Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pc1093 on August 08, 2023, 03:13:00 pm

Title: how to find the negative feedback resistor value for 8 ohm output
Post by: pc1093 on August 08, 2023, 03:13:00 pm

Hi all,
the original negative feedback resistor is 15K for the 16ohm output.
My output is 8 ohm. I have tried 15K and it does not work. The transformer has Self-oscillation.

So what is the resistor value for 8 ohm?
thank you

Title: Re: how to find the negative feedback resistor value for 8 ohm output
Post by: sluckey on August 08, 2023, 03:34:18 pm

10.6K would give the same NFB from an 8Ω tap as a 15K from a 16Ω tap. However, there is no reason why a 15K will not work on your 8Ω tap. I suspect you need to swap the OT primary plate leads.