2020 Monkeymatic Black Butte Desktop #2

[xtian]

Boredom is a good excuse for a beaks-and-feet build, right? Let's make it happen.

2020 Monkeymatic Black Butte Desktop #2 (FYI, here's #1, based on THD Univalve)

This one is based on the simple circuit in the Akai/Roberts M8 amps I turned into Black Buttes, but instead of 1 watt, this build will be closer to 10 watts, I think, using one 6L6GC as SE power tube.

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[EDIT: layout updated 4/13/2020]

[studiodunn]

looks fun! I wish I had a bin of beaks and feet transformers in the garage.

[johnny riff]

looks great! is this a new concept you've arrived at?

[Guy77]

Excited to hear the finished product! I am sure it will sound stellar!

Cheers
Guy

[Colossal]

Xtian, looks like it's going to be another very cool build! I noticed the 470k grid leak is located after the power tube grid resistor.

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[xtian]

Xtian, looks like it's going to be another very cool build! I noticed the 470k grid leak is located after the power tube grid resistor.

Thanks, Dave!

[xtian]

She lives! But I wired the pots backwards.

[xtian]

Got it working. Sounds good. More details to follow.

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[xtian]

Badged.

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[martin manning]

Very clean design there, Aaron! What is the operating point for the 6L6 (Va, Vg2, Vk, Zpri, and idle current)? How does it sound? A clip would be great...

[xtian]

Thank you, Martin. Details shown below. Best output, 12 watts, is using the White OT secondary into an 8-ohm speaker load. Data sheet shows that's a 2K5 primary load.

I'll get a recording posted soon.

I also changed the power tube's screen and grid resistors to 470R and 1K5.

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[martin manning]

Did you try it at 5k Zpri? I think that might be a bit better, that and bringing the screen voltage down some.

[xtian]

Did you try it at 5k Zpri? I think that might be a bit better, that and bringing the screen voltage down some.

In what way, "better"? Do you anticipate dropping the screen voltage will improve the character of the power tube distortion in some way?

I measured each tap against 4/8/16R resistive loads, and the clean wattage output is shown on the yellow post-it, above. So, the yellow secondary, versus an 8-ohm load, gave me 8.8 watts, versus 12 watts using the white secondary.

[martin manning]

Typically you want the load line to be tangent to the max dissipation curve, and centered WRT to the available voltage swing, which is achieved when Z = V^2/Pa, which, for 390 Va-k and 30W, would be 5070Ω. Where you are now you're going over 100% when the signal goes positive. Screen voltage will then determine how well centered your bias point is on the anode curves (between cut-off and grid current limiting). You're biased about 5V to the cut-off side now (assuming a nominal spec tube), so lower screen voltage would bring it closer to centered.

[xtian]

Martin, thanks for helping me with the load lines! I replaced the first dropping resistor (4K7) with 15K. New conditions:

B+ 440
Va 426
Vg2 388
Vk 30.7 (66mA)

and I'm running with the nominal 8-ohm tap (5K primary).

The amp is sounding GREAT. Really smooth onset of distortion. And now I've installed an L-pad for bedroom volumes--works great.

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