Bassman OT questions..

[sergio]

Hi , I have finished my new dumble(non HRM) and I have used Bassman output transformer (hammond type) with 2-4-8 Ohm tap.

My question is : what is the right tap for the NFB?

I wired it on 2 Ohm tap.. it's right?

Thanks

[heisthl]

4 Ohm Tap

[sergio]

Thanks Heisthl for help me, I'll change tap by 2 to 4 Ohm.

Thanks very much

Sergio

[wjdunham]

..

[llemtt]

Different taps have different output voltages: 8 ohm tap has 1.414 times the 4 ohm tap output voltage.

Higher output voltage fed back into the amp feedback loop means a higher feedback level the same way as reducing the feedback resistor, the effects are the same.

cheers
teo

[Sonny ReVerb]

When in doubt, consult 'the Master':

http://www.aikenamps.com/NegativeFeedback.htm

http://www.aikenamps.com/GlobalNegativeFeedback.htm

If you can wade through the math, there's a ton of good stuff there.

The series feedback resistor, in conjunction with the resistor to ground, determines the amount of voltage being fed back. If you want to feed back more voltage, you make the series resistor smaller, or the shunt resistor larger, or you use a higher impedance tap on the output transformer.

The actual resistor values used in the feedback attenuator aren't that important, as their ratio determines the amount of feedback. The shunt resistor value is usually fixed by the phase inverter design requirements, and the series resistor is then sized according to the desired amount of feedback, given the voltage available at the output. Note that Marshall typically uses 100K/5K attenuator, while Fender uses a 820ohms/100ohms. You can get the same attenuation from a 10K/500ohm pair as you would from a 100K/5K pair. In addition, if you were using a 100K/5K attenuator running from the 16 ohm tap, you would get roughly the same amount of feedback if you used a 47K/5K attenuator running from the 4 ohm tap. Note that the tap voltage is not linear with respect to the impedance, it varies linearly with the square root of the impedance. That is, the voltage on the 8 ohm tap is not half the voltage on the 16 ohm tap, rather, the voltage on the 4 ohm tap is half the voltage on the 16 ohm tap. It helps if you think of the equation for power: P = V^2/R.