NFB pot problem: High-pitched noise

[Aldo]

Hello,
I wonder if someone can help with this. I have a noise problem after adding a pot to adjust the negative feedback in my Super Twin Reverb.

The pot is 50k, wired in series with the NFB 820 ohm resistor. If I turn the master over 5 and open the pot (subtracting negative feedback) before half-way I get a constant high-pitched noise. Is this a common problem? I was thinking it could be that I ran really long wires to reach an unused hole... at the other end of the chassis no less! or maybe I wired the thing wrong being this the first time? thanks

[Enzo]

Take the new control off the panel and disconnect the long wires. With a couple short wires, connect the pot very close to the rest of the circuit. Put a piece of cardboard or plastic or a folded cloth under it for it to sit on. DOes the same thing occur? If the amp is now stable, then yes, thiose long wires were the cause. If it acts the same, then the wires wouldn't be responsible.

[Aldo]

Thanks Enzo. It was indeed due to very long wires. Tried short ones and no noise so I drilled another hole closer to the area in question. Everything's fine! EDIT: I hasted victory, it's not fine :-(

[Aldo]

I did all the testing without the reverb tank connected BUT when I plugged it in the noise was back... I would say it's what's know as oscillation noise?. I can crank the master with the NFB pot open and no reverb and there's no problem, but If I turn the reverb past 5 IIRC the same noise as before appears. The noise also dissapears if I turn down the NFB loop pot (as stock) with reverb still on... but I really like better how the amp sounds without negative feedback :-\. So the NFB control works with the dry signal, but not with the reverb. Now I don't have any idea what could be the problem!

[Chuck H]

Did you install the pot or run any wires near any reverb circuitry? It's the same problem Enzo was getting at in the first place. There is probably crosstalk between components and/or signal leads creating a positive feedback loop that is making the amp unstable. Try removing the pot from where you have it now. Put the pot VERY close to the 820 ohm resistor. As in, lift one end of the resistor and solder it directly to the pot. Now solder the other pot leg to the place you lifted the resistor end from. You should have very short, or no leads. Does it still misbehave in any way? Be sure to test it with the reverb hooked up. If the amp behaves now, you need to find a place to put your new circuit that doesn't interfere with the amplifiers existing layout. If the amp still doesn't behave then the new circuit just isn't compatible with the amp, but... You could still experiment with less NFB by using a moderately larger fixed resistor. with the pot in place just adjust it until the amp starts to misbehave. Then back it down a pinch. Now, without turning the pot, remove and measure it, then add 820 ohms. This will be the smallest amount of NFB that circuit will work with. Install a fixed resistor of the nearest standard value.

[greekie]

Also, test the amp out mounted in it's chassis. You never know what it's picking up when sitting on the bench, like a cabriolet.

[Aldo]

Chuck, Greekie, thanks for the answers.
the pot is now in one corner close to the 820 resistor, like 4 inches, far away from any reverb component AFAIK. It was causing problems before even witout reverb when I placed it very far from the NFB resistor. The reverb tube is in the other corner and then the wires to the reverb tank RCA's are like in the middle of the chassis.

With the master and reverb at 10 there's virtually no room for substracting negative feedback, the oscillation appears very early at turning the pot, so it wouldn't be worth finding another fixed resistor as the difference would be minimal. You have to back down the reverb around 4 but that's a very faint reverb! (even maxed theis reverb is not very deep). It seems the amp layout is not compatible with this NFB pot arrangement. I'll make tests with the chassis covered to see if it stiil happens. Thanks!

[greekie]

It is weird, as I've implemented stuff like this in super high-gain amps. I think you'd might want to look elsewhere for the problem, as it sounds to me like the amp might be unstable. The lower amount of NFB can trigger stuff like that.