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My treasure, "Tube amp making course" By Otomeijin(October 14, 2005) Final Chapter: Throbbing to touching moment or nail-biting moment? Since we already finished assembling the kit and checking the wiring so far, we will finally get to the last process, measurement, adjustment and tryout to complete the amp. I feel like praying to God for success.

I exaggerated a bit when I said "measurement" and "adjustment". The good thing about the kit is that you can get a high-grade sound quality because they have years of experience in everything including development even for a little check. I wrote "The pilot lamp lights up upon switching and the heater of the vacuum tube turns red, which looks as if the amp were alive" before in the paragraph of "What is the amp?" Well finally the time has come. Now let's go ahead with it based on the assembling instruction while me explaining as follows. What to prepare: Tester (Check whether the operation voltage of the vacuum tube is in the design value range), Complete set of input equipment and pin cord, Right and left speakers and one pair of cable. Measurement position: Here is the point to be noted. You cannot put it flipped over just like when you did in assembling since the vacuum tubes are large and taller than transformer. So you need to get it up on the side where has no terminal. It would be kind of stable since the transformer is heavy. Please turn the inside of the chassis toward you and make it possible to put a tester sensing pin. Insert five vacuum tubes by aligning them to the designated places. (See "How to align" in the chapter where we learned about the vacuum tube.) Hold up the chassis and be careful since it is pretty hard. Naturally you need to hold the base of the resin part without holding the glass part. And set the input volume to minimum. Connect the AC power supply cord, check if the voltage is 100V on the back of AC socket for measuring. Set the tester within the range of AC/V 100V or more. You watch if there is no abnormal noise or bad smell and if the heater becomes redder and redder after power is on by making sure that you can turn off the power right away if something happens(Am I chicken?). This gives me butterflies in my stomach. If there seems no problem, let's go on to the measurement. (* I wonder if there is any ABC book describing this far in the past.) *Now measure and record the direct current (DC) voltage of each point specified in the circuit diagram. Please don't make any mistake at the ch part on the left and right sides. Although it says "It is OK as long as the difference is not so extreme from the designed value", I wonder how much of the difference can be allowed. I cannot say exactly since that will be different depending on the operation point of the amp. Well what should I do? If I have to draw the line, maybe it would be like ±10%. I know it is ambiguous. But please don't call my amp analog amp. The table below shows my amp's values.

Please turn off the power once and wait for a few seconds until the block condenser etc. discharges electricity so as not to get yourself an electric-shock. And check if there is no abnormally hot part by touching with your fingers. According to my check, it is hot when PT is like 43 degrees at room temperature of 28 degrees. There is no problem in temperature here since the ambient temperature of electronic products is 40 degrees in general. (The temperature 43 is my guess, so if not please forgive me. Anyway I just wanted to tell you the concept. ) Connect the pin cord to the input, short-circuit on the other edge on left and right sides temporarily and set the input to zero for the hum balancer adjustment. Match and connect the right and left phase (polarity) of the speaker. Turn on the power again. Please turn the hum balancer so that the humming sound like mooo or booooon coming from each speaker becomes minimal. Please listen carefully by putting your ear on it. Remember the sound since it is the irritating noise you need to cope with. The volume position as it was when shipped, which is almost in the middle, was good for my amp. You won't hear anything at all within the listening distance. Let's just put it this way, my modification in the heater wiring turned out good. However, it is too early to take it easy since the impression would change depending on the performance of the speaker and background noises etc. I will connect it to the other high-performance SP later on to see what will happen. (There may be less hum with the original route. We better try and see anyway. This is scientific thinking. But if you say "Hum disappeared after the route was changed", that is occult thinking. This way of thinking can be seen everywhere in the audio equipment world of today.) When you are done with the finish touches like turning off power and organizing the intervals of the parts and wiring, put the back cover on and say "Hello" to your new amp. Your work has finally made its debut. Congratulations! Oh I am touched. You just need to check if there is any distortion in sound with your favorite CDs. There will be hardly any distortion except when you set the input to maximum since there was no abnormality found in voltage and the vacuum tube works within a normal operation range. Poor soldering will most likely cause the distortion in sound after the time passes. Fuzzy noises like chiri-chiri or zzzzzz modulated by strong and weak of soft can be heard. Sometimes you can hear them after a few minutes or years. (I am embarrassed to say though, I learned this from my own experiences.) If you want to find which soldering part is poor after you hear the noise, tap the part with an isolated stick etc. And redo soldering for that part. You will feel as if you were a SL maintenance staff.
Well, I noticed something important is missing before the tryout, which is that I don't have any high-performance speaker that can replay well with one amp. I forgot to prepare it totally since I was engaged in making my amp too much. Of course, I have a speaker to put some load as I mentioned above. So I pull out the 12cm-honeycomb flat full-range speaker of my own making for the tryout. Hum can be heard if you get like 30 cm close but usually you won't be hearing any hum from the normal listening position. This is not a SP which can overcome any in-depth evaluation in frequency response etc., but this amp creates soft bass and beautiful mid and high pitch sound. Besides, this amp is pretty high in gain, so there is no problem volume-wise. (The volume position is 10 to 11 o'clock for CD replay.) There is no crosstalk at all. I was only concerned with the hum adjustment, so just in case I checked it by connecting it to the 38cm97dB woofer the bass register of my multi-amp type main system. The humming noise was quiet enough. And just for the record, I achieved a level-balance for bass. First I heard kind of a rumbling bass, but it became surprisingly soft after I put an insulator. Large drums sound pretty powerful at just only 3.5W. Although it sounds a bit loose maybe, it felt pretty organized. I am thinking of bringing my amp to Hi Fi Do Akihabara Store to get an overall rating from third parties through various kind of well-known SPs. So please look forward to Hi Fi Do's report on my amp. (^^♪ Extra talk as usual 1. Often-heard story about amp, NFB: Negative feedback: Negative feedback and Non-feedback There is this desire to match the input signal and output signal together in the speaker driving system. This is a concept of the servo control in terms of various operation systems. Information is returned from the OPT secondary side to the first grade cathode in order to correct after relatively comparing the input and output signal so that the one existing at input is "Exist" and the one not existing at input is "Not exist". In this way, the dumping factor improves with bigger frequency response and less distortion ratio. I think that the Williamson Amp in the diagram below is a typical one of this type. But on the other hand, there are some people who feel suppressive in sound quality even if the response gets improved by NFB. It is because the signal for the microphone function of the speaker is returned to the first grade and which does bad thing. In a word, it creates the disturbance signal. This is why everybody takes notice of non-feedback amp recently. The typical amp of this type is the traditional Olson Amp. I think this kit is an Olson Amp type non-feedback amp. It's been said that the non feedback amp has no abrupt distortion even if it exceeds the rated output. This amp didn't create any distortion feel that I remember from my Williamson Amp even though I turned up the volume a lot. Moreover, you need to have the features improve since the non-feedback amp cannot be improved overall in response. This kit also adopts the film condenser for the power supply and the custom-designed OPT. I think there should be no problem at all, though high region is 30,000Hz - 3dB. These days, people are making a great fuss over the super tweeter because of a revival boom or whatever. I wonder what kind of stuff it would be after I hear about the crossover of 20,000 Hz. I would rather recommend the one that can creates high pitch sound properly within the range that you can hear. 2. Here are some simple properties published by the manufacturer. Used vacuum tube: 6SN7GT x2,2A3 x2,5U4G x1 Output: 3.5W+3.5W Input response: 150mV Input impedance: 100kΩ Output terminal: 8Ω,16Ω *1 Frequency response: 15Hz - 30kHz/-3dB Noise level: 2.5mV or less External size: W360xH190xD270mm (including a protruding part) Weight: 16kg Remarks: 2A3 of the output tube is SOVTEK (made in Russia).

I finally made it to the last after I was given the important task. I'm afraid if my writing sucked a lot or you got bored or tired of reading my writing. But I appreciate you all very much for reading it. It's been a really long time since I made the amp last time. And I could make it pretty well this time since I had to show my work to all of you. And I felt rejuvenated. However, I don't think I will make another amp anymore, so just let me put some pictures of my own amp for the last time please. Yes it is, this is the Otomeijin amp. Thank you very much. Welcome to the "Amp world"!

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