Negative Feedback Loop

[SpareRibs]

Hello,
I am trying to eliminate the negative feed back loop. In (Niels"Petco"Nielsen in Converting An Existing Amp Chassis For Guitar Use) is states, the negative feed back circuit is used to cancel unwanted distortion. The publication says to clip the feedback circuit at both ends and get rid of it.
I clipped the wire and the amp would not work. Is there a reason for that to happen ?

[nickb]

It should work. So....which wire did you actually clip? Also, you might try restoring the connection to check the amp does still work.

[E biddy]

Why not put a pot, or a switch, so you can have it both ways?

[Enzo]

No, there is no reason cutting the NFB line would make the amp stop working. I also worry you cut the wrong wire. In any case, do restore its connection and find out why the amp stopped.

I'd agree that adding a switch or other control is preferable to just yanking out the wire. But get the amp working again first.

[SpareRibs]

No, there is no reason cutting the NFB line would make the amp stop working. I also worry you cut the wrong wire. In any case, do restore its connection and find out why the amp stopped.
I'd agree that adding a switch or other control is preferable to just yanking out the wire. But get the amp working again first.

Hello,
I soldered it back together and it is now working. I think I am going to install a 250k ohm pot in place of the resistor. That would make the feedback adjustable as opposed to just zero.

[SpareRibs]

Hello,
Can anyone look at the schematic and see if there would be any benefit to changing the resistor going to #1 on the 12AX7 from 560K to 270K. Also does the .5 meg pot mean 500K like one of the guitar pots, in addition the Phono pot is 1 meg. Are they to much resistance when using it for a guitar amplifier.

[Enzo]

A pot there means you can turn it down to zero, which is not zero NFB, that means the full 70v output can get to the cathode of the tube. You want to go the other way I believe. In other words, perhaps leave the 180k and add your pot in series. remember, cutting the NFB is like adding a series infinite resistor. SO a pot ranging from none to an additional 500k or whatever is a variable between the NFB of it now, and essentially open.

Yes, 0.5M means 500k.

[eschertron]

+1 to Enzo's explanation. Reducing the value of the 180k resistor increases the NFB and can lead to all sorts of mayhem. Don't go there. Try it, of course if you must, but expect unwanted results.

I'd add a 1Meg pot in series with the existing 180k resistor. The pot would be (might be) big enough to reduce the NFB to observable levels. A 250k or 500k pot on the line might not be very noticeable. I like variable NFB, use it on a Champ as a kind of "mock volume control". Never all the way to zero volume, though. I tried it - but I didn't like it.

[Enzo]

One thing I harp on is this: it is about the circuits, not the parts. A NFB resistor by itself tells us little. People report that some amp has a 100k NFB resistor, while some other has an 800 ohm resistor. Then asks which is better. But of course the NFB is a signal voltage, and it usually feeds through a voltage divider. In this case it parallels the cathode voltage of a triode. You are also dealing with a "PA Amplifier" and the NFB is taken from the 70v output, rather than the much lower level 8 ohm tap. That 180k feeds into a 100 ohm resistor. That makes about a 1800/1 voltage divider.

[nickb]

Hello,
Can anyone look at the schematic and see if there would be any benefit to changing the resistor going to #1 on the 12AX7 from 560K to 270K. Also does the .5 meg pot mean 500K like one of the guitar pots, in addition the Phono pot is 1 meg. Are they to much resistance when using it for a guitar amplifier.

Lowering from 560K to 270K will reduce the gain very slightly but also give you more headroom. I think you need to hit you guitar pretty hard to use up the existing headroom but you might consider that a sonic advantage. Try it and see which you prefer.

Many amplifiers have a 1 megohm or more of input impedance. In this design, depending on where you set the controls, you can get less than that. Therefore I don't think there is any benefit in changing the phono input pot.


On the NFB, rather than adding a pot in series with the 180K, I'd leave the 180K in place and connect it to the wiper of a 25K pot with the CW end to ground and the ACW end to the 70V line. That way you can vary the feedback from the designed amount to zero. If you were to try to increase the feedback by lowering the 180K resistor you could make the amp oscillate. That said, before you go to all that trouble I think you should try it without any feedback by removing the 180K first and see what you think.

[SpareRibs]

On the NFB, rather than adding a pot in series with the 180K, I'd leave the 180K in place and connect it to the wiper of a 25K pot with the CW end to ground and the ACW end to the 70V line. That way you can vary the feedback from the designed amount to zero. If you were to try to increase the feedback by lowering the 180K resistor you could make the amp oscillate. That said, before you go to all that trouble I think you should try it without any feedback by removing the 180K first and see what you think.

Hello,
All right I am going to leave the 180K resistor and wire in a pot. I just don't understand the abbreviations, CW and ACW.

[eschertron]

Hello,
All right I am going to leave the 180K resistor and wire in a pot. I just don't understand the abbreviations, CW and ACW.

CW clockwise
ACW anticlockwise (that's counterclockwise in the western hemisphere)