I had a bunch of these stubby little 6M11 compactrons gathering dust, and decided to see how well they would work as a push-pull spud amp. The triode sections are only rated at 2.25W plate dissipation, so it's not a powerhouse. I get about 350mW per channel max., but the audio is very clean. The original schematic is shown in the first attachment.
THD is about 0.25%, which is almost entirely 2nd harmonic, due to the less than ideal phase inversion method. Frequency response is 20Hz to 70kHz. I used little 4 watt 70V line transformers for the OPTs. They work quite well in this low power application; more than enough iron for 350mW.
Later, I decided to add a tone control which is shown in the second attachment. The tone circuit gives flat response with the control in mid position, and bass boost/treble cut at one end and treble boost/bass cut at the other (see 3rd attachment). The 680pF cap is not part of the main tone control. It's there to cut off the input signals above 20kHz.
After testing out the breadboard version, I decided that it would make a good little amp for my computer audio, so I built a permanent version. The final construction is shown in the 4th attachment.
It drives a vintage pair of Realistic Minimus 7 speakers with more than enough volume.
The final attachment shows the triode section composite plate curves: extremely linear. Orange lines are SOA limits. At high output, it can be seen that SOA is exceeded, but peak levels occur infrequently in my application, so I'm not too concerned.
THD is about 0.25%, which is almost entirely 2nd harmonic, due to the less than ideal phase inversion method. Frequency response is 20Hz to 70kHz. I used little 4 watt 70V line transformers for the OPTs. They work quite well in this low power application; more than enough iron for 350mW.
Later, I decided to add a tone control which is shown in the second attachment. The tone circuit gives flat response with the control in mid position, and bass boost/treble cut at one end and treble boost/bass cut at the other (see 3rd attachment). The 680pF cap is not part of the main tone control. It's there to cut off the input signals above 20kHz.
After testing out the breadboard version, I decided that it would make a good little amp for my computer audio, so I built a permanent version. The final construction is shown in the 4th attachment.
It drives a vintage pair of Realistic Minimus 7 speakers with more than enough volume.
The final attachment shows the triode section composite plate curves: extremely linear. Orange lines are SOA limits. At high output, it can be seen that SOA is exceeded, but peak levels occur infrequently in my application, so I'm not too concerned.
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