What value NFB resistor for 4 ohm spkr load?

[Humbucker]

I have a 1970 Traynor YBA-1 BassMaster 50W head and I'm going to rewire the output transformer for a 4 ohm speaker load. The NFB resistor in there now is a 100K so I'm not sure what to put in its place.

Would I be alright going with a 27K NFB resistor with this 4 ohm speaker load? I noticed that the early Fender Bassman amps had a 27K NFB resisitor and it looks like the speaker load was either 4 ohms or 2 ohms.

What value would you suggest using in this amp?

[MWJB]

Try 27K, 56K and 100K and see which you prefer (or temporarily use a pot wired asa variable resistor to determine your preferred value, then replace with a fixed resistor of that value).

[Chris / CMW amps]

I have a 1970 Traynor YBA-1 BassMaster 50W head and I'm going to rewire the output transformer for a 4 ohm speaker load. The NFB resistor in there now is a 100K so I'm not sure what to put in its place.

Would I be alright going with a 27K NFB resistor with this 4 ohm speaker load? I noticed that the early Fender Bassman amps had a 27K NFB resisitor and it looks like the speaker load was either 4 ohms or 2 ohms.

What value would you suggest using in this amp?

What's the stock output-power and output-impedance?

[Humbucker]

I think that most of these older Traynors were wired for an 8 ohm output and were using a 100K NFB resistor. If that's the case, would a 27K-50K NFB resistor be about right when rewiring this amp for a 4 ohm speaker load?

Here's a schematic:
http://www.schematicheaven.com/barga...aster_yba1.pdf

[Keystring]

I think that most of these older Traynors were wired for an 8 ohm output and were using a 100K NFB resistor. If that's the case, would a 27K-50K NFB resistor be about right when rewiring this amp for a 4 ohm speaker load?

Here's a schematic:
http://www.schematicheaven.com/barga...aster_yba1.pdf

The NFB resistor has nothing to do with the output impedance.

The size of the resistor sets the amount of signal fed back to the phase inverter circuit. The larger the value, the less feedback and more distorted the output, as well as being louder.
The smaller the value, the more feedback and a cleaner signal, and less output.

How are you rewiring the output to change the impedance?
Are you using a different output transformer?

The current transformer has only one impedance according to the schematic so you can't rewire it from 8 ohms to 4 ohms unless there is a 'tap' for the 4 ohm load. But the schematic doesn't show one.

[Humbucker]

I wasn't able to find the exact schematic for my amp, mine is a 1970 model. BTW, my output transformer does have different taps, so I can wire it up for a 4 ohm output.

I'm just wondering what size NFB resistor you guys would suggest installing in this amp when it's wired up for a 4 ohm speaker load. I'm thinking 27K or 50K. I know that it's not written in stone what value to use but I was just putting it out there to see what you guys would suggest.

[Keystring]

I wasn't able to find the exact schematic for my amp, mine is a 1970 model. BTW, my output transformer does have different taps, so I can wire it up for a 4 ohm output.

I'm just wondering what size NFB resistor you guys would suggest installing in this amp when it's wired up for a 4 ohm speaker load. I'm thinking 27K or 50K. I know that it's not written in stone what value to use but I was just putting it out there to see what you guys would suggest.

It's not necessary to change the NFB resistor, unless you want to try to change the 'tone' of the amp by increasing the amount of output distortion.

It has no effect on impedance.

The transformer sets the impedance.

A friend of mine has the yba3 and decreasing the NFB resistor helped a little, but not much.

Your's may be different. You may want to use a 100k pot and adjust it to your liking.

Then take a reading of the pot and put a resistor of the same value in permanently.

[electroid]

When you change the output impedance of an amp, the voltage output changes at the speaker jack. That does indeed change the amount of neg feedback going back to the phase inverter. If you change from 8 to 4 ohms, you will lower the voltage in the feedback circuit so you will need to lower the value of the series resistor. I don't have the schematic in front of me but I think the new value would be around 68K ohms if you want to keep the same amount of neg feedback.

[Humbucker]

I should mention that last year I did a small mod, I installed a 50K linear pot on the back of the amp in the hole that used to be for the ext.spkr and I wired this pot in series with a 50K resistor for NFB. So with this pot, I can go from 50K to 100K, this will probably be good as is, I just need to dial it down to appx. 50k-68K.

Thanks for all your advice fellahs, much appreciated!

[Bruce / Mission Amps]

A common misconception.
The transformer does not set the impedance, it sets the impedance ratio.
However, I also think it is necessary to make a resistor correction when you using the 4 ohm tap on a 4 ohm load.

I didn't read where he was keeping the NFB source connected to an 8 ohm tap... so if he changes the reference point to a 4 ohm tap, then a resistor change is also needed.
In most circuits (like this one) there are two NFB resistors or resistances.
One is usually grounded, or sometimes a pot and cap is used to change the inpedance at specific frequencies for "presence", and these two resistors are wired as an AC voltage divider with the lower value one being the phase inverter DC ground source also.

The amount of NFB voltage has been emperically determined by the designer, with respect to the impedance ratio of the OT primary and secondary load.
In this application, something has changed.
He has gone down from 8 ohms to 4 ohms and the amount of secondary voltage delivered to a speaker load is now a little less... therefore the amount of NFB AC voltage has changed going to the two NFB resistors.
With a lower zed speaker load, there is less AC secondary voltage delivered to speaker and more current.
The result is that there will be less NFB voltage developed so a smaller NFB resistor will be needed to keep the voltage ratio to the phase inverter similar to where it was when stock.
I'd also try a trim pot as Keystring suggested and see where you like it but my guess is it will be around 39K to 56k.

[electroid]

NFB resistor value

A common misconception.


I'd also try a trim pot as Keystring suggested and see where you like it but my guess is it will be around 39K to 56k.

If you look at or compute the difference between the voltage output at 8 ohms vs 4 ohms, the ratio will be about 10:7 I think the resistor will be above to 56K

[Arthur B.]

It has no effect on impedance.

It does have an effect on the output impedance as changing it will raise or lower the output impedance.

[bob p]

if anyone who's interested in this stuff really likes math, there's a "better" way to determine the amount of NFB resistance that's needed when you change taps on the OT: calculate the value of the NFB resistor that's needed to provide the desired number of dB of negative feedback.

when Bruce mentioned that the original designer of the amp determined the value emperically, i think that this is what he was referring to. calculating the number of dB of negative feedback is a straightforward calculation, and the formulae are conveniently published on Aiken's tech pages. its a simple task to plug in the numbers to calculate the amps original feedback in dB, and to change the Z of the OT and recalculate teh value of the NFB resistor to reach the same number of dB.

this is important, because you need to be sure that you don't provide more NFB than the circuit can handle, or you may end up with an unstable amp.

[electroid]

NFB resistor value

Yes, or you can solve for voltage. You'll get the same result.

[MWJB]

What a palava. It is not necessarily true that the calculated "correct" amount of NFB is going to give you the sound 'you' like, this can only be assertained for sure by ear testing. We know that in this instance 27K, 56K, 100K (just suggested as common values, plenty of choices in between) will work.

It is just as quick and guaranteed to sound best if you actually listen to these values, or as has been suggested several times, temporarily install a pot, determine the value you like and replace with a fixed resistor. To stick to a calculation that is theoretically correct is fine as long as it sounds how you want.

Has no one noticed that many 2ohm & 4 ohm Fenders share the same feedback ratio, that many 8 ohm amps have wildly differing ratios?