2 Channel Amp Help with squeal on one channel

[chunkitup]

I am frustrated with finding where the squeal is coming from. The channels share no parts except when they get to the LTPI. The one channel which has almost as much gain is quiet and gives me no problems. The lead channel squeals above 1 on the MV and the pre gain volume doesn't change anything. Ive swapped tubes to no avail. Ive chopsticked til I was blue in the face. I will attach some pics and ask for possible problems I am not seeing.

http://i587.photobucket.com/albums/s..._06_56_Pro.jpg
http://i587.photobucket.com/albums/s..._07_08_Pro.jpg

[nosaj]

I am frustrated with finding where the squeal is coming from. The channels share no parts except when they get to the LTPI. The one channel which has almost as much gain is quiet and gives me no problems. The lead channel squeals above 1 on the MV and the pre gain volume doesn't change anything. Ive swapped tubes to no avail. Ive chopsticked til I was blue in the face. I will attach some pics and ask for possible problems I am not seeing.

http://i587.photobucket.com/albums/s..._06_56_Pro.jpg
http://i587.photobucket.com/albums/s..._07_08_Pro.jpg

What amp is it? and schematic? What are you plugging in the input?

Thanks,
nosaj

[Enzo]

Photos don't tell us much, you need to post the schematic of what you built.

Chopsticks find loose connections, so if a loose connection is not the cause of the problem, the sticks won't find it.

[chunkitup]

Alright. That was a half assed thread starter. My apologies I know better.
I work hard on my lead dress but I sometimes put signal lines too close to high voltage but I cant see anything wrong with my layout. I use pretty much the same layout with every amp I make and this is my first squealer where I couldn't find the problem. Whatever is needed and I will provide it.

[The Dude]

......The lead channel squeals above 1 on the MV and the pre gain volume doesn't change anything.......

Do you mean the pre gain volume doesn't affect the squeal or the pre gain doesn't affect the level of the signal either? If it doesn't change your signal level either, look at connections in that circuit or maybe a bad pot itself.

[g1]

Just for the heck of it, have you tried disconnecting the feedback loop ?

[chunkitup]

the pre gain volume does not affect the squeal but does work flawlessly otherwise.

I disconnected the NFB and the squeal was still there over 1 on the master volume. So frustrating.

[g1]

So the lower channel on the schematic is the one with the squeal? Do any of the tone controls on that channel affect it?
How about removing various preamp tubes to isolate the problem?

[chunkitup]

So the lower channel on the schematic is the one with the squeal? Do any of the tone controls on that channel affect it?
How about removing various preamp tubes to isolate the problem?

Wow! Ok now I have some things reacting. The bass on the lead channel doesn't do anything but the mid and treble definitely have a big effect on that squeal. At high MV if I turn the mid or treble down the squeal subsides then goes away.

I tell myself to learn something new everyday and I believe I am about to learn something new.

[chunkitup]

I found it finally!!! The input to the lead channel was shielded and grounded properly and I used a 68k directly on the triode pin but it ran underneath the crunch channels controls, actually pushed up against them. After I shortened the lead and left it floating to the triode pin the channel is now dead quiet.

http://i587.photobucket.com/albums/s..._25_21_Pro.jpg

[chunkitup]

Ok so now I am really freaked out. The amp no longer squeals on the bench or whenever it is upside down. The moment I put it in the headshell and of course I flip it over, that's when it squeals again. For days I thought it was the box I had built for it because it didn't squeal in another headshell I had built for another amp using the same chassis. But after more swapping the squeal was back whenever I flipped the amp over.

Any ideas????

[Justin Thomas]

I'm guessing this is configured like a Marshall head? Are any of your longer wires flopping around a bit when you turn it over? Did you maybe miss a solder joint and gravity is making a loose connection move out of place? Do you have any sheet metal or metal screen inside the head shell that might be bent or sticking up and contacting anything? Loose bit of something under the board that might be stuck but is invisible to you? I've lost little balls of solder & bits of wire between Fender boards a few times...

Justin

[chunkitup]

Hey Justin
There is always little flecks of solder during the process of building and if its not a lot all I do is turn it upside down and shake a wee bit. As far as configuration yea you could say Marshallesque but not as far as the layout. I use a buss for the preamp thru PI and ground close to the input. If you see the pics you can see I have shielded cable for everything. There is only one lead that is even close to being outside the chassis and I just shortened it so it doesn't drag when I flip and install in the box. I did find a major problem when shortening the lead in question. The 2.2k screen grid resistors where soldered on directly to pin 5 and sticking up in the air and the one resistor just broke off when I moved the lead to those resistors. So I just used the empty pin 1 for the other end of the 2.2k rendering this amp strictly 6L6's.
As far as anything on the headshell I used a newly built box which was only wood and squeal away it did.

[Justin Thomas]

How about any loose connections? If it's upside down in the cab and squealing, can you give it The Enzo Whack and see if anything changes? Or use a rubber mallet... That's about MY last idea...

Justin

[g1]

Is this the same squeal as before? Does the wire you replaced have any room for movement when flipped over? Are there other wires that have room to move when upside down? If so, try pulling up on them with unit upright.
Have you swapped tubes since this second issue began? Worth a try.
And you are no where near the speaker are you? Speaker cable positioned same when inverted?