Help with resistor value!!!

Thread Starter

wlmraziel

Joined May 13, 2019
4
Hi, this is the diagram of an guitar amp, i need to change the resistor but I don't know the value. Here the diagram and a pic100_6561.JPG .g.png
 

MisterBill2

Joined Jan 23, 2018
4,897
The resistor looks fine. Why do you want to replace it?
I also wonder why the TS is considering replacing the resistor, although I do not see the two red bands that I would expect to see on a 22 ohm device. If it has overheated so that the second color band changed from red to black, then there is another problem upstream some place, or possibly the series capacitor has become short circuited. If the amplifier has developed a high frequency oscillation that resistor will certainly get very hot. So I suggest connecting an AC voltmeter across the speaker output connection with no signal applied and check for an output. An oscillation would be far above the audio range and could not be heard, but they certainly can do a lot of damage.
 

MisterBill2

Joined Jan 23, 2018
4,897
To me it looks like the resistor has been overheated a bit. And the TS must have some explanation of why they think that it needs to be replaced. Notice that the second and third band are not the same shade of black, and that the resistor is a very slightly darker shade of gray across the middle portion. It may even be that there is an entirely different motivation for replacing that resistor, having not much to do with anything we are aware of. Very frequently questions are posted with much of the information missing and so we get many guesses at an answer based on an incorrect knowledge of what the TS actually needs to know or discover.
So I am asking the TS to let us know more about why the resistor should be replaced.
 

Thread Starter

wlmraziel

Joined May 13, 2019
4
I also wonder why the TS is considering replacing the resistor, although I do not see the two red bands that I would expect to see on a 22 ohm device. If it has overheated so that the second color band changed from red to black, then there is another problem upstream some place, or possibly the series capacitor has become short circuited. If the amplifier has developed a high frequency oscillation that resistor will certainly get very hot. So I suggest connecting an AC voltmeter across the speaker output connection with no signal applied and check for an output. An oscillation would be far above the audio range and could not be heard, but they certainly can do a lot of damage.
The resistor is burned but the amp still working with a click noise. That is the only part burned.
 

MisterBill2

Joined Jan 23, 2018
4,897
A 22 ohm resistor is a more standard part, and so it should be easier to find, and possibly cheaper. But now I am wondering why the resistor failed. There is always a reason for resistors burning up and the important question is why was there excess current flowing through that resistor.And with that 0.1 MFD capacitor in series, if the cap has not become very leaky then it had to be a whole lot of power. My suggestion is to first put in a lower power rated 22 ohm resistor without shortening the leads, That is so that if the resistor burns up it does not damage the circuit board. Unless the amp was being used for a whole lot of full power high distortion music that resistor should not get hot. I am still suspecting an ultrasonic oscillation. The check for that is simple. When the resistor is replaced connect an AC meter across the output and if there is a voltage without the amp being driven then there is a serious problem.
 

MisterBill2

Joined Jan 23, 2018
4,897
The resistor may look scorched but it isn't that bad.
Change C73 first.
See post 11, apparently the resistor is failed.

AND there are still a few single channel amps around. Guitar amps last a whole long time, and this could be one of those 1972 vintage ones. Or even a 1968 model.
 

MisterBill2

Joined Jan 23, 2018
4,897
Now things get more confusing as I see the circuit more completely. That resistor is more closely associated with the power supply than with the output. Or so it appears at first inspection. Very strange the way it is drawn.
 

Ylli

Joined Nov 13, 2015
878
That is really a weird way of wiring things up. Notice the 'output' of the power amp is tied to chassis ground. Then the speaker is tied between chassis ground the the power supply common. IOW, the 'hot' side of the speaker out is connected to the chassis instead of the 'cold' side.

In any case, the resistor is indeed part of the Zobel network.
 
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