HiFi-Do E-mail Newsletter

	My Treasure - "Tube amp making course" By Otomeijin (September 16, 2005) Chapter 1: Vol. Have your eyes opened. Topics on the amplifier(Part 2) I unpacked the kit to examine the contents to explain each role of them and reviewed a vacuum tube the main character last time. This time I will explain about various parts that help a vacuum tube functioning to finish the "structure/regulation" part as in a driving school and move onto the next practical skill part.
I will just name the parts of the amp sorted out last time again. You may know almost all these parts and not need the explanation, but let me review only the parts related to the electric circuit. Structural parts Chassis, cover, dummy supports, screws, nameplates, etc. Electric circuit parts Vacuum tube, transformer, condenser, resistor, variable resistor, pilot lamp, etc. Coupling parts AC outlet, socket, switch, fuse, tube socket, input pin, speaker terminal, lug terminal, lead wire, solder, etc.
	Transformer (Trans): for changing the alternating voltage (VAC) by using the principle where magnetism is generated when having electricity flow into the conductor and electricity flows into the conductor when having magnetism "change". To get voltage, first wind the parent coil (the first coil) around the central iron core that passes magnetism well and wind the child coil (the second coil) only the number of target turns (target voltage level) (Image) There are power transformer (PT), choke coil (CH) and two output transformers (OPT) from the left in the example picture. (I will write "sign" in the circuit diagram like this from now on in my explanation.) Please note here though, "CH" looks similar to the transformer in shape and material but its purpose is not to metamorphose the voltage but to smoothen the direct-current power source. Because a coil except this coil will not be needed much for the amp, I just put it in the item of the transformer for convenience sake. Transformer manufacturers are producing and selling coils. There would be a case to use resistor (R) instead of "CH" for the purpose of miniaturization in size and reduction in cost. Because, I will go bankrupt if the cost goes over the struggling budget since "CH" is more expensive than R.(-_-;)
This is a circuit diagram of the power supply part in this kit. I will explain the functions of power transformers (PT) and so on here. First I need to wind the first coil and the second coil around the iron core whose magnetic efficiency improved and shape is round. I will explain with the number of coil turns so that you can understand better. This flux of magnetic induction passes through the iron core when the first coil gets connected to 100V-power source for commercial use, whose turns are 100 turns. This flux of magnetic induction generates 100V in the coil of 100 turns oppositely. 1V is for the coil of one turn. The coil of 290 turns can get 290V. Thus, the power transformer (PT) generates the voltage required for the second coils. When electricity starts changing, the coil tries to control it. The iron core is being put so that the coil reacts actively. This is why the coil is called choke coil(CH). The transformer for connecting speakers is called output transformer(OPT). It insulates from the high direct current circuit in the vacuum tube and converts it into the circuit system that exists in the speakers. In short, the output transformer is the insulating transformer and the impedance matching transformer at the same time. There is an output amplifying method amp without using OPT, which is called OTL amp, by putting many vacuum tubes in parallel to tune with the speaker. You can probably imagine that OPT is a part that affects the sound quality a lot.

Condenser (Capacitor): a part that uses the feature which saves electricity when facing electrodes each other and sandwiching insulators. The unit of the energy volume saved is farad (F): the often-used level is μF (micro F) = 1/1000F.) It is shown by "C" in the circuit diagram (in the picture). There are small but mass "electrolytic condenser" using an electric chemical action and (general) condenser I would like you to remember among various condensers. Most of the "electrolytic condenser" is polarized and multi-used in the power supply system. If you made a mistake with the pole, it generates heat and blows out (belch?). And the withstand voltage is important. You need to choose the one with the withstand voltage which is more than the voltage put in the circuit so that the electrode faced each other will not discharge electricity (short-circuit) yielding to the voltage. You can see 350V 47μF on the electrolysis C in the picture. This is used for the 6SN7 plate power supply circuit 280V. And please don't touch this sooner. You may get an electric shock even if the power is turned off since the capacity is pretty large and discharging takes time. And let's see how we can use it. The condenser saves electricity when it is connected to the power source and discharges electricity saved inside when the voltage goes down. Well it is like a purse. (Who says "Mine is always empty? Well…it's me. :-( ) The one using this is the "smooth circuit". Secondly, it has a character ("filtering effect" in a broad sense) not passing the direct current. This can separate the direct-current circuit and the alternating-current circuit. (It seems like someone at Akihabara Store broke a speaker due to a direct current leak from an old transistor amp. I think he/she should have put a condenser in the series.) Moreover, it has a filtering function that passes electricity by changing the alternating frequency (audio signal etc.) according to the composition and capacity value of the circuit using a condenser. Some 2 and 3 way speaker is famous for the circuit dividing bass, mediant and high pitch sound.

Resistor: a part with function not passing electricity. The unit (in the picture) is ohm (Ω): The often-used level is KΩ (Kilo ohm)=1000Ω, which is shown by "R" in the circuit diagram. The resistor doesn't choose AC or DC. It is used to produce the voltage distribution to make the electric circuit work effectively. The relation between the well-known voltage "V" and current "I" and the resistance is V = IR. It also turns electricity into heat as a load. The electric power turning into heat is W=IV=I x IR (I squared R). Therefore, check the thermal capacity not to be burnt because the resistance becomes hot. You don't have to worry about it as long as you install it correctly since it is selected properly in the kit according to the compatible thermal capacity. Naturally, the bigger resistor has a bigger thermal capacity for resistors of the same type and resistance value. They call it one-watt type, half-watt type and so on accordingly. Electrically the greater in thermal capacity embraces the less in thermal capacity. There is a resistance operated by a knob called "volume" among familiar resistances. Now let's assume that there is a "uniform" resistor pillar of 100cm in length connected to 100V power source. It is 10 volts when measured at the point of 10cm from the bottom. It is 70V at the point of 70cm. In this way you can change the voltage of the circuit taken out depending on the point. It is variable resistor "VR" with a knob which is made smaller and roundly based on this image. (The one with an arrow mark: asterisk asterisk) at the zigzag mark of R in the circuit diagram) The rotating angle and the resistance are proportional with the above "uniform resistance", but you can make in various curves if you change the "density and gradation" of the resistance. I remember they are called A, B and C curves.

Coupling parts: parts for connecting circuit. You are familiar with these, aren't you? Anyway let's get to know the composition and the role of these parts while seeing the circuit below. Power supply part: to be the direct current(DC) pulsating the flow through the full-wave rectification with 5U4G the rectifying tube after obtaining the high-pressure alternating-current of 290V from AC100V power supply for commercial use via SPT150 the power transformer. It obtains the good-quality direct current with less pulsating flow (ripple) smoothened by a pie(π)-type filter consisting of the 47μF electrolytic condenser and SCH515 the choke coil. It generates the low voltage for the alternating current ignition of the filament of each vacuum tube. Note) In this luxury kit, this 47μF block type electrolytic condenser has been replaced with the film condenser which is said to be good in sound quality. Signal amplifying part (step, stage): The input signal is amplified by 6SN7 the twin triodes (Two triodes per tube) and sent into the control circuit grid in the following output stage. Output part (electric power amplifying part): Lastly it is amplified in 2A3 the triode output tube, and sent off from SOT525 the output transformer to be electrically matched with speakers. Since it is a stereo amp, it has two pairs of signal amplifying part + output part (electric power amplifying part) sharing the power source part. Note: The circuit diagram shows only one channel.

I think you could have a clear image of "tube power amp" even though my explanation was very rough. I can imagine like this… Mr. Amp is told like "Hey I am going to send you the audio signal" after the power is on to replay music. Mr. Power Source says "Ok then let's work! First I will make rice!" Mr. Amplifier says "The signal is small, so let's make it bigger". Then Mr. Output is like "Oh OK OK. Here I can shake Mr. Speaker by pulling out power! Here we go! Tada! Look! It's a good sound, isn't it?" To be continued

Extra talk: "-- tance" There are various ---tance especially in the alternating-current electric action, so I get confused in my head by using the real number or the imaginary number, and I may go crazy and end up dancing at last. Electric capacity: Capacitance ↓ Alternating current resistance: Reactance↓ Coefficient of induction: Inductance ↑ Alternating-current circuit resistance: Impedance Resistance: Resistance Other than these, Admittance, Conductance, Susceptance, etc. Well you can forget these though.. "Dried-up" Since the electrolytic condenser uses the chemical action, impregnated electrolytic solution stays between electrodes. Therefore if it is not used for a long time, the function of electrolytic solution will be gone. This is called "Dried up". This may happen if electricity doesn't get turned on for 1 year, so please turn on electricity timely for equipments you are preserving. Otherwise, your equipments will face a critical failure.

Used

Used

Consigned

Consigned

Hold

Hold

Reserved

Reserved

Sold Out

Sold Out