Low Voltage Single-Ended Madness!

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This little amplifier was inspired by a simple design found on the ‘net. I had found some Philco output transformers from a stereo record player that had tapped secondaries, which measured to be 2500:8 ohms or 4000:8 ohms. This opened up some interesting possibilities for using different output tubes in the same circuit but maintaining good impedance matching. If a 6Y6 is used, the cathode bias settles out at a current level that pulls B+ to 205V. Connecting the speakers to the 2500:8 taps gives the best impedance match. If a 6W6 is used, the current draw will settle out lower, pulling B+ down to 230V. This is where 4000:8 is the best match. The amp was built on a hammertone finish-painted steel chassis, with the power transformer under the diecast aluminum cover, and the output transformers under the chassis. This will output 3.5 watts per channel with 6Y6, and 3 watts per channel with 6W6.

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Here is the schematic of the Madness, which is actually my first SE design. Overall similar to the previously posted 6Y6 amplifier, there are several important differences. This is connected as single-ended pentode, which will deliver more output but requires some sort of global feedback. It is also cathode-biased, which lowers output but simplifies design. The power supply is CRC filtered solid state, with a large R (680 ohms). This gives the power supply a lot of what is termed as “sag”, where the output voltage varies greatly with load. The interesting effect of this power supply is that the amplifier never ‘clips’ when overdriven, it simply compresses! This, like several of my projects, was a budget build that almost shouldn’t work out, but with a little tuning, it worked out beautifully.

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