Low Power 6U8A Vacuum Tube Audio Amp

Published:2013/3/10 22:11:00 Author:Ecco | Keyword: Low Power, Vacuum Tube, Audio Amp | From:SeekIC

The line-level audio signal is sent through a 100K audio volume control and into the grid of the 6U8A triode section. The triode is wired as a standard class-A tube amplifier. The B+ line to the triode is isolated from the final amplifier's B+ through a 5.5K/22uF low pass filter, this prevents interstage feedback and oscillation. The output of the triode amp is sent to the control grid of the 6U8A pentode section through a 10nF capacitor. The pentode is wired as a class-A amplifier with a transformer output. A negative feedback loop linearizes the amplifier's frequency response, this consists of a 120K resistor and 470pF capacitor from the speaker side of the output transformer to the cathode of the 6U8A triode. Feedback is kept to a fairly light level to preserve the amp's dynamics. The R/C network across the output transformer primary was adjusted for a flat frequency response. The pentode's screen grid is pulled up to the B+ voltage through a 1.5K current limiting resistor and AC-bypassed with a 10nF capacitor. If a tone control is desired, it can be built onto the feedback loop, see my Squirrel Monkey amp project for an example of this type of tone control. If you build the amplfier and it oscillates, swap the output leads on the output transformer to correctly phase the feedback signal. The power supply is not shown, this amp is designed to be an audio module that can be combined with other modules to build a radio or other audio devices. Any standard vacuum tube power supply should work here, it just needs to provide 6.3VAC for the filament and 250VDC for the B+ circuit. The amp should work fine with B+ values between 150VDC and 300VDC. Again, see my Squirrel Monkey amplifier for a power supply that will work with this amplifier.



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