I learned tube circuit the hardest way, by using 6n2p-ev (high gain dual triode, 6.3v heater) at 12V.
Why is it the hardest, because at 12v, the "tube theory" doesn't work at all :
- biasing is hard, no negative power supply, very small anode current for cathode resistor
- gain is limited, i got only 10 from
- headroom is limited, i lost around 2V on both side, makes only 8v room to play
- output inpedance still very high
On the good side, the limited gain works enough for line level amplification for a small watt application (headphone amp, etc), therefore i build my first "tube amp" for headphone amp.
My headphone had only 16-32ohn impedance, it need current driver more than voltage amplification. Well, such current can't be provided from tube circuit at low voltage supply. Therefore i use BUF634 chip for the output. The tube act as voltage amplification, the buffer act as current driver.
6n2p-ev is dual triode high gain (similar to 12ax7 but with 6.3v heater), the biasing is done by resistor to positive 12v (??, amazing discoveries by the person who created attached pdf file) to make sure anode voltage at around 6v-7v, with stereo 100kohm pot at input side as gain control, 100kohm plate resistor, 15kohm grid stopper resistor, and buffer at output side (coupled by 2200uf 16v , grounded with 1kohm resistor) to the headphone. This makes me have to wait for the capacitor to fill up (to make sure output at ground level) before i connect the headphone or else loud pop exist, the same need to be done in reverse when power off, headphone need to be removed before power being turned off.
The heater is powered by lm317 set to 6.3V output, slow start done by adding big capacitor at adjust pin to ground, the problem is it waste a lot of power on thr lm317, proper heatsink is needed (300mA x 5.7V, around 2 watt).
The sound is warm, i believe i hear more detail of the sound (like breath of the singer, detailed drum playing), and this save my flac player battery as i can play it at low volume and let the tube amplify.
Let it glow..
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