Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on February 25, 2024, 08:19:26 am

Title: Math question (NFB calculations)
Post by: dwinstonwood on February 25, 2024, 08:19:26 am

Hello, I'm trying to calculate voltage divider resistor values for a NFB circuit.

The 6G6-B (https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_6g6b.pdf) uses 56K and 4.7K resistors with the NFB coming off of a 4 Ohm secondary tap. For the sake of math, let's say there is 10 volts at that tap. So, the above voltage divider reduces that 10 volts to 0.77 volts.

Now, if I'm using an 8 Ohm tap would I calculate using 14.14 volts? If so, then using a 56K and a 3.3K would result in 0.78 volts. Close enough, yes? Or, am I missing something obvious?

Thanks!

Title: Re: Math question (NFB calculations)
Post by: sluckey on February 25, 2024, 09:31:59 am

Your math looks good and should result in the same amount of NFB using either the 4Ω or 8Ω secondary tap. But math is an exact science. NFB is not. Your ears may be better at determining NFB resistor values.

Leo was not as precise as you are. For example, in his AB763 amps with a 4Ω or 2Ω speaker he used an 820Ω/100Ω NFB ratio, but for an 8Ω speaker he used an 820Ω/47Ω ratio.

Title: Re: Math question (NFB calculations)
Post by: dwinstonwood on February 25, 2024, 10:13:19 am

Thank you sluckey!

I'm finishing up drawings for a single input 6G6-B using your Dual-Lite channel switch (the 6G6's channels are out of phase).