2,4W SE amplifier with 6P9 (6AG7) or 6P15P TV video amplification tubes
It is very simple amplifier, which uses only one tube per channel. Not two tubes in one glass or metal envelope, as usual in many amps, but single, literally. By the way, tube must have high amplification level and work as a pentode, this way it is possible to reach required level of amplification. Tubes, designed to work in video signal amplification circuits should do the trick. One of them is 6P9 with metal envelope 6P9 (6П9) or it's little analogue 6P15P (6П15П). 6AG7 and Telefunken EL83 is very similar to tubes, mentioned above.
Amplifier's load depends on how output transformer secondaries' sections are connected. If they connected in parallel, as shown in scheme, amplifier may work with low impedance speakers (or low resistance headphones). If secondaries are connected in series (don't mess up with phases), then it may work with high resistance headphones.
Almost all parts, used for this amp may be found in local radio shack store. Only output transformer requires little more time to work with, because it is crucial component and overall sound quality depends on it.
I have used very popular OCM-0.1 steel core (25x40mm) for output transformer. Primary winding: 0,15 mm wire, 2 sections, 1800 windings each, connected in series. Secondary winding: 0,5mm wire, 3 sections, 57 windings each, connected in parallel. Scheme of winding: S57-P1800-S57-P1800-S57. Layers are separated with 0,1 mm transformer paper, sections are separated with 0,2 mm transformer paper. Transformer is assembled with air gap. Air gap keeps the core from overloading by magnetic field and distortion. For this reason 0.05mm thickness paper spacers are glued between halves of core. Don't even think of glueing aluminium foil or similar stuff!!!
Tube is connected as a pentode. Shielding grid voltage is stabilized (D1), this way distortion level is reduced and anode voltage is used at optimum. Grid bias (-3V) is created using battery. There is almost no battery drain, because there is no current flow. PC motherboard's battery works like a charm.
If amp is used only for headphones, it may be simplified: tube is connected as triode, this way sound quality is better. For this reason R3, C1, D1 are not used. Shielding grid and anode is connected through 270ohm resistor (resistor is between 6'th and 8'th tube pins). Output transformer requires some changes too: primary windings are reduced from 1800 to 1400, 0.18mm wire (1400+1400). Amplification level and output power is reduced, but amp becomes more linear.
I have used two (TR1, TR2) toroidal 50W transformers, designed for 12V halogen lamps, which are popular and you don't have to waste your time on making one. Primary TR1 winding is connected to the outlet, secondary 12,6V winding connected to the same TR2 winding. What do we get? Transformation 220>12>12>220. This way of connection gives one great benefit, reduces network noises, because 12V windings work as shield/filter. Both tubes' filaments H1 and H2 are connected in series (12,6 = 6,3+6,3) and are connected to 12V windings.TR2 220V winding is connected to rectifier bridge, DC voltage is filtrated with CLC (C3, DR, C2). Instead of DR it is possible to use choke from vintage TV.
So, what are the conclusions? We get an amplifier with no capacitor in signal path, like it is said: best capacitor ir signal path - no capacitor! Sound quality using high quality heaphones is exceptional, and it should be, because signal path is as short as possible, just a single tube and output transformer.
Tuning is very simple. First of all, you turn the amplifier on with no tubes. Check anode and filament voltages. They will be higher than according to the scheme, but that's because of no tubes, in other words, no load. Filament voltage should be ~14V, anode voltage after choke up to 330V. That's all the tuning, if there are no mistakes in assembly, amp should work.
This amplifier was created very very long time ago, when digital cameras did not exist, so there are no photos. By the way, this project is very suitable for rookies.