skipwave
Well-known member
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
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