A DIY Hi-Fi Supply Ltd Product

Joplin

Push-Pull 2A3 Vacuum Tube Amplifier

Builders Guide DIY Hi-Fi Supply

www.diyhifisupply.com

Contents

Chapter 1 – IntroductionChapter 2 – Before You BeginChapter 3 – UnpackingChapter 4 – Identifying the Fitted PartsChapter 5 – Fixing the HardwareChapter 6 – Mains Transformer PrimaryChapter 7 – Mains Transformer SecondaryChapter 8 – Power Supply PCB ConnectionsChapter 9 – Socket Wiring and PartsChapter 10 – Signal-in Wiring Chapter 11 – Output TransformersChapter 12 – TestingChapter 13 – Set-upChapter 14 – TroubleshootingChapter 15 – Final Word and UpgradesAppendix -- 45 Modifications – Circuit Diagrams

Chapter 1 – IntroductionThe Joplin amplifier uses 2 x 2A3 DHT (directly heated triode) tubes per channel in push-pullconfiguration typically providing 9W per channel class A amplification. The first stage in theJoplin uses a single 6DJ8 (ECC88) tube directly coupled to a phase splitter circuit, morecommonly known as long tailed pair, using the 5687 (ECC99) tube. This single 6DJ8(ECC88) tube at the input is a dual triode device and is shared between the left and rightchannels, one half of the tube being used for each side. A full 5687 (ECC99) dual triode tubeis used per channel for the second stage.

Choke smoothing is employed for the HT B+ supply providing a stable and filtered powersupply. AC heaters are used throughout the design and on the main 2A3 power tubes. Withhum balance pots and 2.5V heaters on the power tubes, hum at the output is not a problem,with the sonic advantages of AC heaters.

The chassis is very well made and of robust construction. It comes pre-assembled with aBlack aluminum front fascia, ready made power supply PCB, power transformer, choke,output transformers and point-to-point wiring around the signal tubes. This simplifies the unitassembly with less components to assemble and mostly wire links.

The design is such that the unit can utilise output power tubes other than the 2A3, such asthe 45 and 300B with a few changes. This new revised Joplin also features the hum balancepotentiometers and power tube anode balancing potentiometers mounted directly through thechassis for ease of set-up. Featuring an input volume control the unit can be driven directlyfrom a line source such as a CD player eliminating the need for a separate external pre-amplifier there by providing a more direct audio path from source to amplifier.

Specifications: (may change without notice)

• Power output: 8/5/10+W x 2 RMS @ 1KHZ Class A with 2A3/45/300B• Frequency response: 5-48kHz (-3dB@5W)• Total harmonic distortion: <1% (5W@1KHz)• Signal to noise: 90dB• Input sensitivity and impedance: 1V, 50K ohm (w/NFB)• Output impedance: 4 or 8 ohm, selectable at build• Power requirement and consumption: 110/123/220/230/240V AC +- 10%, 90W• Tubes: 4 x 2A3 (or 45 or 300B), 2 x 5687, 1 x 6DJ8

It is recommended that you read through the manual completely before starting the build. If youplan on using the 300B or 45 power tubes consult chapter 23 before starting the build.

Warranty

The kit cannot be returned for a refund. Defective parts will be replaced provided they arereturned within 30 days of purchase and are confirmed defective and not misused. No furtherwarranty is expressed or implied. This is to protect you the buyer to be assured of receiving abrand new kit and not one that has been returned by someone else which may have beenmisused.

Chapter 2 – Before You Begin

Requirements

The kit builder must have basic soldering skills, a basic understanding of electronics and beable to read and understand the wiring diagrams and photos presented in this guide.Carefully refer to the text and photos at each step throughout the manual and ensure youdouble check all wiring after each stage. It is also advantageous to refer to the circuitdiagrams at the back of the manual throughout the build. Take your time and do not try andrush the build or work when you are tired since that is when you are most likely to makemistakes and will not enjoy the construction. It is worth detaching the circuit diagramsfrom the back of the manual and keep them in front of you during the build to helpclarify and understand each step a little easier. If you have no electrical or electronicsknowledge at all consult a professional before attempting the build due to the high voltagesinvolved in this project.

Recommended Solder and Iron

It is highly recommended that solder with flux, designed for electronics is used. The betterelectronics solder usually has up to 2% Silver which prevents the joint from becoming brittleover time. It provides a much strong solder joint but it can be difficult to work with due to itslower ‘wetting’ properties, it does not flow as easily as normal 60/40 tin/lead solder. Someelectronics solder also has a small amount of copper instead which does have very good‘wetting’ properties. Which ever solder you use make sure that you avoid poor soldertechniques. If you are using the silver loaded solder a slightly hotter soldering iron can beuseful. Normal hobby soldering irons are set to work at 370 Degree C but for silver loadedsolder a variable iron is ideal and the temperature can be increased slightly to 400 Degree Cto improve the flow of the solder.

Solder Technique

Solder means to apply the iron tip to the point that needs to be soldered to heat it and at thesame time apply the solder. The iron and solder should come together at the same timewhen soldering any joint or tinning a wire. The solder should start to flow evenly around thejoint or wire. Allow the solder to cool naturally and do not blow on it. The finished joint shouldappear shiny and not dull. Many problems are due to poor solder technique. Clean the iron tipregularly on a wet sponge in between soldering steps. Do not be tempted to apply the solderto the iron tip first and then move this to the joint to be soldered. Solder and iron on the jointat the same time.

Connections

Connect means to dress a wire by stripping off enough insulation at both ends and solderingit to the connection points, first tinning the wire. When connecting a wire to a valve base tab itis recommended that enough insulation be removed so the wire can be wrapped around thetab to form a good mechanical joint before soldering. When attaching wires to any of thePCB’s or switches you should strip off a small amount of insulation and then apply solder tothe exposed wire to tin it before fixing it. A mechanical joint to a valve base before solderingwill provide the best contact and minimise the likelihood of poor connections but they can bedifficult to remove at a later date if you plan to do any changes and component upgrades.

Optional Wire

The following describe the optional wire types which may be purchased as extras with the kit and usedduring the build in place of the standard insulated tinned copper wire for all audio signal and audioground connections.

CCC Single Strand Magnet Wire (OFC Wire)

This wire comes with an enamel coating so the part of the wire, which will form part of a joint, musthave its enamel removed before fixing and soldering. This can be done by boiling off the enamel in asolder pot or by holding the tip of a hot soldering iron with a little solder on the end against the wireuntil you see the enamel start to boil away. Clean the iron tip on a wet sponge afterwards and then tinthe newly exposed wire with fresh solder. You can also use a fine grade sand paper to remove theenamel. This wire is ideal for all signal ground connections. You may also use it for the signal sideconnections if you did not purchase any solid silver hook-up wire.

Solid Silver Hook-up Wire

If you are using this wire it is recommended that you use silver loaded solder when making any joints.This wire is ideal for the signal side connections but may also be used on the signal ground connectionsif you have enough.

Throughout the manual the signal side and signal ground connections requiring the use of awire link will be mentioned. At these steps you may use any high grade wire like the abovetypes in place of the supplied regular tinned copper wire if desired.

** Disclaimer **

WARNING: The high voltages present in this kit can KILL and the high operatingtemperatures can BURN. Observe all precautions and never connect the kit to anelectrical supply until it has been fully assembled, checked and ready for testing. Youassume total responsibility and liability for the build, use and operation of this kit bothfor yourself and others around you. Always discharge power supply resistors with a>100ohm resistor across them before working on the kit as these can store enoughresidual charge to kill.

Chapter 3 – UnpackingCarefully remove the unit from the cardboard packing. It may have some tubes already fittedfor transit, so remove these and put them to one side. Turn the unit upside down and undothe bottom chassis cover as shown in Figure 1.

TIP: To avoid marking the transformer covers it is recommended that you place a soft clothor towel down on the work surface to rest the unit on during the build.

Take out the bags of loose kit parts from inside and check them against the list below forcompleteness. Take care not to mark the front wooden faceplate when undoing the base andunpacking the parts. We will remove the front cover later before starting the build.

Standard Parts List

• 1 off 50K PEC Stereo Potentiometer (Volume control)• 2 off 220uF 100V Electrolytic Capacitors (2A3 cathode bypass)• 10 off 1M Resistors (Phono test points and long-tail pair grid resistors)• 4 x 1k resistors• 4 x 360k (can be 390k more or less, not critical)• 2 x 47k• 2 x 51k• 2 x 470r• 2 x 15k• 2 x 10k• 2 x 20k• 2 x 0.47uf caps• 4 x 0.1uf caps • 1 off LED packaged with 1 off 1K to 5K Resistor (Front panel led indicator)• Solder Tags (For chassis star earth point and IEC Earth)• 1 toggle switch for selector function• Bundle of Cable Ties (To tidy up finished wiring)• Bundle of Hook-up Wire• Length of Multi-core Screened Cable (Optional for audio signal path)• Heat Shrink Tubing• 4 off Insulated Phono Input Sockets (1 pr Red, 1 pr Black, audio signal input)• 4 off Speaker Cable Binding Posts (2 Red, 2 Black)• 1 IEC Mains Input Socket• 1 Mains Rocker Switch• 1 Brushed Metal Volume Knob• Bag of Screws and Nuts (Fixing hardware)• 4 x 2A3 Power Tubes (if purchased with kit)• 1 x ECC88 Tube (if purchased with kit)• 2 x 5687 Tubes (if purchased with kit)

Your kit should come with the power supply PCB ready built, this is the small long PCB nearthe front of the unit. It may be missing one or both of the large smoothing capacitors (markedC1 and C2 on the PCB) so you may have some large electrolytic capacitors for this in the kitof parts. The value should be 150uF 450V or 220uF 450V. There should also be 2 x 47uf/400vsmoothing caps on the pcb as well as 1 x 220r and 1 x 24k

Chapter 4 – Identifying the Fitted PartsFigure 2 shows the tube socket locations. The rear of the amplifier holds the smaller tubes forthe pre-amplifier input stage. The power tubes are at the front.

Figure 3 shows the rear of the amplifier. The hardware fixing locations are shown. Next pageshows the internal chassis layout. Next section will describe the parts to be installed.

Figure 2 - Joplin Top View

Figure 1 - Joplin Rear View

Chapter 5 – Fixing the HardwareRefer to previous page to reference hardware mountingIt is highly recommended that you cover the face before you start to avoid accidentallymarking it during construction.

Phono Sockets

Install the 4 insulated phono RCA sockets on the rear of thechassis in the punched holes shown in figure 3 previously .Use the Red for the right channel and white for the leftchannel. When inserting the socket through the hole makesure that the socket has a plastic insulating washer on bothsides of the chassis. Order is as listed below.

Phono socketNutPlastic Washer (with ridge)ChassisPlastic WasherPhono socket ground section

Make sure that the cup of the centre solder pin on the socket is facing up (with the unit stillupside down on the transformer covers). Use a hot solder iron at the notch and solder the redpr together with a short connecting wire as shown. Repeat for the white pr. TIP: Flow thesolder at the notch then put the wire in place and flow solder over both to avoid a coldsolder join. Now tighten the outside bolt to fix in position. Also now is a good time to mountthe small toggle switch which will serve as selector

Speaker Cable Binding Posts

Install the loudspeaker cable binding posts in the punched holes shown in figure 3 previously.Make sure the plastic insulating bush is fitted from the outside and the plastic washer is fittedon the inside. Screw the posts in finger tight only for now. Use the black colour coded postfor the zero ohms and red colour coded post for the 8 ohms output. The metal contacts of theposts should not make contact with the chassis. You may check this with a multi-meter to besure. The posts should only be tightened with a spanner at the end of the build when all wireshave been soldered to them.

Attach tagboards to standoffs4 or 6 point tagboard doesn't matter. The hole without solder is mounted to the standoff whichis mounted to tube socket bolt.

Figure 3RCA sockets

Mounting the IEC Socket

Install the mains IEC socket on the rear of thechassis in the punched hole shown in figure 3previously. A small round solder tag must also beplaced under one of the nuts. Scrape away thechassis paint around the hole until you see cleanmetal as shown in figure 7. This ensures that thechassis has a good earth. This is very important.The solder tag must touch the chassis metal. TIP:A very small flat blade screw driver or craft knife isideal to remove the paint. You will need to removeboth the top coat and sometimes a few coats ofprimer to reach clean shiny metal.

Connect the earth tag of the IEC socket (sometimesmarked E on the plastic) to the solder tag under the nutas shown in figure 8. The heat shrink tubing on theearth is not essential but should be used for the live andneutral mains connections later.

Mounting the Mains Switch

Before you mount the mains switch to the chassis, bridge eachside of the contacts on the switch with a wire link as shown inFigure 9. Only 1 set of contacts are actually required to switchthe live of the supply so bridging the contacts increases theswitching load capability and life of the switch.

Clip the switch into place on the rear of the chassis in thelocation indicated in figure 3 previously (next to the mains IECsocket). The switch mounts with the rocking movementsideways.

Confirm that all the chassis parts including tube sockets, tagboards, IEC jack, switch, toggleswitch for selector, adjustable pots are in their respective positions.

Figure 6 - Mains Switch Contacts

Figure 5 - IEC Mains Earth

Figure 4 - IEC Chassis Earth Point

Remove Paint

Chapter 6 – Mains Transformer PrimaryThe primary of the mains transformer has 2 separate windings with 3 wires per winding making a totalof 6 wires (2 red, 2 blue and 2 brown). These enter the chassis in the top left hole (with the unit upsidedown and front facing away from you) as shown previously in Figure 4.

** NOTE : Your transformer primary may have 2 green wires here in place of the 2brown. They should be treated and connected as the brown pairs in this chapter **

You will need a multi-meter switched to the ohms setting to determine which of the primary wiresbelong to each winding. Select one of the blue primary wires and connect this to one of your meterprobes. Now connect the other probe of your meter to each of the brown wires until your meter readscontinuity. This blue to brown reading should be about 4 ohms. Leaving your meter probe on the bluewire, now move the other probe to each of the red wires until your meter again reads continuity. Thisblue to red reading should be about 4.5 ohms. Group these 3 blue, brown and red wires together asthese form one of the primary windings which we will now call winding 1. The 3 remaining blue,brown and red wires are the second primary winding so group these together also, these we will callwinding 2. You now need to connect these wires depending upon the mains voltage of your country asshown in the following diagrams.

NOTE: Make sure that any unused primary wires at the completion of this stage are cut off atthe ends and are individually insulated with heat shrink sleeving or electrical tape.

110V Operation

Connect Blue to Blue. Connect Brown toBrown. Both Red are unused. Both Blueconnect to the IEC socket neutral tag.

123V Operation

Connect Blue to Blue. Connect Red to Red.Both Brown are unused. Both Blue connect tothe IEC socket neutral tag.

Extra_Caption 1 - 110V

Winding 1

Winding 2

To IEC Neutral

To Power Switch

Extra_Caption 2 - 123V

Winding 1

Winding 2

To IEC Neutral

To Power Switch

220V Operation

Connect Brown of winding 1 to Blue of winding 2. Both Red are unused. Blue of winding 1 connects to the IEC socket neutral tag.

230V Operation

Connect Brown of winding 1 to Blue of winding 2. Red of winding 1 is unused and Brown of winding 2 is unused. Blue of winding 1 connects to the IEC socket neutral tag.

240V Operation

Connect Red of winding 1 to Blue of winding 2. Both Brown are unused. Blue of winding 1 connects to the IEC socket neutral tag.

The connections you make on the primary windings should all be insulated with the heatshrink tubing. The unused connections should be cut off at the ends and separatelyinsulated so they do not touch the chassis or each other. They should also be moved andcable tied so they point away from the pre-amplifier stage i.e. point them towards the frontface.

The wires may not be long enough to reach the IEC socket neutral tag and the mains switchat the rear so you may have to extend them. You may use the trimmed off unused primarywires to extend those that you do need. Twist together any wires that run from thetransformer primary to the rear of the chassis.

Mains Switch Connection

You should now have at least 1 blue wire connection to the IEC socket neutral tag and the IEC earthtag connected.

Winding 1

Winding 2

To IEC Neutral

To Power Switch

Winding 1

Winding 2

To IEC Neutral

To Power Switch

Winding 1

Winding 2

To IEC Neutral

To Power Switch

Chapter 7 – Mains Transformer SecondaryRefer to following page drawing for the connections to make in this section2A3 Filament (Heater) Wiring

The mains transformer secondary 1 winding enters the chassis as shownin figure 4 previously. In this winding you will find 2 green, 2 blue, 2white and 2 red wires (thick wires). These supply the 2A3 power tubeswith 2.5V AC for the filaments (heaters). All 4 pairs will be used tosupply all 4 power tubes. The filaments of the 2A3 are pins 1 and 4(large pins). Each power tube having its own filament feed from the mainstransformer.

Make the connections to the 4 (4-pin) power tube sockets and the three (9-pin) signal tube sockets asshown by the previous page diagram. Trim the wires to the length required to reach the tube socket.

At this time install the 2 x 1meg resistors across the 2 nine pin sockets as shown, pins 2 and 7.Later it will be difficult to insert it into this position.

Refer to the previous page diagram to make and complete the connections for the secondarywires. There will be 3 remaining wires which are connected later.

Unused Secondary Wires

The brown pair of wires (with a white cloth sheath covering)entering the chassis from the mains transformer secondary 2winding (refer back to figure 4) are not required. This pair ofwires supply 70V HT and are only needed if 300B power tubesare to be used as a later modification to lift the HT a further70V. They can also be used in for the 6CG7 version of the ampas shown on the last page of schematics. Trim off the ends ofthese wires and individually insulate them with insulation tapeor the supplied heat shrink tubing as shown in figure 16. Youmay twist them together and as before, with the unused primarywires, they should be moved and cable tied so they point awayfrom the pre-amplifier stage PCB, i.e. point them towards thefront face.

Figure 7 - 2A3 Pin-out

Figure 8 - Unused Secondary Wires

Insulate Any UnusedSecondary Wires

Chapter 8 – Power Supply PCB ConnectionsA common star earth point on the chassis is now required before we can continue any further. On thePS PCB one of the feet has a resistor connected from the ground cluster on the PCB. Remove the topnut and loosen the bottom so that that painted surface of the chassis below the nut is exposed. Sand orscrape away some of the paint so that when the nut is tightened it is contacting bare metal. Make surethe feet has metal connection to the chassis.

PS PCB connections:Refer to the wiring diagram on the following page to complete the connections to the PCPCB.At this step you should complete:1. single purple wire from transformer to ground cluster on PCB.2. Two sheathed (gray) wires connect to '310v' holes on PCB by the 4 rectifiers.3. One length of wire from each output transformer 'B' tab to B+ position on PCB4. Wire from PS PCB B+1 to indicated position5. Wire from PS PCB B+2 to indicated position6. Two wires from choke one each to 'L' position on PS PCB7. Two ground wire connections from 25w resistors and two ground wires from centre tab of

adjustable resistors as shown to the ground cluster between two large caps.

Chapter 9 – SocketWiring and PartsIn between the power tube sockets you will see a22K wire-wound potentiometer mounted on eachside of the chassis. Also note the position of the47 ohm hum pots (x4).For this stage the following should be completedreferring to the wiring diagrams on the nextpages:

1. 4 x 1 meg resistors soldered from tab 2 oneach power tube sockets to position on jack asshown

2. Connections from 47r pot to tabs 1 and 4 ofpower tube socket as shown

3. 220uF/100v capacitor acrossresistor shown. Note the lineon the capacitor. This isnegative and must position asshown

4. As shown, one each side,connect 1k, 1meg x 2 and360k resistors on each pair ofsmall pcb over the powertube sockets.

5. Finish making the wireconnections around each prof the power tube socketsand 47R to 25W resistor asshown.

6. From the middle of the 2 x1meg resistors make aconnection to the B+2 pointon the PS PCB

7. From the small pcb at the power tube sockets bring out two wires to the 9 pin tube sockets as shown.8. At the red speaker binding post connect a wire as shown9. Now install the parts and make the wire connections around the 3 signal tube pcb. This includes 4 x

0.1uf, 2x 0.47uf capacitors; 51k and 47k resistors, 2 x 470r resistors; 2 x20k resistors; 2 x 10kresistors; 2 x 15k resistors;

10.Make the connections from + speaker posts to 20k resistors as shown11.Check the following illustrations for a close up view of parts and wire connections around signal

tubes.12.Install the LED connection as shown

Front Panel LEDYou should have 1 LED packaged with a resistor (typically 1K to 5K). This is the front panel powerindicator. Connect one lead of the resistor to one lead of the LED.

Both leads of the LED should be separately insulated with heat shrink sleeving. Then cover the entireresistor and LED leads with a single piece of heat shrink sleeving. Twist the wires together. Connectone wire to Pin 1 and the other to pin 4 as shown

The LED should be inserted into thefront face when it has been fixed backonto the chassis front. You may gluethe Led in place.

Refer to the following page diagramfor completed wiring and checkagainst your work.

Figure 9 - Front Panel LED

Chapter 10 – Signal-in WiringRefer to the following page wiring diagramThe RCA and selector connections are made as follows:

– connect the RCA jacks to the positions shown on the toggle switch

– connect one lead of the shielded cable as shown to the common ground of the RCA groundtabs. If your RCA is of the type that doesn't have the ground tab, just use a length of wire tomake the same common connections between the perimeter of each RCA jack.

– At the other end of the shielded cable (cut it to the proper length), make the connections asshown to the volume control pot. Notice the colour of the wires to which point. Notice theblack wire across the last tabs that must be soldered in place

– The leftover cut pc of shielded cable will be wired as shown to the volume controlpotentiometer. The other end (follow to the 'S') will be wired to the tube sockets as shown.Again notice the wire colours to keep the connections in order.

– Try to keep the two shielded cables as far to the left side of the chassis as possible. This willreduce noise pickup. Making the cable a little longer to achieve this is better than short-cutting diagonally across the chassis.

Illustration 1: in position should look like this

Chapter 11 – Output Transformer

Refer to the following page wiring diagramsThe OPT connections are made as follows:Remove the round transformer covers for the left and right channel output transformers (rearof the unit). Each cover is held in place by three small nuts on the inside of the chassis. Notethat there is no need to undo the three hex headed bolts on the round transformer cover itselfsince these only hold brackets on the inside of the cover which in turn bolts to the chassis bythe 3 nuts. They are held in place in exactly the same manner as the mains transformer coveryou removed earlier to create a star earth.

The primary of the output transformers have 5 solder terminals labelled P1, G1, B, G2 and P2in that order. Terminals P1 and P2 (the outermost terminals) connect to the anodes (pin 2) ofthe two 2A3 power tubes for each channel. G1 and G2 are unused. The centre tag Bconnects to B+1 on the HT PCB.

For the left channel connect P1 of the left output transformer to L1 pin 2. Connect P2 of theleft output transformer to L2 pin 2. Connect B of the left output transformer to B+1 on the HTPCB (Refer back to figure 22). The B+1 connection point on the HT PCB should have anumber of holes drilled in it for connecting multiple wires.

Repeat the process for the right channel. Connect P1 of the right output transformer to R2 pin2. Connect P2 of the right output transformer to R1 pin 2. Connect B of the right outputtransformer to B+1 on the HT PCB (Refer back to figure 22). Figure 30 shows theseconnections for both channels. The connections are a mirror image of each other.

Figure 10 - Output Transformer Primary

Output Transformer Secondary

The secondary of the output transformers have 3 solder terminals labelled 8 ohm, 4 ohm and0 in that order. Depending upon your loudspeaker impedance that you wish to drive you nowneed to connect either the 8 ohm or the 4 ohm output tag of the transformer to the positive(red) loudspeaker cable binding post on the rear of the chassis for both channels. The 0 ohmsolder tag connects to the ground (black) loudspeaker cable binding posts.

Both black (ground) loudspeaker cable binding posts for left and right channels also connectto the star earth point tagboard as shown following page diagram.

P1

G1

B

G2

P2

8R

4R

0

P1

G1

B

G2

P2

8R

4R

0

Output TransformersSecondary Secondary

Primary Primary

Loudspeaker CableBinding Posts(Chassis Rear)

Star Earth

Chapter 12 – TestingDouble check all your connections and refer to the circuit diagrams at the back of the manual whilstdoing so. Inspect all solder joints for completeness.

DisclaimerWARNING: The high voltages present in this kit can kill and the high operating temperaturescan seriously burn. Observe all precautions and never connect the kit to an electrical supply untilit has been fully assembled, checked and ready for testing. You assume total responsibility andliability for the build, use and operation of this kit both for yourself and others around you.

Testing

A voltage measuring device such as a multi-meter is required to test the unit. Take care not to short outadjacent contacts with the meter probe. When measuring the HT clip the ground of your meter probe tothe star earth and use the positive meter probe to measure the test points. Keep one hand in yourpocket or behind your back and just use one hand holding the positive of the meter probe. Up to 10%variation in voltages measured from those quoted are acceptable. A mains variac is very useful at thisstage to bring the mains voltage up slowly but it is not essential.

*** DO NOT INSERT ANY TUBES JUST YET ***

• Connect the unit to a mains power source and switch on. If you see any sparks or smoke thenswitch off immediately. Check all the wiring again especially around the mains primary andsecondary connections. You may have also blown the fuse in the IEC socket or the mains plug.The mains plug should have a low rated fuse of about 5A maximum.

• If everything is OK measure between pins 1 and 4 for each of the 2A3 power tubes. Theseshould all read 2.5V AC. Measure between pins 4/5 of each signal tube. These should measure6.3V AC. Switch off.

• Clip the negative probe of your meter to the star earth point on the chassis. Switch on. With onehand behind your back measure the B+ solder pad on the power supply PCB. This shouldmeasure between 290 and 320V DC. Switch off.

• Insert the single ecc88 tube and the two 5687 tubes. Do not insert the power tubes at this stage.Switch on. The tubes should glow orange as the heaters come on. With the negative probe ofyour meter still connected to the star earth point on the chassis, pins 1 and 6 on the single ecc88tube should both measure around 70V DC. Pin 1 of both 5687 tubes should measureapproximately 170V DC. Switch off. Remember that pins are numbered clockwise whenlooking at the socket base.

• Insert the power tubes. Connect a suitable load to the loudspeaker cable binding postsfor both channels. An old pair of loudspeakers is ideal or a high wattage resistor. Theload should match the impedance which you wired the output transformers for, that is8R or 4R. Switch on. You will probably hear some hum. The amplifier now needs to beset-up. If you hear any buzzing then you may have a fault somewhere, usually aground problem. See the troubleshooting section.

Chapter 13 – Set-upThe hum balance pots for each of the 2A3 tubes on both channels needs to be adjusted forthe lowest level hum. You can do this by ear or use a multi-meter for greater accuracy. To usea multi-meter, clip the negative probe of your meter to the black loudspeaker cable binding post(ground). Connect the positive probe of your meter to the red loudspeaker cable binding post. You canset-up which channel you like, the order is not important. Make sure your meter is switched to AC (mVrange). You still need a load connected such as an old pair of loudspeakers or a high wattage resistor.Switch the amplifier on. You will now see a reading on your meter display. Adjust the 2 hum balancepots for the channel you are working on until the reading is at its lowest. Switch off. Repeat the processfor the other channel.

The power tubes on each channel also need to be balanced. You can use the test phono sockets on topof the chassis to do this. Short the signal input RCA for each channel. Insert one of your multi-meterprobes in one of the test sockets and the other probe in the other test socket for the same side. Ensurethat the multi-meter is switched to the DC mV range and adjust the 22K balance potentiometer betweenthe tubes until you get the lowest reading. The reading may be changing all the time so you need to getthe reading as close to zero as possible. You will not get a steady zero reading and most likely have areading that fluctuates in the single digit range and the low 10's of mV at the very best setting.

Once you have balanced both sides it is worth re-checking the hum levels again and finely adjusting thehum balance potentiometers if required. It is possible to get the hum down to <=0.9mV per side.

Chapter 14 – TroubleshootingIf you are picking up hum and noise (buzzing) then follow these guidelines. Make sure you haveadjusted the hum cancel potentiometers and the tube balance potentiometers first as best youcan.

Ideally, ensure that all audio signal wires and audio ground wires cross the power supply and chokewires at roughly 90 degrees. They must not run close and parallel with any power supply wires. Allpower supply and choke wires should have been twisted in pairs during assembly. The signal wires canbe raised so they don’t touch the chassis and directly touch any of the power supply wires. The audioground from the volume potentiometer is very important. It must be routed clear of the HT PCB andmust be connected to the star earth. This can pick up most of the hum.

Ensure that the heater wires for the 2A3 power tubes do not touch the chassis. Make sure there is noground connection other than the star earth (and the IEC ground tag). Make sure that these groundpoints make good solid contact with clean metal and that the chassis paint is not acting as an insulator.A multi-meter switched to ohms can help sometimes but it is not a guaranteed indicator of a poor earth.Measure the resistance between the star earth and the IEC earth. This should read zero (taking intoaccount the resistance of you meter leads). Loud buzzing is usually a sign of a poor ground connectionor a bad layout of the earth (ground) wires.

You may optionally scrape the paint off the chassis on the inside near the hole which the potentiometerthreads pass through. This ensures that the potentiometer body has a good earth contact on the chassisso it does not pick up any hum when you touch the metal knob when in use. This is not normally aproblem with the Joplin so this should be a last resort and only if it proves an issue.

You can remove the ECC88 and the 5687 tubes from the pre-amplifier stage and power-up theamplifier with just the power tubes in place. This will isolate whether the hum or noise is originatingfrom the pre-amplifier stage or the power output stage. You may also set the hum balancepotentiometers for each of the 2A3 power tubes without the pre-amplifier stage running to eliminateany hum from the output and completely rule out as a problem.

If you are only getting a problem on one channel then swapping tubes between the channels (1 at atime) can isolate whether you have a bad tube. Do not rule out a bad solder connection and check allsolder joints throughout the unit.

Under normal operation the Joplin is very quiet with about 0.9mV per channel hum measured at theoutput with the hum balance potentiometers having been adjusted. This is not audible at normallistening position.

Finally, help and advise is available in the diyhifisupply forum if you are still having problems and haveexhausted all the suggestions here. Visit www.diyhifisupply.com and click the forum link.

Chapter 15 – Final Word and UpgradesThe Joplin requires running in especially if you are using brand new tubes. Like all high end audiocomponents the sound will improve with playing time. Power tubes generally require a few 100 hoursbefore they fully settle down and open up. Expect at least 50 hours run in time for the initial change tothe sound and character of the amplifier.

Replacing capacitors to higher grade types like paper-in-oil will push the unit to new heights. Howeverthe standard build will produce a high end sound that can stand with the best. Similarly changing the220uF/330uF 100V cathode by-pass capacitor across the cathode resistor for the power tubes to ahigher quality electrolytic will provide sonic gains.

Using more exotic NOS tubes can also improve and change the character of the amplifier. Using NOStubes for the ECC88 and 5687 pre-amplifier signal tubes can change the sound considerably and youcan select tubes with particular sound characteristics to match the rest of your system, room and taste.Most British and American NOS tubes like Mullard and Sylvania are good replacements but arebecoming quite rare.

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45 Modifications45 Power Tubes

If you plan on using 45 power tubes instead of the 2A3 then it is not necessary to connect all4 of the 2.5V AC filament supplies as shown in chapter 7 (figures 14 and 15) previously. The45 has less heater current demand than the 2A3 so you may use just 2 of the windings andlink the filaments of L1 and L2, R1 and R2 as shown in the caption below.

In this example the white pair has been used for the right channel power tubes and the bluepair for the left channel power tubes. You may use any pair you wish and you can just aseasily use the red and green pairs.

Note : In chapter 13 make sure that you use the 820R 25W wire-wound resistors for the45 Tubes. The 470R 25W resistors are for the 2A3 tubes only.

1

23

4 1

23

4 1

23

4 1

23

4

R2 R1 L2 L1

TransformerSecondary 1

Circuit diagrams:

This variation is for experimentation only and not supported.