What value NFB resistor for 4 ohm spkr load?

[electroid]

NFB resistor Value

I agree 100%. It was my impression that the original poster was trying to figure out what value to use in order to keep the same amount of feedback that the amp originally had. By all means, experimentation is good!

[bob p]

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.

hmm. i have to admit, i'm so ignorant that i don't know what a "palava" is. maybe I don't want to know!

i also interpreted the original poster's question the way that @electroid did. that is to say, it seemed that he was trying to change the OT taps on the amp while preserving the amount of NFB that the amp had before making the switch.

if that's the case, then this is a "textbook" problem, and using math to find the "correct" value for NFB will lead the user to determine a "safe" equivalent level of NFB. if he's switching OT's for example, and wants to maintain the function of the amp during a transformer swap, the math-laiden approach is the RIGHT way to approach the problem. that is, if i correctly interpreted the problem. of course, its possible that i'm totally wrong.

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.

i agree with you 100% that this approach doesn't help the experimenter find the sweet spot that their ears would consider magic tone -- its not supposed to. if modding is the goal, then "experimental" approach of using a pot to experimentally determine resistance values is a great idea -- as long as the user doesn't make the mistake of wiring the pot in place of the original NFB resistor instead of placing it in series with it -- too little Z in the feedback loop will really screw things up.

[vince76]

interesting post to me...
but I would be happy understand better.
I hope someone would explain
example:
I have an OT with 2, 4,and 8 Ohm taps and I'm using the 4 Ohm tap with 4Ohm speaker load.
The original schem shows OT with 2 Ohm tap and 2 Ohm speaker load.
Following Bruce and Electroid I have to increase the resistor value to mantain the ratio. let's say from 27K to 56K.
where I have to solder the wire coming from the NFB resistor (wich tap of the OT) to mantain the ratio? the original 2Ohm tap or the 4Ohm tap?
I expect 4Ohm.
Soldering that wire to different OT tap will affect the tone? Which way?

Excuses for all these questions,
Vincenzo

[electroid]

interesting post to me...
but I would be happy understand better.
I hope someone would explain
example:
I have an OT with 2, 4,and 8 Ohm taps and I'm using the 4 Ohm tap with 4Ohm speaker load.
The original schem shows OT with 2 Ohm tap and 2 Ohm speaker load.
Following Bruce and Electroid I have to increase the resistor value to mantain the ratio. let's say from 27K to 56K.
where I have to solder the wire coming from the NFB resistor (wich tap of the OT) to mantain the ratio? the original 2Ohm tap or the 4Ohm tap?
I expect 4Ohm.
Soldering that wire to different OT tap will affect the tone? Which way?

Excuses for all these questions,
Vincenzo

If you still have a 2 ohm tap, use the original resistor value and connect it to the 2 ohm tap and connect your 4 ohm speaker to the 4 ohm tap.

[Robert M. Martinelli]

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.

Check this out, it includes some factory mods and several versions...

( Sorry for the document's low quality, that's all I currently have )

Hope this helps

Best regards

Bob

[vince76]

Thank you Electroid for the reply.
That's the way I wired... Anyway I was interested to learn about variations in the circuit and tonal response.

vincenzo

[Regis]

I'd like to know where the OP got a YBA-1 with a 4 ohm OT tap? Every YBA-1 amp and schem I've seen only has one 8 ohm tap, unless it has an aftermarket OT.

Also, wouldn't the Presence control pot need to be changed if the NFB resistor is changed? In learning about presence controls in another thread I was told the value of the presence control pot is usually a ratio/value of the NFB resistor.

Thanks,

Regis

[MWJB]

Hi Regis,

Tha value of the presence pot is determined by the value of the NFB load resistor (smaller value to ground), rather than the NFB dropping resistor (in series to OT secondary) which is what most folks would call the "NFB resistor".

[HTH]

how about this way?... in early Marshall plexis they connected the NFB resistor directly to the speaker output jack rather than the impedance selector switch.

if I understand this correctly, this now means that the NFB resistor will be connected to either the 4, 8 or 16 ohm tap depending on the speaker load being used. would that maintain the amount of NFB any better?

[Chuck H]

how about this way?... in early Marshall plexis they connected the NFB resistor directly to the speaker output jack rather than the impedance selector switch.

if I understand this correctly, this now means that the NFB resistor will be connected to either the 4, 8 or 16 ohm tap depending on the speaker load being used. would that maintain the amount of NFB any better?

No. The amount of NFB would change with each setting even though the voltage division ratio would stay the same. Each ohm tap on the OT would give a different voltage but each voltage would be divided by the same number. This would change the amount of applied NFB voltage for each impedance setting while the other parameters in the loop, like voltage supplied by the PI above the NFB shunt resistor, remains the same. So while the voltage division ratio of the NFB remains the same the whole circuit NFB ratio does change.

Chuck