Sound City 120MK4 Bias mod, a slight return

[01redcobra]

I know this has been touched on before as the 120's don't have enough negative voltage on the bias to run newer EL34's safely. I happened to be reviewing a bunch of schematics of SC amps and found some differences in the bias circuit on the Bass 150 that I thought may be transferable to the 120 to get the bias better suited to newer tubes. I give you the 2 bias schematics for your review and comments. Thanks!
120Mk4bias.pdfBass150bias.pdf

[blindboybenton]

Hi Look up a Voltage doubler circuit. This should give you about -70v and then you can bring it down to whatever you want.



BBB