How to Build a tube amplifier - Practical realization
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On: 06 Agu, 2020
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It is obvious that the first thing to do is to get all the material, electrical and not necessary. The next step is to "present" the components on the aluminum plate, and to decide the position to give to each of them, taking into account the dime...
It is obvious that the first thing to do is to get all the material, electrical and not necessary. The next step is to "present" the components on the aluminum plate, and to decide the position to give to each of them, taking into account the dimensions and the necessary connections.
Some arrangements make wiring easier (eg all valves on the same line), but are less aesthetically interesting; it is advisable that the tubes and transformers are clearly visible (for the tubes there is also a question of dispersion of the heat produced; especially the final ones reach very high temperatures).
Once the dislocation has been established, we move on to marking, pointing and drilling the plate; the valve sockets, the tabs for the ground connections, and some insulated tab-holder bases are then fixed, which can be useful for connecting the components.
Now you can start the wiring, connecting with cables of adequate section and insulation (0.5 sqmm; conductors for electrical systems are fine too), first the pins of the filaments of the valves, then the other components (when it is possible to use the terminals directly capacitors and resistors for connections, without interposing any conductor).
However, try to make short connections, well laid out, and possibly using cables of different colors; respect these simple rules for both mono, which will essentially be a copy of the other; this to facilitate checks and any measures.
The LM317 integrated circuit (used as a constant current generator, see Clarifications for more information) will be mounted directly on the aluminum plate, near the EL84s, insulating it properly with the accessory small parts. For the pinouts it is advisable to refer to the specific table in the diagrams; resistors and capacitors, ceramic and polyester, have no polarity, the electrolytic ones show the + and - poles. The transformers also have the IN and OUT conductors with different colors; in order not to make a mistake, pay attention and check again several times.
As regards the two power transformers (see photo), they are two identical components; 220V input, 250V (120mA), 6.3V (3A) outputs, for a total of 50VA. The first feeds the anode and filaments of the two 12AX7s, the second feeds only the filaments of the EL84 tubes; in the latter, in fact, the 250 V winding was interrupted and, therefore, was eliminated. Nothing prevents the use of a single transformer of adequate power (eg 100 VA), or two separate transformers, one for the anode (40-50 VA), the other for the filaments (30-40 VA).
The power supply and the timer are mounted on an integrated pitch board; we have separated the two circuits, but nothing prevents them from being assembled together. For welding it is advisable to use a 25/30 W soldering iron and 60/40 tin wire, 1-2 mm in diameter.
Once the connections are complete, a systematic, visual and instrumental check is carried out using a tester to verify the accuracy of the circuits. It is convenient to check several times.
Remember that these circuits are powered with three different voltages: a very low voltage, 6.3 V; the second at "low" voltage 250-300 V DC; the last, the input line voltage, 220 V alternating current. Due to the presence of the latter values, the components and the entire circuit can be extremely dangerous, it is therefore recommended to pay the utmost attention, connecting the equipment to power sockets protected by an earthing system and by a highly sensitive differential switch ( 30 mA).
Personally, I first mounted the power supply and an amplifier, tried them, made some adjustments; then I mounted the second mono; therefore the same tests first separately then together. Testing a single mono the voltages are a little higher, because obviously the drops on the power supply resistors are lower. It is important, however, to use the delay provided by the timer, so that the valves go to temperature and therefore in conduction before feeding them with the anode voltages. With the values indicated, the delay is approximately ½ minute.
It is advisable not to insert the valves during the first live test; if there are no short circuits (fuse intact) the valves are mounted, obviously with the circuit disconnected; it is recommended to be careful when mounting the pipes, trying to keep them vertical, pushing firmly, without exaggerating to avoid any cracks.
Connect the speakers, power up the circuits, after the preheating delay, turning the potentiometer towards the maximum, no sound should be heard; at most, however, a very slight hum can be noticeable. In this regard, it is obvious to remember that all the connections, filaments and anode, should not be close to the IN conductors (bipolar cables of good quality, shielded, the braid to earth), and that the casings of the two potentiometers are connected to each other. and then to mass.
It is now necessary to adjust the two potentiometers indicated with R10 in the amplifier diagram; we will have previously adjusted the cursor to the central position; placing a tester on DC voltage full scale 100-200 millivolts and reading alternately the voltage across the two resistors, R11 and R12, adjust with small shifts R10 until the two voltages V R11, and V R12 are "equal" (about 30-32 mV, with Ian = 62.5 mA). Further details in this regard are explained below in "Clarifications".
At this point you can send the audio signal to the inputs (eg the output of a CD player) and listen with satisfaction, starting from a minimum volume and gently increasing, the sound coming out of the speakers.
Attention the valves reach very high temperatures.
Some aspects remain to be clarified or explained in a more widespread way. Small circuit variants have not always been highlighted. For example, in our constant current circuit (LM317), the 25 ohm potentiometers were not used; this value is difficult to find. We had found 50 ohm 3 W potentiometers, which can be used by putting in parallel to each of the same two fixed resistors placed in turn in series. With this circuit the values return to be equivalent to the original value of 25 ohms.
We therefore opted for fixed resistors, then adjusting the values of the two 1 ohm resistors by trial and error (the values can vary from 1 to 4-5 ohm, 0.5 W). The result does not change much, even if obviously, the potentiometers allow a more precise and faster adjustment.
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