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HOW TO BUILD A SIMPLE TUBE AMPLIFIER 1. Safety first 2. Schematic with the ECL 80 or 6AB8 (tested & on YouTube) 3. Schematic with the 6BM8 (not tested) Ko Tilman/Radioam232 December 2008

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1. SAFETY FIRST A tube operates with an anode voltage. That voltage can be very high, and is high with the shown schematics. Battery tubes can operate on anode voltages of 12 - 60 Volt, the tubes used here are designed to work on 200 volt anode voltage. With this 200 Volt a capacitor of a high value (between 50 uF -microfarad - and sometimes 4000 uF) is charged. This means that an enormous amount of electric power/charge is present on that capacitor that can suddenly discharge through your body when you touch the + of the capacitor accidently, or a place "somewhere" in the circuit where 200 Volt is present. This can be lethal. On the other hand: when you are careful and do'nt touch the anode wire there is less risk. So read these instructions first. If you are not an adult, let your parent(s) or an older person that takes care for you in your family read this text and ask for permission. 1.1. Workbench & carpet, enough light Work on a workbench of wood. This does not conduct electricity. Put under your workbench a carpet of wool or cotton. This also does not conduct electricity. Do'nt work with electricity in a moist room or on bare feet. Put on shoes or socks. Don't work in a room that has no or less light. Mount a good lamp above your work bench. 1.2. Take out the plug of the wall 110-230 Volt wall socket Always take out the plug of the 110-230 Volt wall socket when you work on the tube circuit or solder components into the circuit. Don't solder components with the wall socket plugged in. The reason to take out the mains plug totally is that on one of the 110/230 Volt wires there is a lethal voltage. This means that, though you can switch off the 110/230 Voltage with 1 wire, on the other wire there can be still a lethal voltage. It looks as if your device is switched off, but that is not the case. So take the wall plug totally out when you work on the circuit and use a switch that switches off both 110/230 Volt wires from the wall socket. 1.3. Isolate all contacts on the 110-230 Volt side Isolate these contacts (in the schematics named "A" and "B") with the wall plug taken out totally. Do this with good quality isolation materials that are designed and fabricated to isolate 110-230 Volt. Ask it in an electronics shop.

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Do'nt use cellotape or paper isolation tape, paper etc. You can also isolate with silicon kit (normally used in bathrooms etc, colourless), let it dry first for 24 hours. Don't use isolation materials that can take fire on places where the circuit can get hot, for instance: around the tube, around power resistors, lamps, transformers, etc. Mount a transformer and the tube in an way that air can get along and cool the transformer and the tube. 1.4. Safety ground wire Use the safety ground wire. In the Netherlands this is the green-yellow wire. In other countries the colour code may differ. Ask this and research it. Connect it to the ground/chassis. In the described circuits this is named "ground, chassis, zero". If you do'nt know which wire is the safety wire in your country, please ask someone educated in electronics/electricity. If you construct the schematic in a metal cabinet or on a metal chassis, connect the metal chassis or box to the safety wire. Take the safety wire approx. 3 inch (10 cm) longer on the location where the power cable enters the cabinet. If the power cord is torn or the electronic device falls from a table, the device keeps connected to the safety wire and this is safety wire is in the ideal situation the last cable to break. Fix the power cord well to the cabinet in which you built the device. The schematics showed here are designed in a way (total DC) that they can be constructed on wood or multiplex without problems. This is much safer. 1.5. Adequate wiring material Use well isolated wire with good quality electric isolation, to connect your components. Ask your dealer. Do'nt use thin wires when a substantial current (more than 1 A) has to be transported. 1.6. Electrolytic capacitors Electrolytic capacitors have a + and a - (minus). Never reverse + and - (minus). Be absolutely sure that the capacitor is mounted with plus and minus in the right way. Never connect an electrolytic capacitor to an alterning current (AC) source, like the mains wall socket in your home, or the secondary side of a power supply transformer. It can explode. Never exceed the working voltage that is printed on the capacitor, on the contrary: always use a capacitor rated for 20 % more voltage than the voltage that comes parallel to the capacitor. Be aware that (in the schematics shown here) 200 Volts DC on a capacitor of 50 uF (microfarad) or more can be lethal. Always use a bleeder resistor parallel to an electrolytic capacitor.

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Don't use very old electrolytic capacitors. In these type of capacitors a "wet" process (aluminiumoxide) realizes the capacitance. If the capacitor dries out (is internal dry, can happen after approx. 10 years) it will not function very well. If you use an old electrolytic capacitor: put it on a voltage source and slowly put up the voltage to the level that is printed on the capacitor. Hold it this way for half an hour and see if no strange things happen (explosion or so). 1.7. Always use a bleeder resistor Always use a bleeder resistor parallel to an electrolytic capacitor. This is always needed with voltages higher as 20 Volts. This resistor discharges the capacitor when the power is switched off. When the value of the bleeder resistor is high, it takes substantial time before the capacitor can be handled safely. Measure with a voltmeter how much time it takes till the voltage on the electrolytic capacitor is lowered to a safe value (20 Volt). A bleeder resistor may never get hot, so take a heavy one. If it burns out, the charge stays on the capacitor. In that case it can take many many hours till the capacitor is discharged to a safe value. On 200 Volt a good value for a bleeder resistor is 470 K (470000 Ohm). Take a 2 or 5 Watt type.

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2. SCHEMATIC WITH THE ECL 80 OR 6AB8 This schematic is tested and you can find it in a YouTube video on my channel.

2.1. DC voltages As you can see, I used DC voltages everywhere. Also for the filament, though this tube was designed for AC on the filament. The reason is to prevent hum. A tube amplifieris very vulnerable to hum, because the input grid has a very high input resistance. In our daily life "hum" is everywhere, because our mains supply is 110-230 Volt at 50/60 Hertz. This electromagnetic field on 50/60 Hz can cause problems in sensitive amplifiers, like electron tubes. As you can see the filament voltage comes via a bridge rectifier with a very high capacitor (10.000 uF). The reason is to suppress all hum coming from the mains supply. 2.2. DC filament Voltage The 6AB8 or ECL 80 tube needs 6,3 Volt at 300 mA (tube handbook). Point X and Y are not grounded anywhere in the circuit, they go directly to the filament.

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If you cannot get a 10.000 uF capacitor, put two 4700 uF capacitors (20 Volt types) in parallel. The value of R 2 depends on the chosen transformer. With 6 Volt AC on the secondary side you will find 8,4 Volt after the bridge rectifier (the output of a bridge rectifier is always 1,4 x the AC Voltage). With 12 Volt AC you will find 17 Volt after the bridge rectifier, parallel to the capacitor. R 2 has to be taken in a way that parallel to 4 and 5 (filament) 6,3 Volt occurs. So the Voltage has to be dropped to a lower level with this resistor. With a 6 Volt transformer it is also possible that no R2 is needed, because the 8,4 Voltage lowers when the transformer is loaded with the filament that takes 300 mA current (the real output voltage of a bridge rectifier in a loaded situation is theoretically 0,9 x the AC voltage). So figure it out. I presume it will be in the 10 Ohm range (?). If you use a resistor, take a power type that can get hot. 2.3. Negative grid Voltage What is negative grid voltage? There is a very informative video about this on the YouTube channel of Allamericanfive radio. Study this. On this channel you can also download (for free) a book about shortwave radio's. In this book there is also very well explained what negative grid voltage is all about. Generally spoken we can say that negative grid voltage sets the tube to the right "working point". This means some part in the tube characteristic where a sine wave (coming on the input grid) is amplified competely, pure and exact on the output circuit (anode circuit, where the transformer is inserted). Many tubes don't need a negative grid voltage. Sometimes a tiny negative voltage (0,8 - 1,5 V) on the input grid is necessary, you will find this for instance in antique radio's (1920-1930 etc.) where they use a small battery that had to be renewed after two years or so. Generally spoken almost no current is needed for the negative grid voltage, only "voltage" must be applied. We can also say that negative grid voltage is necessary everywhere where we need power, so in power (audio) amplifiers. The 6AB8 or ECL 80 tube needs a negative grid voltage of 8 Volt (tube handbook). Be aware of the fact that the diodes in the bridge rectifier in this part of the circuit are reversed (!), also the capacitors are reversed (!). Don't make mistakes when you construct this. The value of R3 is not critical, because no current is needed to supply the grid (pin 8). So take for instance 1 K. Reason to mount R3: the filter (hum suppression) works better, so take the highest possible value for R3, in such a way that the negative grid voltage (approx. 8 Volt, measure it) stays intact. The reason to construct a seperate negative grid voltage circuit is that also other triode-pentode tubes can be tried, tubes that need a higher negative grid voltage. In this case you can set this voltage between 0 and 22 Volt negative. I presume that is plenty enough to let other tubes operate. The reason is also that with a lower anode voltage the negative grid voltage must be adapted. So this is maximum versatile.

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2.4. The anode voltage This part of the circuit is dangerous. So read the instructions in chapter 1. The resistor of 470 K is the bleeder resistor parallel to the supply capacitor. The 6AB8 or ECL 80 has a normal anode voltage of 200 Volt and an anode current of 17 mA. Take the value of R1 as high as possible, in such a way that approx. 17 mA flows in the anode curcuit . So measure the anode current on point "M" in the circuit. Connect the ampere meter between the dots in the circuit. You can also forget this and experiment with different values for R 1 and listen whether the sound stays good. Try for R 1 for instance: no resistor, 100 Ohm, 220 Ohm, 470 Ohm, 1K etc. And listen everytime to the sound. Always disconnect the wall power cord totally when you change the resistor R 1 and wait every time till the capacitors are discharged to a safe value. It can take some time, be patient. 2.5. Hum on the anode voltage In the worst case situation hum stays on the anode voltage. You will hear it out of the loudspeaker box. So you have to make the value of the electrolytic capacitor in the anode circuit bigger. Be careful, the charge becomes bigger and more dangerous. You can go up to 4000 uF (microfarad). Though you will seldom see such high values in factory made devices. Always use capacitors that can handle 200 - 250 Volt (!). Do not compromise. You can switch two capacitors in series to reach the right working voltage. For instance/example: 2 capacitors of 1000 uF each 100 Volt in series build a capacitor of 500 uF that can withstand 200 Volt. The voltage goes up in this case (x 2) the capacitance is divided by 2. But, as said, the better way is to experiment with the value of R 1 (thus making the filter work more effective) and make this resistor as high as possible. That is also done in factory made tube circuits. 2.6. The audio transformer I had no real tube type audio transformer. These kind of transformers have a DC Ohms resistance of approx. 7000 Ohms on the primary side (anode circuit) and a Ohms resistance of 4-8-16-32 Ohm on the secundary side (loudspeaker). They are specially made to reproduce the complete audio spectrum (20 Hz - 20KHz) from the primary to the secundary coil. This means: a heavy core, high quality wire, less capacitance (capacitance damps high frequencies) etc. So I took an old power supply transformer ( 220 V ~ to 12 V ~) and reversed it. The primary coil of this transformer was 700 Ohm (DC resistance) and the secondary (loudspeaker side) was 2 Ohm DC resistance. It worked rather well.

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But I presume that the audio output volume is not very strong (approx. 300-400 mW) because of the too low resistance (700 Ohm) of the transformer. I am almost sure more audio output power will be generated with a better transformer. Always use a good loudspeaker box connected to the transformer. It gives the real "tube" sound. 2.7. The input circuit It consists of the 18 K resistor, the 10 N capacitor, the coupling capacitors (680 N and 330 N) and the volume control of 10 K. I saw oscillations on the scope (parallel to the loudspeaker). The oscillations disappeared when I mounted the 18 K resistor and the 10 N capacitor. In fact this is the easiest way to prevent oscillations (put the input resistance down) but it als has consequences for the audio characteristic. The 10 N capacitor muffles/damps the high frequencies. So do some experiments, try for instance a 4N7 capacitor or a 100 K volume control resistor.

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3. SCHEMATIC WITH THE 6BM8

3.1. Where you can buy this tube First I have to say that I did not test this schematic, but I am sure I show you the right way to develop the circuit. Reason to publicate this schematic is that this tube can be bought normally in the shop. For instance via the website of "Conrad" (electronics etc. shop) you can buy this tube for approx. 10 Euro. You can also download (for free) the datasheet of this tube there. 3.2. Differences between the 6BM8 and the 6AB8 This tube has more power, the anode current is higher. The 6BM8 needs an anode current of 35 mA at 200 Volt. The consequence is that the value of R 1 has to be adapted (lower resistance, higher anode current). How to do this is described earlier. Again: be careful, disconnect the circuit totally from the wall socket when you change the resistor, wait till the power supply capacitor is discharged. Be patient. Because the 6BM8 creates more audio power, the filament current is higher. The filament current is 6,3 Volt at 780 mA. So change the value of R 2 till you measure 6,3 Volt parallel to pin 4 and 5 and till the filament current is 780 mA (take the DC ampere meter into the circuit).

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For the rest this tube does not differ much from the 6AB8. There is an internal connection between the highest grid in the pentode part to the cathode (pin 2). And the cathode is "split". The input grid of the triode part of the 6BM8 also does not need a negative grid voltage, just like the 6AB8. Perhaps you have to do some experiments on the input circuit to adapt it for a specific input device (tuner, CD player, etc.). It is possible that you have to do some experiments with the input circuits (everything hanging on grid 1 to earth) because the input resistance of the 6BM8 is lower (20 K) opposite to the 6AB8 (150 K). This means that the 6AB8 is (generally spoken) more sensitive at the input. 3.3. tube data 6BM8 (ECL 82) Input resistance: 20 K Max. power from the pentode part of the tube: 7 Watt Max. power from the triode part of the tube: 1 Watt Normal voltages applied to the pins: Pin 6: anode from pentode part: 200 Volt at 35 mA Pin 7: grid from pentode part: 200 Volt at 7 mA Pin 3: input grid from pentode part: 16 Volt negative opposite to ground ("0") Pin 2: cathode from the pentode part (ground, "0") Pin 8: cathode from the triode part (ground, "0") Pin 4 and 5: filament: 6,3 Volt at 780 mA Pin 1: input grid from the triode part Pin 9: anode from the triode part: 100 Volt at 3,5 mA

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6AB8 (ECL 80) Input resistance: 150 K Max. power from the pentode part of the tube: 3,5 Watt Max. power from the triode part of the tube: 1 Watt Normal voltages applied to the pins: Pin 6: anode from pentode part: 200 Volt at 17,5 mA Pin 7: grid from pentode part, connected externally to cathode and ground ("0") Pin 3: cathode,connected to ground ("0") Pin 2: input grid from triode part Pin 8: grid from pentode part: 200 Volt at 3,3 mA Pin 4 and 5: filament: 6,3 Volt at 300 mA Pin 1: anode from the triode part: 100 Volt at 8 mA Pin 9: input grid from pentode part, 8 volt negative opposite to ground ("0")

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