Squealing

[dtdionne]

My 12xxx series build started squealing when i get close to the amp after working great for a few months. The only correlating event or change around the time this started was the reconfiguration of speakers in the cab. There's no difference in the squealing when i put the head in it's case.

I've tried the following:
1. new pre and power amp tubes
2. separating the speaker cab from the head
3. multiple guitars
4. multiple attenuators
5. multiple instrument and speaker cables
6. multiple picks
7. multiple known good speaker cabs
8. totally redoing my grounding (I first tried SDM's method and then I tried Larry's)
9. taking the attenuator out of the loop
10. searched the forum
11. moving the presence wire around
12. new vol1 pot and bright cap
13. removing the bright cap - which actually helped a bit
14. chopsticked

Here are some observations:
1. The closer i get (with the guitar of course) the worse it gets
2. It goes away when I get ~20 feet away
3. When I stand ~1 foot away from the amp while facing the amp and i go side to side while still facing the amp the pitch of the squeal drops and gets louder as i near the center point of the amp (ie neck pickup aligned with mids knob) and then goes back up as i cross center headed to either edge of the amp.
4. When i stand ~2 feet away on either edge of the amp and spin in a circle the pitch changes but doesnt really change in volume.
5. When i take my hands off the strings the squeal gets louder and it hums pretty loud.
6. I also hear a kind of crackling every now and then on notes, kinda sounds like a deep frier i guess.


I guess the thing that's got me most is how sudden it started. Before this happened I was able to pretty much stand right in front of it with it out of the head cab and upside down while dimed without any squealing.

What can i do to help narrow down the culprit? For example when i ground the tabs on some of the tone pots it gets very quiet which I think is a normal behavior but im not sure.

Thanks guys

David

[neikeel]

Does it do it on all inputs?

Does it happen with the volumes on the amp turned down?

Does maxing the mid pot make it go away?

http://forum.metroamp.com/viewtopic.php ... =squealing

http://forum.metroamp.com/viewtopic.php ... =squealing

[dtdionne]

Does it do it on all inputs?

Does it happen with the volumes on the amp turned down?

Does maxing the mid pot make it go away?

http://forum.metroamp.com/viewtopic.php ... =squealing

http://forum.metroamp.com/viewtopic.php ... =squealing

Hey Neil,

Yes, but vol1 bright is the worst.

No

Well, i always have the mids maxed so i guess not. The mids knob didnt affect the squealing much at all just now when i tested it, as a matter of fact i just tested it and with presence on 10, vol1 on 7, high mids and lows at noon it barely squeals at all...it seems that the high knob is much more closely tied to the squealing then mids or bass, in fact its almost as closely tied as the presence knob.


In that first link is he talking about the preamp cap that's under the ptp board? I've kinda suspected that but was trying other stuff cause of how damn hard that sucker is to get to. I gotta head back to work so ill read that second link when i get back to my desk.

[neikeel]

It can be a pain, but:

-if you drain the caps
-undo the pinchbolt on the clamp and try to raise the can you may be lucky enough to be able to apply haemostats (I have a few - perks of the job ) to the wires to stop them dropping back in, resolder and you are away. I usually make the wires long enough to do this when I wire an amp from scratch.

[dtdionne]

yeah...the ground wire's long enough and i made the red ones long enough at first but then accidentally cut them totally down so much so that i cant get clippers or an iron in there...i think what i'll do is unsolder from the board and then extend each wire with a much longer piece. Hopefully this will allow me to pull the cap out all the way while pulling the new extensions into place, then ill just cut the original wires off. I sure hope it's something with this cap.

[dtdionne]

I don't have another 16/16 cap but I do have a couple 50/50's, would one of them work in place of the 16/16 if its bad?

[neikeel]

Yes it will be fine.

btw my early 68 SL has dual 32 and you will find that a much more common preamp cap than 16s if you find the 50 a bit stiff c/w the 16.

[dtdionne]

K i swapped the 50/50 in and it sounds different, like better different but the squealing is still there. I am getting all of the pick harmonics that ive felt ive been missing.

How can i test this 16/16 cap?

I also moved the ground for the preamp can to that bolt on the left.

What else can i try?