Sopht-y Mini Tube Amp

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Last night I built a little amp inspired by the Sopht amp. It has 12AJ6 and 12AL8 tubes. Powered by a salvaged 12V 1.1A wallwart.

EDIT: It's working fairly well now. I added three RC poles to the power supply filtering and lost the old cap in the wallwart. No hum. Surprisingly loud through a Jensen Mod 8" speaker, which is not so efficient.

No schematic needed, it's dead simple. Just a 12AJ6 triode with Rp=100k Rk=4.7k (bypassed with a 47uF cap), then into the 12AL8 circuit shown in the TungSol Datasheet (200k PDF). Output transformer is a Triad S-79Z 70.7V line transformer.

There is a bit of unpleasant distortion, almost sounds like modulation. It's not so noticeable in the low notes, but higher notes are more affected. It's the kind of thing that seems to stay at a lower volume, so it's masked on the attacks but cuts through as the notes decay. I think I need to dumb down the circuit and limit the bandwidth. Maybe a smaller cap between the first two gain stages.

Doh! I just realized the output is phase reversed from the input. Should I swap the transformer secondaries to correct that?

Thanks!

Brian

 

[NewYorkDave]

Most so-called "DC" wallwarts put out a waveform that's so ripply that "PDC" (pulsed DC) would be a more honest name for it. Add filtering or try a regulated wart. Have a scope? Take a look at your B+... probably ripply as hell. Single-ended amps don't like that one bit.

 

[mikka]

Curious how it sounds hooked up direct to a car battery...... still hum?

 

[NewYorkDave]

Good idea... or a couple of lantern batteries in series. (Do they still make those?).

 

[skipwave]

Yep, I think you are absolutely right. On a second listen, it seems like 120Hz hum. I bet the filter cap in the wallwart is dried up. It's from an '80s Marantz 4-track. Puts out 15V unloaded. I didn't want to go regulated, but added a 12000uF cap for filtering. A seperate RC pole for the heaters and the B+ to each tube would be good as well.

A test on a battery is a great idea, after all that's what these tubes were made for.

Building this little toy has bolstered my confidence about finishing my 2-Watt amp. That'll be done shortly.... much to my neighbors' chagrin. :grin:

 

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12AJ6 Datasheet (100k PDF)

 

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Bump.