Super Triode connection Circuit (STC) Vacuum Tube Amplifier

6BM8 in 'Yaki-nori' amplifier

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Last update of this page was Jun. 1 2002.

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In early '70s, it was an era of vacuum tubes and transistors were alive both. I remember that the tubes were mainly used in RF equipments eg. AM wireless microphones and radios, and high power equipments. By the way, 6BM8 was a cheap tube used as a typical AF power amplifier, and used also in AM modulation of AM wireless microphones, and AF power amplifier in the radios, I remember.

Time passes to year of 2002. The tube is very nostalgic, but I have no tube equipment now. And it is no good to make just the same circuit in old days, I feel. I come across "Super Triode connection Circuit (STC) Vacuum Tube Amplifier" in the internet.
Page of a hand-made vacuum tube amplifiers---The site of hand-made tube amplifier club having much information.
Evolve Power Amplifiers ---Mr. Kamijo's site, he invented STC amplifier.

I don't understand detail theory, but I'm impressed that this is easy to make with few parts and having nice sound quality. Further I'm impressed much that this is not nostalgia but the new circuit invented in '1990s. This is a great invention to revive old device by putting new life. And more, this circuit is easily made only one 6BM8 with sweet memories. I want to make it because of the characteristics that nice sound quality with reasonable price in small, easy to make, with 6BM8. The circumstances like this and situations in "AKIBA" are agree with Mr. Hayashi in a next link.
Essay of making vacuum tube amplifier (Japanese)---Mr. Hayashi's site, he finds the site of hand-made tube amplifier club, goes shopping parts to Akihabara after 30 years absence, and makes 6BM8 amplifier; just the same experience of me.

Parts shopping

Now we can buy parts in easy by mail-order, but I went to "AKIBA" (abbreviation of Akihabara), because it is a part of essence of making tube amplifier to taste an atmosphere in underpass of JR lines at Akihabara station. In old days, I lived near Akihabara, but I move to suburb of Tokyo now. So not very easy to go. I cannot go there in weekdays, and I hear that some shops are closed on Sunday in Radio-Center or Radio-Department-Store recently. Therefore, there is only chance on Saturday. Further, the great purpose of this shopping is to show my sons for tasting the atmosphere of small parts shops (not a PC parts but a conventional electric parts) in "AKIBA". I have no idea how children will feel there. But this unique atmosphere may be valuable cultural heritage. It is very clear in Japan that electronic DIY hobby including vacuum tube, audio and amateur radio are declined very much in nowadays. I think that it is meaningful thing to experience it.

But, if they will go into engineering or go to the science course, whether they will be happy or not..... is very difficult question, I think. If the Japanese society itself gives this feeling to the people, or the society does not open the possibility to children, and if the result is a decline in the number of births, the future of this country is miserable........ Not only anxious of children but also even in our generation we are taxed much and will not receive pension in the future maybe .....

Anyway, D-DAY(?) is March 30 (Sat), 2002. This day is the day of the 30th anniversary of opening an operation of JE1QMS amateur radio station. (But I have been QRT for a long time.) First, we receive a handbill at a road just the side of JR Akihabara station. We walk about Radio-Center, Radio-Store, and Radio-Department-Store across the street in many times, and get small parts, vacuum tubes, and transformers. Because we do not go to PC parts shop intentionally, go to only conventional old parts shop, we taste the atmosphere very much. For the children, they watch vacuum tubes and other parts for the first time. It is unknown that they remember these parts, but will remember that the transformer is heavy. (This is simple, I don't like to carry a heavy one...) Maybe because the STC amplifier is famous, seller in Toei-transformer store understands the "set" of transformer to make 6BM8 amplifier. There is no need to explain in detail to him ! Of course, the lunch is not Jangara-ramen but "Nami-To-Tamago" (a beef bowl medium and raw egg) at Don-Don. (It is pity that not "Niku-no Manse" luxurious restaurant.... But "OOmori-Tamago" (a beef bowl large and raw egg) at Yoshino-ya on 30 years ago... It is also pity that medium becomes just my size because I am aged..... ) After shopping and lunch, we go to Akizuki-Denshi-Tsusho (I went frequently in the day of Shinetsu-Denki-Shokai), and Sengoku-Tsusho (I went frequently not Sengoku but Ado in the day of Ado-Denshi existed.), and buy some ICs. This is also an event for children that they will find the difference between the parts before and after lunch, and feel the stream of era. The children will only remember the taste of the beef bowl...

Selection of parts and case

Frankly speaking, the selection of visible parts and case is important, because I want to make not only electronic quality but also even the taste. Most important matter is well visible vacuum tube. Sound quality is important, but it is the first thing to watch the shape of 6BM8. And simple is best, there is nothing except a power switch, a power indicator, and a volume. The design of the shape is like '70s. The shape of the switch, the knob of the volume is like oldies. The power indicator is not an LED but a neon tube. It seems that a vacuum tube and a neon tube are the set. (A constant voltage discharge tube was expensive, so I substituted a neon tube for a discharge tube, I remember. But does constant voltage effect exists in a neon tube or not ?? There is a way of thinking that dischage phenomenon itself is like zener effect... ?) I wonder if which case is the best. The height of the case is needed because there are many transformers. At last, the case is decided to "Yakinori" (baked laver) can. Therefore, a name of this amplifier is decided as "Yakinori" amplifier. The good point of this Yakinori can is this color and this luster. The relationship of red light from vacuum tube and neon tube, and green color of this can corresponds to red color of the flower of an azalea and leaf color green in a Japanese garden. an azalea in Japanese garden, and reflected to the surface of the pond. And in the dark room, there is a taste that heater light of vacuum tubes is dully reflected by the surface of this can. It looks like a flower in a Japanese garden is reclected to the surface of the pond. This is just the same that the taste of "Seibun-gyo" (some kind of goldfish) is the reflected red color of red goldfish by a scale of Seibun-gyo. (I kept Seigun-gyo in old days....)
And, there is more taste in peel off green paint because of my poor craft skill....


Yakinori can and parts First, think an arrangement of parts by putting them near the can. The direction of the tube is decided by the thought of relationship I/O pins and Line IN / SP OUT terminals..... this is a lie. The direction is only decided for looking good 6BM8, it is simple ! The arrangement of transformers is decided automatically because of these shape and weight. (click for magnify the photo.)
dig a hole. tired. Dig a hole by a drill. Digging is little easy because the material of Yakinori can is softer than usual aluminum chassis. But an attention is needed for not peeling off the paint, by digging a hole at painted surface is down. (Fault is done only by me ??)
a two-story transformer Output transformer, I put together two like many people done. The height becomes just for Yakinori can. Inside of the can is little tight because there are many big parts. It is allowable that the transformers outside, but you need to pay attention for electric shock if the terminal of the transformers are naked. You must remember fear only in vacuum tube circuit. Because I bought a skirt for a chemical condenser, using this skirt was possible. But there is no room in the can, I attach it by an adhesive couple-face tape. Bridge diode for rectify is also by an adhesive couple-face tape.
2SK30-GRBIt is unknown Y is better than GR ?? And 2SK30 board is also an adhesive couple-face tape. In this situation, some people are reminded repair hell using BAUMKUCHEN, so I don't like it, but will be forget by cover... The 2SK30 board is attached by tape to near volume. (I will write that 2SK30 will not be needed after all.)
ugly wired.... Soldering is done by turns. Attention is needed that the case for the last position for soldering is the most depth place... Do not remember GND lines. Pay attention to one point grounding technique. First, you make only power section, and make amplifier section after that. In heater circuit, you must twist lines for suppressing common mode noise because AC current is directly used.

Circuit Diagram

I almost copy the circuit diagram from several sites. But at last I do not use FET, but use Tr. The situation like this, I write in the next.

Circuit Diagram for Yakinori amplifier


Finally, Power ON ! Please use not your main speaker but cheap speaker prepare for the worst. Input Line OUT signal from CD player, and make adjustment VR. First you will hear only very small distorted sound. More rounding the VR, suddenly sound will be big and clear. This is good point but check the voltage between the cathode resistor, and calculate cathode current. Cathode current must adjust under 50 mA. 35 mA is ideal but it is not so easy. In my case, changing power transformer tap for down the voltage and adjust 38-40 mA. But I found that it is clear sound in small volume, but sound is distorted in large volume by clipping. Unfortunately, signal is distorted even in the case of not so large voltage amplitude. I tried to change feedback resistor value in triode and cathode resister value in pentode. But all attempt makes no result. I try to turn VR at 2SK30 for cathode current to 70-80 mA, suddenly waveform become clear, but it is not allowed because tube will be dead.

I have an trouble.... and check more site... In Mr.Uda's page, there is a sentence that FET is critical. I don't surprise at all for the most people have nice result with a good luck, and only me is not nice. This sutuation is very common. Therefore, changing the circuit immediately.
After all, I check Fig. 2 of Mr. Kamijo's circuit (Japanese). Change FET to Tr, and it can decrese the number of parts and the number of changing points. The result is the circuit above. Most people make good result by 2SK30 circuit, but if someone comes across the symptom that there is distortion in not so large volume like me, I recommend this circuit. Because Tr is 2SC1815, this circuit is cheaper than FET's one. If you do not have 33 ohm, you can make 3 parallel 100 ohms for 33 ohm.


This is not listening but photo gallery. I put several Yakinori amplifier photos. (click for magnify)
Whole view of Yakinori Whole view of Yakinori 6BM8
Whole view of Yakinori amplifier. You can see the damage made by digging holes. In the photo you can see the wood, the amplifier is set at the side of backloaded Horn Speaker made at the next corner.
Whole view of Yakinori 6BM8
In the dark room, you can see the light of the heater.
I put N guage model train "Toden" (Tokyo streetcar, which was serviced mainly early '60s and abolished late '60s.) between two 6BM8s. This is good photo for the people who knows those days toden, I think.
In real listening, please look the next corner Backloaded Horn Speaker.

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