Assembly Procedures for the Single Ended Triode Tube Amp
Wiring the Tube Amp
More Assembly notes and photos. A Schematic "looks simple", but the actual hookup can become a nightmare . These images and notes will help :
The Schematic for my Blue Berry Tube Amp ( again - too small to see well- Full size copy for $100 to build one amp.
Once you study the wiring photos below, you will see how difficult it is - for not a single wiring error is tolerable. After all, you do not want to buy many hundreds of dollars worth of components, only to see them go up in smoke, now do you? If you are like most people, you will get one chance at this, owing to the cost.
The Blueberry SET Audio Tube Amp
Click on thumbnail for a full size image. Not only is every single wiring connection critical...placement of the wires is critical to keep the AC filament wires for the tubes as distant from the other signal wires as possible. You can see that the AC filament wires in green are twisted to keep down the"hum".They are kept to one side of the tubes, and the other wires approach from the other side of the tubes
Close up of the circuit board; and over to the right column a drawing of how the wires connect.
I can make available both the schematic AND parts if so desired-which include the planks for the chassis. You can design your own chassis of course. This chassis was meant to be simple as possible...but also to be very strong - for the amp weighs about 25 pounds, and must be built very stoutly so as not to fall apart easily. Of course a person with a wood shop, and experience, can design a much more beautiful chassis, with exotic woods, and some fancy woodworking skills...the woodworking tools themselves involve considerable expense. . it is possible to build one of these amps with just thekit supplied, and a soldering iron, and a voltmeter, and a variac....and extremely careful and flawless wire hookup, so as to avoid burning up any expensive components
First...the close up of the circuit board; followed by an image below that to show a drawing of how the components are connected, and run to pins on the tubes. A printed circuit board is supplied with purchase of the parts - or with ONLY the schematic, you can do this on your own-but only if you have some experience.
The twisted, heavy green wires are the filament voltage at 6 VAC. They arephysically distant enough from the signal path so as to not induce a hum. I thappens that a good schematic is # 1 priority; followed by judicious placement of hookup wires. At first, I thought I would service the amp from the bottom - but that turned out to be impossible to trouble shoot by tracing signals..too little room. so I made the control panel on the front of the amp to be unscrewed, and dropped down for a full view of test points you see above. This required leaving some signal wires "loose", in mid air" to allow for enough flex. It should be noted here, that solid wire will break after only a couple of 'flexes', so stranded wire, though more messy to strip and solder to pins, etc...is recommended.
Below here is the same Blueberry amplifier-but with an Art Deco style Chassis.This was designed just in case one wanted to keep toddlers in the household from getting their hands on hot tubes...with lethal voltage on them.
Here is a close up of the first stage of the amp. When the amp is first turned on, a meter will show ca. 450 VDC on the plates immediately, but as the filaments warm up and start a current flow, the DC voltage on the plates will drop to the numbers you see there. Current "flows" through the tube for only an instant as the voltage on the plates fall to a "quiescent state"...and current flow falls to a small , but steady flow-delivering no sound to the speaker, since it is steady . This very important current flow causes the cathode resistor to "exhibit" a positive voltage at the top...and correspondingly, the ground end of the cathode resistor would be the negative "pole" so to speak.
I trust you paid attention to the 450 VDC above. The Power supply Transformer swings peak to peak 720 VAC! And is "Rectified" to 350VDC. But the 500 volt capacitors to smooth out the "Ripple" of residual AC component on the rectified DC - can discharge, and together with the tansformer iron holding charge, and the capacitors holding charge...the effect of 700 volts can be felt on you if you are not extremely careful to avoid touching anything after turning off the amp...those devices still hold charge, and is "Lethal"..deadly.
I repeat here, that experienced technicians have a wire with alligator clips at each end, and a One Watt, 1 thousand ohm resistor between. First one end is clipped to the circuit ground, and the other end is carefully, gingerly applied to anything in the circuit with high voltage - which is all across the "Rail" it is called, which supplies the anodes of the tubes. This will quickly bleed down the charge on the capacitors, and thus make it safe to work on the circuit some more.
This all important feature of a positive charge at the top of the cathode resistor "causes" the wave signal at the grid to appear to be 100% negative. This "restrains" the tube from overconducting, and burning up-which it would do with any kind of positive charge on the grid. You see there is a powerful positive charge on the anode, and the only way to regulate or control the powerful current flow through the tube is to restrain the current flow with the "all negative sound signal".
This is all a result of the positive charge at the top of the cathode resistor...and is called "Self Bias"...instead of a separate positive DC source applied there...the cathode resistor "supplies" that charge. ...providing the all important restraint on the tube which has the potential to "unleash" a large current flow, with only the aid of a small, low voltage signal from your audio source...typically less than one volt from most audio sources. If you followed this little speech...you understand how the tube "amplifies". I explain this again in greater detail on the other pages on this website.
All the tubes for this amp have the same tube pin arrangement as shown in the drawing below here. The ceramic sockets for these tubes are marked , as seen from below, where one does the soldering...marked clockwise one through eight. The 7 th and 8th pins are always the points to apply the 6 VAC filament voltage. Note the color code...since it is difficult to actually see the numbers on the sockets, it is a great help to use colored hookup wire...red for high voltage, green for signal voltage, yellow for cathode , and purple for the filament voltage...although I did not have available heavy guage wire for the filaments, which carry a LOT of current...the other wires can be much lighter guage...I used Light green for my filament wires in the photos above. This is a big help when using the voltmeter probe to test and read voltages, easily identified via color.
This diagram applies to all the"Twin Triodes" popular in audio and guitar amps - the 6SL7, the 6SN7 6BX7, and 6BL7. Popular owing to the inherent "Musicality" of the Triode Tube; and the simplicity of the circuit. The goal of HI FI audio is to make the signal path as simple and uncluttered as possible. You will learn that the amps featured here, the "SET", or Single Ended Triode" amplifiies the signal 270 times larger in only three steps. Transistor amps by comparison can have many dozens of components involved in the signal path.
