Continuous problems with Fender Tone Stack

[EETStudent]

I went in today to replace the tone controls from my old tube PA head with standard Fender tone controls, and though I did my part correctly, the amp did not do its. Its kinda hard to describe exactly whats happening, but maybe someone experienced with know whats goin on.

Basically, with the new controls installed properly, the amp develops a limit. When I start to turn up the volume and reach the 5, a loud hum comes though. At 4 1/2 it sounds fine, but at 5 it hums like crazy and and it fizzes out. I can smark a chord and ill get the biggest fart from the amp Ive ever heard. A similar thing happens when I adjust the treble control. When i turn it to about 8 that same hum appears. This time instead of farting, I get a sound that literally sounds like someone is beating on my speaker cab. Its a loud thud that sorta oscillates.

All in all with the volume at 4 and the treble at 7, it sounds great. But with the volume at 5 and the treble at 8 it sounds very, very strange. Whats up with that

[Amp Kat]

That's called motoboating and it sounds like you either have a pot not grounded that should be or a supply cap or potential that is not grounded. Check your wiring good and your grounds.

[EETStudent]

well according to schematic, the only ground comes off the Middle control(however I used a fixed resistor). And that seems to be tied ok. I guess I will go through it again though

[Amp Kat]

That is most likely your problem as pin 1 is usually tied to ground so you most likely lifted the ground.

[EETStudent]

I assume that by ''lifted'' you mean disconnected at one end of the ground wire. however, the tone controls ground connection is solid and the circuit ground connection at the other end is solid. still motorboats.

[Steve Conner]

Could well be parasitic oscillations too. Use your oscilloscope (you do have one right?) to hunt them down.

[EETStudent]

I actually do not have one. Ive only used them at school a couple times, and not quite that advanced to have and operate one. As for the tone controls, the motorboating must lie in the tone stack, right? The original PA controls didnt produce this result, however the Fender stack does. They respond correctly though. The bass increases when turned up, and the same for the treble. The volume however limits out and starts the motorboating at a certain level. I mean, whats up with that??? its quite annoying

[EETStudent]

Well, turns out it is NOT the tone controls. I replaced the new ones with the original ones and got the same result. Then, instead of plugging into the 1/4" jack I installed, I plugged into a mic input and problem solved. Must be an issue with the jack, I assume. That is good news

[Manic]

Yeah, I have had simular probs out of shunting input jacks that are not shunting. That can cause all kinds of undesirable sounds.