Microphonic type squealing

[Darth Infernus]

Hi guys,

I recently encountered what I thought was a microphonic preamp tube in my MP-1 so I went out and ordered three new Hi-Gain, Balanced preamp tubes from Eurotubes. I installed them and even lowered the gain trim pot to about half way and now at loud volume even with my Decimator gate on, I heard a high pitched squeal quickly passing through the gate. In the past when this had happened, I would tap the display screen on the MP-1 and the squeal would go away but now that doesn't work. I tried turning down the MP-1 output level and it seemed to work until I raised the volume on my power amp and it started to squeal again!

What is going on??

[MarshallJMP]

What input do you use,front or rear?

[Darth Infernus]

Right now I'm using the front one because the rear one stopped working a while back. I had done the rear jack conversion mod but one day it stopped working so I switched to the front jack and never fixed the back one

[rnolan]

My guess is it's input socket related. MikeB had this the other night on his MP1. Fiddled with the lead/jack all came good, taped up with gaph for now/gig, new socket coming soon .

[MarshallJMP]

Well try to clean out the contact of the front jack,it has been discussed here before,just do a search on it.Because the rear jack problem is comming from the front jack,and i guess the squealing also.

[Darth Infernus]

Ok, I'll check out the jacks and clean them as well as look to see if there's any wiring issue with them.

Thank you very much guys, I'll let you know what my findings are.

[MarshallJMP]

See attached pic,clean the contact with a piece of business card

[Darth Infernus]

How do I clean the contacts with a business card, by just rubbing it against the contact points and scrapping off any grime?

[MarshallJMP]

Well just take a piece of card and put it between the contact and just rub it a few times.Best is you plug in a jack untill the contact opends ,then put the card in and remove the jack.Don't use sandpaper,it will destroy the contact.

[rnolan]

And best use a non waxed/shiny business card (just the standard ones should be good), and yes you are just (gently) scraping off grime (but as MPMP says if you use sandpaper (even 1200) it will destroy the contact (removes the "shiny" coating/plating on the jack)). Or you could replace the jack with a new one ? But I suspect MJMP's advice will work (for now).

[Darth Infernus]

Ok, I'll try that first and if that fails, I'll replace the jack. Thanks again guys!

[Darth Infernus]

I think I found the problem with my rig and it wasn't the input jacks, preamp tubes or anything like that. The problem was being caused by ground loops. I use these plastic tabs called HUMFREES but on some of my rack units, some of the humfrees tabs broke off over time due to wear and me taking rack units out and back in etc..

Anyway, I was using plastic washers in their place but I've now discovered that they weren't working since the units were still contacting the screw, chassis and rack so I replaced them with new Humfrees and so far, I have ZERO issues and much lower noise, it's amazing what these stupid plastic tabs do to protect my tone.

Thank you all for your suggestions and on a side note: On a guitar rack set-up where should the level switches be set, to -10 or +4?

[rnolan]

Good news  :thumb-up: , level switches will generally be set to line level (+4) for rack gear, -10 is for instrument level. But it depends on what you are combining and how you've patched it.
So list how you've set it all up (patched it together) and I'll work through the best level settings and gain structure with you.

[MarshallJMP]

Wow you learn everyday,never would have tought this could be a groundloop problem.Usually this causes hum but not squaeling.

[rnolan]

+1  :thumb-up: So does this mean the HUMFREES are designed to stop the units earthing together through the rack rails (as they already earthed together through the power board they're plugged into (well in Australia/Europe/UK)), is this a US/Japan problem coz gear isn't earthed (much)? Or is it because there are 2 earth paths between the units (power board and rack rails) so loop sets up ??