Presence squeal is really starting to irritate me.........

Flames1950 · Post by **Flames1950** » Tue Oct 04, 2005 4:28 pm

thetasigma wrote:
Speaking of which the MoJo head cases George gets don't come with a metal shield under where the chassis sits, while many Marshall head cases do. Have you tried a metal shield under the chassis? (Fenders often use screen stapled to the head case for this purpose if you don't have a sheet of metal around.) Just makes sure it is large enough to touch the chassis edges for ground without interfering with the chassis bolts.
Well that could be it. No, I don't have any metal sheets hanging around. Could lining the head box with some tin foil work?

It may work for testing but you won't want to do that as a permanent cure. See if it helps, and if it does some heavy window screen may get the job done; works in Fenders anyway.

thetasigma · Post by **thetasigma** » Tue Oct 04, 2005 5:43 pm

Thanks Flames, I'll give that a shot.

NY Chief · Post by **NY Chief** » Wed Oct 12, 2005 10:52 am

Flames1950 wrote:It's the last thing I had to work on on the JTM45.
And no matter what I try, I can't shake that damn presence squeal.
I re-did some of the grid wires. I re-did all the control wiring after scouring through pics of '65-'67 Marshall '45's and 50's. I've re-routed the presence wire any which way but loose.
I even got so anal that I went back to the pics I dredged up of mid-sixties Marshalls and went through the grounding on the control buss, and copied that (well, changing the grounding on the buss of my Super Bass improved presence squeal issues....)
It's not even that I set the amp up in such a way that it squeals when I'm playing. It's simply that it does it when the High Treble volume, treble and mids are cranked and the presence gets to around 6 or 7, and it shouldn't do it, so it's irritating me.

Anybody spots anything obvious that I'm to groggy to see, just holler.......

On the subject of ground buss grounding it looks like the sampling of mid-sixties Marshalls I found had the inputs and first stage cathode grounded at the first pot; the second stage/cathode follower cathodes, plus the chassis ground at the second pot; the board-mounted filter cap at the third pot; and the bias ground at the fourth pot. These observations were from just a smattering of JTM45's and JMP50's -- can anyone who's seen more of them verify the same? (PS thanks to Brian Wallace's website for many of the pics I hunted through.)

I got the same problem on my new build, Flames. Gonna read through this thread. Can't get presence over 6-7 or treb over 4 without squeal when vol cranked. Sounds great 'till then though!

Flames1950 · Post by **Flames1950** » Wed Oct 12, 2005 11:51 am

Best of luck, there's lots of ideas in this thread -- and mine proved resistant to all but shielding the bottom of the chassis. I cut some steel for the bottom of my head case to insure that whatever interference wasn't an issue anymore.
Keep us posted!!

NY Chief · Post by **NY Chief** » Wed Oct 12, 2005 12:06 pm

So that did work, Flames? I have a JTM45 RI that I'm A/B'g together with the kit. I planned on swapping the chassis' to try that. (The RI has the plate and is quiet as hell)

Flames1950 · Post by **Flames1950** » Wed Oct 12, 2005 12:08 pm

Yeah, with the shield in place I can set the chassis out and dime it all with no squeal. Remove the plate and she squeals like a little schoolgirl. Not that I'd know, I'm not that kinda guy.

NY Chief · Post by **NY Chief** » Wed Oct 12, 2005 12:15 pm

I'll give it a shot tonight. It will interesting to see what the RI does without the shield plate.

Ricky Lee · Post by **Ricky Lee** » Wed Oct 12, 2005 3:26 pm

Aluminum flashing is typically used in Marshall heads, your local hardware should have it since it's also used on flash work for houses. I don't know why Slojo don't use it in their cabs, I'm sure Marshall had a reason for doing so. To secure the flashing in the cab just use staples like Marshall. This squeal thing seems to be consistant with all these 45 builds, I just wonder why a few others including myself don't have that problem.

Post by **VelvetGeorge** » Wed Oct 12, 2005 3:37 pm

Ricky Lee wrote: This squeal thing seems to be consistant with all these 45 builds, I just wonder why a few others including myself don't have that problem.

I'm wondering that too???? I'll see about adding the aluminum to the cases.

George

Flames1950 · Post by **Flames1950** » Wed Oct 12, 2005 5:41 pm

I wish I'd had aluminum handy, the stell doesn't exactly staple. I didn't realize that roof flashing would be thin enough to staple through though, so I may have a lookie at it.

Ricky Lee · Post by **Ricky Lee** » Wed Oct 12, 2005 6:01 pm

Flames1950 wrote:I wish I'd had aluminum handy, the stell doesn't exactly staple. I didn't realize that roof flashing would be thin enough to staple through though, so I may have a lookie at it.

That's exactly what's in Marshall amps. I assume it comes in different gauges, in fact I've seen it about the same size as the cabs themselves. I've used it before for other projects and it works really well. It's almost like tin foil and staples penetrate with little effort.

mightymike · Post by **mightymike** » Thu Oct 13, 2005 8:26 am

Good Idea.
Anyone know the guage?

Ricky Lee · Post by **Ricky Lee** » Thu Oct 13, 2005 9:32 am

I don't know the gauge but it's thin enough to pop in staples.

NY Chief · Post by **NY Chief** » Thu Oct 13, 2005 11:06 am

WORKED LIKE A CHARM!

First I took my RI chassis out of it's Marshall box and put the Metro in. All dimed and not a hint of squeal. I had a roll of flashing handy so I put a piece of flashing in there and it did the trick. It's .010" thick Depot variety. Bitch to staple though, I stuck in down with thin double face tape.

While I had the RI out I took a look at the way it was wired. See pics. Shielded wire on inputs and tone, AC not twisted too tight (where it is twisted)...

Ricky Lee · Post by **Ricky Lee** » Thu Oct 13, 2005 4:08 pm

I think what's happening is the 45's are prone to antenna effect, which may defer from household to household. In other words, just because my 45 doesn't squeal without the flashing doesn't mean it won't when played elsewhere, would I be right in taking the precaution of installing the flashing? I myself think so!

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Presence squeal is really starting to irritate me.........

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