6M11 push-pull spud amp - diyAudio
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Old 13th November 2013, 01:22 AM   #1
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Join Date: Sep 2013
Default 6M11 push-pull spud amp

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.
Attached Images
File Type: png SpudAmp6M11nt.PNG (53.9 KB, 116 views)
File Type: png SpudAmp6M11.PNG (44.9 KB, 116 views)
File Type: png Tone_AT.png (56.7 KB, 113 views)
File Type: jpg Complete.jpg (60.1 KB, 112 views)
File Type: png PlateCurvesComposite3.png (81.7 KB, 111 views)
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Old 13th November 2013, 04:56 AM   #2
tomchr is offline tomchr  Canada
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Location: Calgary
Push-pull spud. That's pretty cool! Thanks for sharing.

Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits.
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