Negitive Feedback Tap Equivalent Chart

[RockinRocket]

Looking for a chart that shows Negative feedback equivalents from one tap to another with the necessary resistor change. Say a TW Express 100k 8 ohm tap is equal on the 4 and 16 tap. Or 100k 4 ohm and 47k 8 ohm for all taps for examples.

I read Aiknen's site but the math has gone over my head.

[tubeswell]

The VAC on a 16R tap will be 1.4142 x the VAC on an 8R tap, and the VAC on an 8R tap will be 1.4142 x the VAC on a 4R tap, and so on.

The Pr:Sec VAC is 1/1.4142 of the Pr:Sec Impedance ratio.

If you have an OT that reflects 8k with a 16R secondary, that is a Pr:Sec impedance ratio of 500:1, and a Pr:Sec VAC ratio of 22.36:1. (So, 100VAC on the primary will result in 4.47VAC* on the 16R tap.)

If you have an OT that reflects 8k with an 8R secondary, that is a Pr:Sec impedance ratio of 1000:1, and a Pr:Sec VAC ratio of 31.62:1. (So, 100VAC on the primary will result in 3.16VAC* on the 8R tap.)

If you have an OT that reflects 8k with a 4R secondary, that is a Pr:Sec impedance ratio of 2000:1, and a Pr:Sec VAC ratio of 44.72:1. (So, 100VAC on the primary will result in 2.23VAC on the 16R tap.)

* 4.47= 3.16 x 1.4142 and so on

The amount of VAC on the OT secondary tap where you source the NFB from will be affected by the voltage divider in the NFB loop. If you figure out all the ratios that apply to the stock amp you that are dealing with, you should be able to determine a suitable voltage divider that will produce an equivalent amount of NFB for any given OT secondary tap.

[RockinRocket]

Hi, Thanks for the detailed reply. I think I only processed the first sentence though. For me, an example along with the math usually helps a lot.

So lets say for a Marshall these are equivalent for the same OT transformer.
100k 4 ohm tap
141.42k 8 ohm tap
200k 16 ohm tap ?

And to complicate things-
A JMP Plexis 47k 8 ohm tap are equivalent.
33k 4 ohm
66k 16 ohm ?

So a Soldanos SLO 39k 4ohm tap has a feedback level only slightly more feedback than a JMP Plexis 47k 8 ohm tap?

I think I got it... But not surprised one bit if I messed all that up

[RockinRocket]

Looking for conformation on the above post

[tubeswell]

Hi, Thanks for the detailed reply. I think I only processed the first sentence though. For me, an example along with the math usually helps a lot.

So lets say for a Marshall these are equivalent for the same OT transformer.
100k 4 ohm tap
141.42k 8 ohm tap
200k 16 ohm tap ?

And to complicate things-
A JMP Plexis 47k 8 ohm tap are equivalent.
33k 4 ohm
66k 16 ohm ?

Quite likely, but the way to be sure (if you don't have a schematic of the OT with wires colour coded etc) is to hook up the windings to a VAC source and measure the comparative VAC across each set of winding ends. Be careful.

So a Soldanos SLO 39k 4ohm tap has a feedback level only slightly more feedback than a JMP Plexis 47k 8 ohm tap?

Global NFB of the sort in these examples is sourced from the VAC which occurs on an OT secondary. This is the starting point for working out relative NFB circuits. So you need to concern yourself with VAC, and not DC resistance. Relative DC resistance may indicate which windings are which, but it doesn't tell you what the source AC voltage will be for the NFB loop.

On any transformer, the Pr:Sec VAC ratio = the Pr:Sec turns ratio,

BUT The Pri:Sec VAC ratio is the square root* of the Pr:Sec impedance ratio. (and conversely, the impedance ratio is the square of the VAC ratio)

'Impedance' is 'resistance to alternating current'. (This is different to DC resistance, which is a measure of how much direct current will occur at a defined voltage).

Impedance cannot be measured with a R-meter. R-meters can only measure DC resistance.

The only way to measure impedance is to measure VAC, and then calculate the impedance.

The only way to measure the impedance ratio of transformer, is to measure the VAC ratio between the windings, and then multiply the VAC ratio by itself (i.e. to square the VAC ratio) - or to know what the turns ratio is of the windings.

You measure the VAC ratio by hooking one set of winding ends up to a (live) VAC source (of a known VAC quantum), and then measuring the VAC across each of the other winding's respective ends (which is why I said 'be careful' above. This is a dangerous task)

* Hence my reference to '1.4142' in my earlier post. This is the constant that you need to use in order to compare relative differences in VAC output between each secondary tap.

[roberto]

You also need to take care of the resistor to ground on the PI, when comparing the resistors.