distortion and highest source is used to regulate the bias configuration, giving greatly enhanced output power, because of the `pseudo for the input and driver valves. common-mode rejection ratio. As a result pentode' operation. 3. The output valves are individually the common-mode component at the out- Although I recommend the ultralinear provided with adjustable bias, to put of the driver stage is greatly reduced. configuration shown, together with bias- allow crossover distortion to be The bias currents in both halves of V1 ing for class AB operation, some minimised. and V2 will not be exactly equal, because audiophiles may wish to experiment with 4. Negative feedback is derived from the two halves of both valves are general- other configurations and bias levels, in the same winding on each output ly not identical. As a result the voltage at order to determine for themselves the ef- transformer that is used to drive the pins 1 and 6 of V2 can vary by between fect on sound quality. speaker, to reduce overall distortion. 5V and 20V; however this is nota matter One possibility is setting the biasing so 5. The amplifier uses the ultralinear of much significance as these points are that the output valves operate in class A configuration and class AB mode of biased at about 180V In any case, mode. The effect will be lower distortion, operation, to maximize power out- capacitive coupling is used to transfer but also lower power output a maximum put and minimise distortion. between the plates of the driver of 25W total per channel. The operating 6. The power supply provides a high stage and the output valve grids. life of the output valves will also be con- value of reservoir capacitance, to The push-pull output stage of each siderably shortened. improve the dynamic response. channel uses two high-power Another possibility is to change the Refer to Figs.1 and 2 for subsequent EL34/6CA7 pentodes (V3 and V4). output stage configuration, so that the explanation of the circuit. These are individually fed with adjus- valves operate in mode. This is The two halves of V1, a 12AX7/ table fixed bias, using four preset poten- done by simply connecting the screen ECC83 `medium-mu' low noise twin tiometers (two per channel). This allows grids (pin 4) to the plates (pin 3), instead triode, form the input and feedback the quiescent current for each pair to be of connecting them to the transformer mixing stage. These are directly coupled balanced, for optimum performance. taps. Again, the power output will be to the two halves of V2, a 12AT7/ The quiescent current in each output considerably reduced, in either class A or ECC81 twin triode, which forms the valve is typically set for between 25 class AB mode, compared with the push-pull driver stage. As can be seen the and 30 milliamps, depending on the ultralinear connection. two stages are connected in series, using desired compromise between perfor- The triode configuration is strongly the `cascode' configuration. mance and valve life. With each valve's favoured by many valve amplifier The total bias current I through both bias set for a quiescent current of 30mA, manufacturers, including Audio Re-

T7 I ca4V B• (480V) + C10 F 1.5H 7 0k UU~~ Fl 350V ▪~e 2505~ s 450V 1W lA B1 1C9 +~ IC111 10A/1000V 47,uF 4R700 680 450V 1W 25V T T I P2 - P5 50k 0.25W 240V 02 ~n MULTI -TURN MAINS 75V TRIMPOT INPUT 1A/400V iR2G 57V Tire10}IF 0k I (-515) ffirt EO C12 BIAS V3 HEATERS 4 60yF /s57 6 3 RIGHTV RIGHT CHANNEL

6 3V HEATERS 1 -2351 BIAS V4 LEFT ► RIGHT CHANNEL R23 10k v. BIAS V3 LEFT BIAS V4 — LEFT

Here Is the power supply circuitry for the complete amplifier. bridge B1 is used to produce the high tension voltage, while diode D2 is used to generate the negative bias for the various valves. Preset pots P2-P5 are used to set the bias on each of the output valves, and ensure that each output stage is balanced.

36 ELECTRONICS Australia, September 1992 search Corporation. With this design, Components R7, R8, R11, R16 and The power supply (Fig.2) uses a the builder is free to experiment with R17 are `stoppers', used to ensure single untapped HT secondary winding the different output configurations and stability and reduce the amplifier's sen- on the transformer T2, with a voltage of bias conditions, in order to achieve the sitivity to RF interference. Similarly 350V. This feeds high-voltage silicon best compromise in terms of desired resistors R20 and R21, in series with the bridge rectifier B 1. Series-connected sound quality and valve life. output valve screen grids, are included to capacitors C8 and C9 are used for the Despite this flexibility, the circuit is improve stability and minimise distor- capacitor-input L-C filter, which uses in- quite straightforward. Under no- signal tion. The latter have a nominal value of ductor Ll (1.5H) to provide excellent conditions, the grids of both halves of V1 lk, but those who require greater power hum filtering. Output reservoir capacitors are very near ground potential. The input C10 and Cll provide a total of around signal is fed to the grid of Vla, while the 340uF of capacitance, to cope with peak negative feedback signal from the output MEASURED PERFORMANCE current requirements and achieve good transformer secondary is fed back to the Frequency response dynamic response. grid of Vlb via the divider R13/R10, (1W to 8 ohms): Resistors R29 and R30 are used to en- with components R12 and C6 used to 3Hz - 40kHz, +4-3dB sure that the capacitors share the applied shape the phase response and ensure loop Power bsr~dwidth DC voltage equally, despite differences stability at high frequencies. More about (30W into 8 ohms): in leakage current. these shortly. 10Hz - 35kHz (- 3dB) Negative bias for the valves is The 12AX7/ECC83 has a medium Signal to noise ratio produced from a separate 57V trans- and low noise, making (relative to 30W): former winding, with a half- wave rec- it ideal as an input valve. Its transconduc- 93dB tifier using diode Dl and capacitor C13. tance variation from unit to unit is not as Maximum '6ugput R-C filtering can be used here, due to the (both channels driven, into 8 ohms): widespread as with 6DJ8/ECC88, which 38W RMStchånnel very low current requirements. Using has higher transconductance. This Input sensttivlty: readily available 100uF capacitors for enables the circuit to have a consistent 700mV input for, 35W output C13 and C12, together with a 10k resis- open loop . The transconductance of into 8 oknio (Oset approx. 28) tor for R24, we are able to maintain each half of V1 is about 1.6mA/V. Damping factor the hum on the bias supply to a very Each plate of V1 is directly coupled to (1kHz, 8 ohm load): low level. the of the two halves of driver 2.57 (Output valve Iq'a Resistor R23 is used to limit the range stage V2, which has its grids grounded 3.76 (Output valve lq 60mA) of the output valve bias adjustment pots for audio frequencies via Cl and C2. In Totathannonio distortion: P2-P5, to make adjustment easier and this configuration, the effective AC load Power level Output.valve Output valve also prevent the risk of damaging the val- for each half of V1 is close to the physi- Frequency lq = 30mA lq=69mA ves due to accidental removal of bias. cal driver load resistor (R1 or R2), in 1W at 1kHz 0.12% 0.03%• The input valve current sources, based 3W at 1kHz 0.20% 0.05% parallel with the top resistor of each 10W at 1kHz 0.70% 0.18% around Qi, are supplied directly from the driver's grid divider (R3 or R4), and also 30Wffi lkHz 2.30% 0.70% negative rail available across C13. the bias series resistor for the associated 35W at ikHz 2.70% 0.95% The valve heaters for each channel of output valve (R18 or R19). As a result 30W at4OHz 3.6% 1.6% the amplifier are powered from separate the voltage in of each input stage/driver OM* I.OkHz 3.1% 1.8% • windings on the power transformer, to stage pair is about 100, measured at distorts minimise any possible crosstalk Each points A and B. To improve linearity in (two tones, 4:1 ratio): winding has a centre tap, which is ear- the circuit, the gain at points A and B Power level Output valve Output valve thed to ensure low hum coupling into the should be equal. Iq = 30mA lq = 60mA valve cathodes. lOW . 2.4% 0.3% 30W 6.0% 1.3% Finally, a few words about grounding. Low feedback Some care must be taken in grounding The benefits of negative feedback are any valve or transistor amplifier. Here a widely known and will not be elaborated output can reduce their value to 470 `star' grounding technique is used here. The circuit configuration used ohms if desired. throughout. The PCB ground, power employs a low feedback design of less Although the grid of Vla is nominally supply ground, chassis ground and mains than 8dB (return difference). The feed- at ground potential, coupling capacitor earth are all connected at a single point back is taken from the secondary wind- C7 is used to prevent any possibility of on the tagstrip supplied, which has one ing of the output transformer, and results noisy volume control operation due to leg screwed to the case/chassis. in an overall gain for the amplifier of grid current. Using this method, ground loop hum is about 25 - 28dB. Resistors R25 and R26 in series with extremely low at the speaker output ter- It is the author's belief, based on ex- the output valve cathodes are basically to minals, and virtually inaudible. Note that perience, that a low feedback amplifier allow convenient monitoring of each the input stage grounding is decoupled has a more `open' sound. valve's quiescent current, using a DMM. from that for the rest of the circuit via Since this is a low feedback design, Since the resistors have a value of 10 R27, to minimise earth loop currents. circuit stability is not a real problem. ohms, they will have a voltage drop of The grounding technique used will be Capacitor C6 and resistor R12 are used 100mV for each 10mA of quiescent cur- discussed again in the next article. for `lead' compensation, to improve rent. With this low value the resistors do stability at high frequencies. However not make any significant contribution Component quality for those builders who prefer an extended to valve performance, although in As mentioned earlier, components play high frequency response these com- theory they will introduce a small an important role in the sound quality of ponents can be removed without affect- amount of negative current feedback any amplifier. Here good quality com- ing the overall stability. and protective self-biasing. ponents are used throughout, without ELECTRONICS Australia, September 1992 37 Valve amplifier

going `overboard'. If the very best com- ponents were used, then the amplifier would not be within reach of most music lovers. But by making a sensible choice of components, we are able to achieve good sonic quality without allowing the cost to escalate unduly. Metal film resistors are readily avail- able nowadays, at a very reasonable price. While there are even debates among audiophiles on the type of metal film resistors which gives the best sonic quality, in this project we use the normal type — i.e., no special brand. These resistors give a cleaner sound over their carbon counterparts. A rear view of the amplifier, showing the input connectors at far right, the speaker The capacitors are the most critical terminals in the centre and the mains input connector at the left. The power and components which affect the sound output transformers are locally manufactured and of high quality. quality. It is not advisable to use polyester capacitors for signal cou- logarithmic potentiometer is available. mance. In fact in some areas, it was pling, because of their high distortion. This pot is especially matched to enable even judged to sound better. A com- The normal choice is polystyrene for the same output to be achieved from both ment from one reviewer was that it has low values and polypropylene at channels. Also included is a volume con- 'the romantic sound of a Leak, but with higher values. The latter have been trol knob. better bass response'. chosen for this project as coupling No doubt there is still room for im- capacitors, while polycarbonate Sound quality provements in the circuitry, so as to im- capacitors are used for bypassing. Now, the question that by now, prove the sonic quality. This could well The 12AX7, 12A17 and EL34 valves everyone will be asking. how does it become a subject for further discussions have been chosen for their sonic quality, sound? The attractive feature of this in future issues of Electronics Australia, while at the same time being readily valve amplifier is the `warm' sound, with if there is sufficient reader interest available off-the-shelf. emphasis on the midrange. (Editor's Note: Mr Tan loaned us a The prices of these valves are While it may lack the deep bass prototype of his amplifier for a couple of reasonable (by today's standards), with response of some transistor amplifiers, weeks, and we were able to both test it the 12AX7/12AT7 costing about $10.00 the latter cannot give the same degree of with our lab instruments, and conduct each and the EL34 around $18.00 each. rich warm sound. Mid-priced transistor listening tests in our homes with suitable The circuit is designed such that the out- amplifiers can also tend to sound `flat', speakers and signal sources. The loan put valves should last for a long time, if by comparison. unit was also used to take the the bias is set for low quiescent current Without being at all biased about this photographs. class AB operation. amplifier (after all, I am its designer!), The results of our measurements are The critical component in any stand- the following claims are made: shown in the table. As may be seen, it ard transformer-coupled valve It offers a `clean' sound, without being gave a good account of itself even by amplifier is of course the output trans- too clinical through the use of quality modern standards, with wide frequency former. The output transformers used in resistors and capacitors. and power response, a good signal to this project are designed and manufac- The midrange is warm and rich, noise ratio and genuine 38W RMS output tured in Victoria, Australia. terms synonymous with traditional per channel into 8-ohm loads with both They have a very impressive specifica- valve technology. channels driven. tion, with a frequency response of 13Hz The bass response is much fuller than Understandably, for this kind of — 60kHz +/-1.0dB. A grain-orientated many traditional valve amplifiers. Al- amplifier, the total harmonic distortion iron core is used to give this frequency though not as powerful as some transistor and intermodulation figures were both response. Under testing, each trans- amplifiers, it blends in well with the quite dependent on the quiescent current former can deliver full rated power lower midrange. level chosen for the output valves. In- (35W) from 23Hz to 20kHz, without ex- The treble response is well behaved creasing the quiescent current levels to cessive distortion. and can sound a bit bright on some 60mA for class A operation reduced The power transformer comes from the speakers. But after a sufficient warm- the distortion by a factor of four same local source, and is a husky unit ing up period, the brightness tends times, for example. designed to provide more than enough to disappear: However this will undoubtedly shorten power for both channels. The total power The overall sound brings out the vocal the life of the valves, as the author points rating for the transformer is 250VA, qualities in many recordings. It is not em- out — even though the dissipation of which gives a capacity for at least 100VA barrassed by recordings with wide each valve (30W) is still within the offi- per channel, plus the power for the dynamic range. cial ratings. heaters which is another SOVA. In an A-B comparison with a commer- The design therefore seems a good one, For those who require a volume con- cial valve amplifier which cost over in that constructors can decide for them- trol, a high quality dual-ganged $4000, it gave very comparable perfor- selves how much they are prepared to 38 ELECTRONICS Australia, September 1992 'pay' for lower distortion, in terms of Obtaining the kit reduced valve life. A complete kit for the Stereo Eighty In our listening tests, the Stereo amplifier can be obtained from Contan Eighty provides some very pleasant Audio, of 37 Wadham Parade, Mt Waver- and relaxed listening. It can drive ley 3149; telephone (03) 807 1263. reasonably sensitive loudspeaker sys- The price for the kit, complete except tems with ease, and provides very for the special volume control poten- smooth and well-balanced sound. With tiometer and knob, is $999 plus most kinds of programme material postage/freight if applicable. If the there is very little, if any, `edginess' due volume control is required as well, the to distortion — even with the output price is $1035 plus postage. These prices valves running at only 30mA quiescent include sales tax. current. No doubt this is due to the If required, the amplifier can also be relatively `soft' nautre of the distortion supplied in fully assembled and tested produced by a valve amplifier, with form. In this case it is priced at $1249 predominantly low-order harmonics without volume control, or $1284 with being generated. volume control. These prices again in- Only with more complex programme clude sales tax,, but postage/f eight must material like that produced by a full or- be added if applicable. chestra and/or a choir, could we really Contan Audio can also supply in- detect the effects of THD and IMD, at the dividual' components such as output lower current setting. However our im- transformers, power transformers, chas- pression is that even this becomes vir- sis, valves and so on. Prices for these tually inaudible when the quiescent items are available on request. All parts currents are increased. are guaranteed for one year, except the In short, we're very happy with Mr valves which are only covered for six Tan's design, and delighted to present months and against manufacturing it to our readers. We believe he has defects only. provided today's audiophiles with an In the second of these articles, the as- excellent way to build a flexible, high sembly and adjustment of the amplifier quality stereo valve amplifier, at a will be described. reasonable cost and with minimum (To be continued) o'~ hassle.) Construction Project: HIGH QUALITY STEREO AMPLIFIER USING VALVES - Z

In the first of these articles, the author explained the philosophy behind the project, and described in detail the circuit design and operation. This month he moves on to cover the construction and testing of the amplifier. As he explains, the design is quite straightforward and should not give any problems providing you follow the steps described. by TEAN Y. TAN, B.E. (Hons.) To ensure that constructors don't ex- includes a special chassis, manufactured become apparent under the section on perience any problems getting the parts in Australia. safety precautions. to build this amplifier (some of them are I estimate that builders with some con- The kit can only be obtained directly now rather harder to obtain than they struction experience should not take from Contan Audio; for further details once were!), I have made arrangements more than 10 hours to build the amplifier. please refer to the end of this article. to source them myself, and supply them The tools required for the construction direct to readers as a kit. are a soldering iron, solder, screwdrivers, Safety precautions Only the highest quality components a small adjustable spanner, a multimeter It must be strongly emphasized at this and parts are supplied, and the kit is com- and a jumper lead with alligator clips at point that builders must take due care plete as shown in the parts list. It even both ends. The use of the last item will when constructing this project. Unlike

Here is a general view looking under the chassis. The output and power transformers are visible at the bottom, near the rear, with the filter choke at upper right. The PC board at upper left supports all of the low level circuitry.

34 ELECTRONICS Australia, October 1992 other amplifier projects using solid state 5. Switch off the amplifier before By tackling the assembling in stages components, the present design involves making any adjustments — e.g., and testing as you go, any problems very high voltages — 350V AC on the changing components, or correct- which may arise are likely to be isolated secondary side of the power transformer ing any faulty soldered joints in and located easily, before they result in and as high as 500V DC for some of the the amplifier. any component damage. plate and screen wiring and components, 6. Do not plug in any valve when the To help you in assembling the quite apart from the usual 240V AC on power is ON. amplifier, we are providing a diagram the primary side of the transformer. Always turn the power OFF before showing the location of all parts mounted All of these voltages are potentially plugging in any tube; if necessary .on the PCB (Fig.!), plus another showing very dangerous. check the B+ voltage before plug- the wiring between the PCB and all of The following precautions should ging it in. • the components mounted separately therefore be taken before applying mains 7. Do not make measurements unless (Fig.2). These plus the photographs voltage to the amplifier, and making sub- you are wearing suitable footware. should make the assembly quite sequent measurements: This means ideally rubber or plastic straightforward. 1. The mains earth should be connected soled shoes, although leather-soled to the chassis, in a reliable and shoes are suitable if you are on a securelashion. perfectly dry floor. STEP 1: 2. All of the OV terminations shown on. The builder should make sure that he Mechanical assembly the schematic as going to chassis or she takes the above safety precautions. First of all, mount the two pairs of ground should be connected to the The project is designed so that if con- octal sockets for the output valves (V3 chassis. This will be elaborated structed as shown, you should be able to and V4) underneath the chassis, with the upon in the next section. measure any voltages on the amplifier socket rings and screws provided. Note 3. The correct fuse must be inserted in without removing or touching any com- the correct orientation (Fig.2). the IEC socket fuseholder. ponents, parts or the PCB. All the vol- Then mount and tighten the following 4. Use only one hand when measuring tages are within easy access. components to the chassis: any voltages preferably the right A. The front panel. hand. The left hand should not be B. The RCA input terminals. touching any part of the amplifier, Construction C. The earth screw terminal. or any earthed metal objects. The You are encouraged to build this D. The speaker terminals. common terminal of the multimeter amplifier step by step, using the follow- E. The IEC captive power plug and is connected to the chassis ground ing procedure to reduce the risk of fuseholder. using the alligator jumper lead. making any mistakes. F.The power switch.

Here's a close up of the PC board assembly, with the four output valve sockets lust below it. Note the way that the valve heater wiring is twisted together, to minimise any external field and prevent hum injection into the signal circuitry.

ELECTRONICS Australia, October 1992 Valve Amplifier - 2 mount about 3mm up from the sur- complete the HT supply wiring in- face of the PCB. volving C8, C9, C10 and C11, Ll, B. The valve sockets. These mount on R29 and R30. G. The volume control pot, if you are the reverse side, orientated as Double check your wiring, to make fitting one. shown in Fig.1 so that the pins pass sum you have not made any errors. H. The output transformers, together through the holes in the PCB and Do NOT connect the output of this with their protective covers solder to the pads on the copper side. supply to the PCB, as yet. (provided). Refer to Fig.2 for the C. The BC546 (one per The following step is advisable, but not correct transformer orientation. channel — Q1). compulsory; it involves powering up the J. The power transformer, again with D. The bias trimpots P2 - P5. Fit the amplifier at this stage, to check that the its protective cover. Fig.2 gives the centre two, P3 and P4 first, then the correct HT voltage is being produced. If correct orientation. outer P2 and P5. Each trimpot you do this, first connect your multimeter K. The 8-way terminal strip, for sup- should be mounted with its adjust- carefully between the chassis (earth lug porting C8 and C9, etc. Make ment screw towards the front panel of the tagstrip) and the `+' side of C10, sure that you scrape a small and C12/C13. with the multimeter set to its 1000V DC amount of the lacquer away from E. All of the electrolytic, coupling and range. the inside of the chassis, around bypass capacitors. Take care with Then connect the mains icord, and each mounting hole, and use a `star' the polarity of electrolytics C5 (x2), apply the power. The meter should read lockwasher between each foot of C12 and C13 — this is shown clear- approximately 500V, if all is well with the tagstrip and the chassis, so that ly in Figl. your wiring. If the voltage is correct, when the mounting screws are With everything thus fitted to the remove the power and allow the. tightened, there is a really good PCB, you should now make a final capacitors at least 20 minutes to dis- metal-to-metal contact via the lock- check that everything is in its correct charge before proceeding further. washers and bared metal. place and orientated correctly. The only way to avoid this wait is to fit L.The filter choke Li. This mounts on Then you are ready to mount the com- an additional temporary discharge resis- the end of the chassis; ensure that plete PCB assembly under the chassis, tor from the `+' terminal of C10 to chas- the terminal connections are located with the components facing inwards and sis, BEFORE applying the power for the underneath and not exposed. the valve sockets passing through the test. M. The main reservoir electrolytic clearance holes in the chassis. A 100k,1 watt resistor used in this way capacitors C10 and Cll, with the Use the sticky tape provided as will reduce the capacitor discharge time rings and screws provided. Mount spacing, and fasten the PCB to the down to about two minutes. Don't forget the rings underneath the chassis. chassis firmly with the machine screws to remove this resistor, though, after it N. The main rectifier bridge Bl. This and nuts provided. has done its job. mounts under the chassis between By the way, while you are carrying out C10 and Cll. the above test, you can check that the Before mounting, bend each lead STEP 3: On-Off switch is illuminated when carefully about 8-10mm from the Hard wiring power is applied. body, into a small loop (say 2mm You should now be ready for the `hard' 3. Wire up the output valve heaters, for ID), to allow easy soldering of the wiring — that which connects between V3 and V4 in both channels.'Con- connection leads. Then cut off any the major items, and between the PCB nect lugs 2 and 7 of each V3 socket excess lead length. The bridge at- assembly and the rest of the components. to the same lugs on the V4 socket in taches to the chassis using a single The suggested sequence of wiring is as the same channel, and then connect machine screw, nut and lock washer. follows. Note that pairs of wires should pins 2 and 7 of the socket nearer the This completes the assembly of the be twisted together where appropriate, power transformer, for each pair, to main items, apart from the PC board. and that although the description below the outer lugs of one of the 6.3V describes the wiring for one channel, heater windings (i.e., the '0' and STEP 2: both need to be wired up. Fig.2 should be '6.3V' lugs). In other words, the PCB assembly used as a guide, along with the heaters for the output valves of the A single PCB measuring 255 x 85mm photographs. right channel go to one winding, is used to mount the majority of the A. First fit the 240V AC wiring be- and those of the left channel to the smaller components, along with the tween the IEC plug/fuseholder, the other winding. sockets for the input and driver valves On-Off illuminated power switch Note that the centre-tap (CI') of each (V1, V2) for each channel. Note that the and the primary connection lugs of heater winding on the transformer must components are mounted on the same the power transformer. Make sure be connected via a short insulated side as the tinned copper tracks, except that this wiring is all in suitable wire, to the `chassis ground' lug of the for the valve sockets which are mounted cable, with mains-rated insulation. main tagstrip. The heaters for Vl and V2 on the reverse (top) side. Also fit the earth lead (green or yel- of each channel am then connected to the With reference to the circuit diagram low/green insulation) between the output valve heaters for the same chan- and Fig.1, fit and solder the components IEC plug's `E' lug and the earth lug nel. in the following sequence: at the Ll end of the terminal strip, Lugs 4 and 5 of each V 1/V2 socket are A. All of the resistors, and zener so that it makes a reliable connec- connected together, and connect to say diodes. Note that the power dissipa- tion to the chassis. lug 2 of V3 or V4, while lug 9 of V1/V2 tion for R1 and R2 exceeds 0.3 2. Connect the high voltage (350V AC) connect to lug 7 of V3 or V4. watts, under quiescent conditions, secondary winding of the power All of the heater wiring is shown in so for adequate ventilation, bend the transformer to the bridge rectifier basic form in Fig.2. However it is very leads of these resistors so that they B 1, as shown in Fig.2, and then important to twist together each pair of

36 ELECTRONICS Australia, October 1992

CHOKE 1 POWER TRANSFORMER( IEC SOCKET 0 A N E WITH FUSE HOLDER

2L0V A N C8 .2L0 ON/OFF SWITCH 0 Bl 6 3%. C9 •350 AC • 0 CHASSIS CT GROUND • .55 6.3 TERMINAL STRIP C11

R/C,0 63 L/, / R/C,V3 p-188 ohm red L/C,0 ~f RIGHT CHANNEL RIGHT CHANNEL PCB 0/P TRANSFORMER SPEAKER PC B TERMINAL GNO CHASSIS GROUND'IS"""1 —1 Corn block '+wAz ! R20 i+ SC? OP1 VL~ 0

OP2 V1 8 ohm red _R10 R21 F/B. SC1 LEFT CHANNEL SPEAKER F/B TO TERMINALS R/C. PCB Inbu R16 V3,0 Corp block R17 2 R25 65 O.P gt3 CHASSIS P3® R 6~'~ GROUND 6.3 PL ($:3 .__ R 6 P5 LEFT CHANNEL 0/P TRANSFORMER

R1' CHASSI R16 GROUNI 60 A2 R10 R20 ~+• SC2 OP1 F/B 0 ~ B VL,© OP2 TO RIGHT CHANNEL R21 R28 INPUT VG SCISCS F/Ba R/C m, 4 ~~_• Al . 6C) TO PCB TO LEFT CHANNEL R28 INPUT L/C

Miss u GND

Use this overall wiring diagram as a guide in making the connections between all of the main items, and also between the PC board and the rest of the circuit Note that for clarity, the heater wiring is not shown here twisted together. Note too, that the wiring for the optional volume control is not shown — but is clearly visible in the photographs.

ELECTRONICS Australia, October 1992 37 Valve Amplifier - 2 modern speakers are nominally of ther I suggest that you check the above eight ohms. steps again to ensure that nothing has For eight ohm operation, connect the been missed. (insulated) wires used to make the two secondary windings of each output various heater connections, from the transformer in series, as shown in Fig.2. Testing time! power transformer to the output valve For two ohm speakers, the connections Now it is time for you to test whether sockets, between the output valve sock- should be connected to parallel — that is, everything has been wired correctly. As ets and to the V1 and V2 sockets. This is connect '0' to '0' and `OP!' to 'OPT. discussed earlier, the safe method to per- necessary to prevent induction of hum Once you've connected the two form any measurements is to have the into the signal circuitry. output windings together in either common terminal of the multimeter con- 4. Connect lug 3 (plate) on each V3 series or parallel, then wire them to the nected to chassis ground, via the aligator socket to the `A2' lug on its cor- speaker terminals as shown, and connect jumper lead. responding output transformer; then each `common' speaker terminal to the First, plug in the valves V1 and V2 for connect lugs 4 (screen grid) on each chassis earth. both channels. DO NOT plug in the V3 valve socket to the matching 8. At this stage, DO NOT wire the power tubes just yet. Then apply power `SC2' lugs of the transformers, via feedback (F/B) lugs of each out- to the amplifier, and the following vol- resistors R20. When this is done, put transformer to the PCB. This tages should be checked withr the help of connect lugs 3 of the V4 sockets to is done later, after troubleshooting the circuit diagram: the `Al' lugs of the matching output the circuit. 1. The B+ (at say the + terminal of transformers, and fmally connect 9. Connect the `B+' pad at each end of C10) should be about + 495V lugs 4 of each V4 socket to the the PCB (next to R22) back to the DC +/-10V. remaining 'SC1' lugs respectively, positive terminal of reservoir 2. The B- (at the junction of C13, via resistors R21. Again these con- capacitor C10. Also connect the '0' D2 and R24) should be about -77V nections are shown in Fig.2. pad at the centre rear of the PCB DC +/-2V. 5. Connect lugs 5 (control grid) of each back to the chassis ground lug of 3. The emitter voltage at each transistor V3 socket to R16 of each channel, the tagstrip. Ql (where it connects to R15) on the PCB, and lugs 5 of each V4 10. Connect '55V' and, '0' pads on the should about -50V DC +/-2V. socket to the corresponding R17. PCB (between C12 and C13) to the 4. Pins 1 and 6 of V2 should about Then connect lugs 1 (suppressor similarly marked terminals on the +220V DC +/-10V DC. Note that grid) and 8 (cathode) together on power transformer. this voltage will change when the each V3 and V4 socket, and connect 11. Connect the RCA input connec- power tubes are plugged in, as the each V3 socket's lugs to the R25 for tors to the PCB inputs, using B+ drops. As discussed in part one, that channel (on the PCB), and each shielded leads (outer braid to the difference in voltage between V4's lugs to the corresponding R26. earth). Note that when the pin 1 and pin6canbeashighas 6. Connect the `B+' lug of each output volume control pot is not fitted, 20V DC, due to mismatch of the val- (O/P) transformer to the positive ter- resistors R28 are connected across ves. By swapping the valves minal of C10. the PCB input lugs along with the around, it is possible to minimise 7. The output transformers are op- leads from the input connectors. this voltage difference. timized for either eight ohms or 12. Connect the earth terminal to the 5. The bias voltage for each output two ohm loads. (For those who chassis ground. valve should be adjusted via the ap- require four or 16 ohms, please 13. Connect the copper flux reduction propriate trimpot. Pots P3 and P5 ask for a different transformer straps of each transformer (three control the bias current for the V3 type.) It is believed that the total) to chassis ground, also. valves, while pots P2 and P4 control transformers supplied will cater The amplifier should now be fully the bias for each V4. Before plug- for over 90% of users, as most assembled, but before going any fur- ging in the output valves, the pots

Placement of all of the parts In their correct places on the PCB should be a straightforward Job using this overlay diagram as a guide. Note that all components except the valve sockets are mounted on the copper side.

38 ELECTRONICS Australia, October 1992 should be adjusted such that the the polarity of all electrolytics (espe- 6.3V AC between pins 4 and 5 and voltage at lug 5 of each output valve cially C8, C9, C10 and Cll), and pin 9, for each of valves V1 and V2 socket measures approximately - the heater wiring. in each channel. Also check that the 44V DC. This is to set the bias cur- bias current is correct, by checking rent for each output valve to not the correct orientation of zener more than 40mA when they are PARTS LIST diodes Z1 and signal diodes D1, and plugged in. Resistars each transistor Ql. The voltage When you have performed all of the R1,R2 220k 1W 5% carbon across each resistor R15 should be above tests and adjustments, you are R3,R4,F15,F R9 about 27V DC +/-0.5V. ready to turn off the power and plug in 1M 0.25W 1% metal film 3. If positive feedback occurs when R7,R8 100 ohms 0.25W 1% metal film the output valves. Then you can turn on R10 510 ohms 0.25W 1% metal flim you connect the feedback loops, the power again, and after waiting for R11,R27 10ohnis 025W 1% meal flim then change over the wiring at the them to warm up (allow say one Rt.2s -1k 015W 1% metal flim 0/P transformer. That is, swap the minute), re-check the main voltages. R13 22k 0.25W 1% metal film wires connecting to the Al and A2 With the output valves plugged in the R14 . 5ök0.25W 1% metal film. lugs, and also those connecting to 1415 't2k 0.25W 1% metal film B+ at C10 will normally have dropped to RAWy 2.2141 W 5% carbon the SC1 and SC2 lugs. Normally about +475V DC, while the voltage at 1418.1319 479k 0.25W 1% metal film this should not be necessary. pins 1 and 6 of each V2 should be be- R20,R2t: lk tW 5% carbon tween about +180V and +200V DC. R22,R23;11241 Listening tests 10k 025W 1% metal film Trimpots P2, P3, P4 and P5 should R25,R26:10 ohms 1W 5% carbon Now that you have finished the con- now be adjusted so that the quiescent R28 •. -.100k 0.25W 1% metal film struction and testing of your amplifier, it cathode current for each output valve is R29,ä. 4 1 w 516 carbon is time to reap the reward and listen to it. set initially to about 30mA. The builder Cipighltrif At normal listening levels, the can experiment with other current levels C1:,C2 luF 250V polycarbonate sound should as described in part later, once the amplifier is operational C3,C4 0,47uF 400V polypropylene One. Otherwise, there is something (although I suggest that you do not use C5,C8,C947UF 450V electrolytic wrong with your construction. C6 39PF ceramic currents higher than 60mA, as this would C7 0.68uF polypropylene The slight brightness reported in part considerably shorten valve life). G10,C1:1,680uF 250VW electrolytic one of these articles is mainly due to the The easiest way to monitor the (chassis mtg) output valves. After replacing them the cathode currents is by measuring the C12,C13 100uF 160V electrolytic brightness should disappear altogether. voltage at each valve cathode (lug 8 Sankdad uctoss The sound is now more balanced and as a on each V3/V4 socket) — this reflects D1 : ` IMAM N4148 signal diode result there is better imaging. the voltage drop across the 10 ohm D2 ' '1114004 1A/400V rectifier For those builders who like to experi- cathode resistors R25/R26. Bl ' 9R101010A/1000V bridge ment, there are a number of options Z1 114971B 27V/400mW zener For 30mA, each trimpot should be ad- 01 BC546 NPN transistor worth exploring. Examples include set- justed to produce +300mV at the ap- ting the output valves for Class A opera- MONO propriate cathode lug. Make sure you V1 12AX7/ECC83 dual triode tion and connecting them for triode check and if necessary adjust the voltage V2 12AT7/ECC81 dual triode operation. These options were discussed at each of the four cathodes. V3,V4 . 6CA7/EL34 power pentode in part one. Free advice will be given to If all is well so far, you are now Aikleeliarrealia those who purchase the kit. ready to connect the negative feedback to Tt •- Power transformer, with I hope you will enjoy building and each amplifier channel. Turn off the T2 Output transformer, with listening to this amplifier, as much as I power, wait 30 seconds or so and then Lt Filter choke, 1.5H have enjoyed designing, testing and P1 ed 100k bg pot, connect the `F/B' lug on each output . Dumåtr å manufacturing it. transformer to resistor R13 for that chan- P24,5 50k 0.25W multi-ttun timpot nel, on the PCB. Fl ' IEC captive mains plug Obtaining a kit Use insulated hookup wire, of course. with lA fuse As noted in the first article, kits for the SW1 SPST 250V mains switch Then turn the power back on. If there are ,with neon Stereo 80 valve amplifier are only avail- no funny noises from the output trans- Four Vital valve sockets; four miniat ire able from Contan Audio, of 37 Wadham formers, then the polarity is correct. 9— valve sockets; four insulated screw Parade, Mount Waverley, Victoria 3149; Otherwise there is a 180% phase shift be- tams nAls(two red, two black); two RCA phone (03) 807 1263. tween the output and input, resulting in audio sockets; one screw terminal for The price of the kit, including all parts earthing; mains cord with 3-pin plug and positive feedback. IEC socket; one 8-lug tagstrip for mount- and valves, is $999 plus postage without C8;.C9etc; hookup wire, shielded volume control, or $1035 with the spe- Troubleshooting mounting clips for large cial matched dual-gang volume control. If the above voltages are not right, , nuts, bolts, lock washers, These prices include sales tax. check the following: solder etc. If required, the amplifier can alterna- 1. If the B+ line measures zero volts, tively be supplied fully assembled and check the mains fuse. If the fuse is If the B- line measures a positive volt- tested for $1249 without volume control, there but has blown, you may have age instead of the correct negative volt- or $1284 including volume control. a short circuit somewhere or have age, you have probably wired D2 the Individual parts for the design are also made a mistake in your connections. wrong way around. This may have available, such as the output and power If the fuse blows again, when you caused damage to either D2 or C13. transformers, chassis, valves etc. Please replace it, there is almost certainly a 2. If the voltages at pins 1 and 6 of V2 ring for prices. All parts are guaranteed short somewhere. Check your are too high, check that your heater for one year, except for the valves which wiring around the rectifier bridge, wiring is correct — there should be are only covered for six months. ❖ ELECTRONICS Australia, October 1992 39