Amp's squealing when the guitar cable is not plugged in - ideas?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sickboy79, Jan 9, 2008.


  1. sickboy79

    sickboy79 Member

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    Hey everyone

    Looking for some ideas to look at here. Here's my scenario: I recently purchased a Matchless Chief head (basically a higher powered Chieftain - 4xEL34s instead of 2 and a SS rectifier). When I received her, the head cab was a little more banged up than I liked so I purchased a new Matchless head cab from forum member V-Verb (great guy to deal with as well). The amp is fine and sounds fantastic.

    So last night I recieved my new head cab and installed the chassis. Everything is good to go except she is squealing quite a bit when the guitar cable is NOT plugged into the amp. Plug it in, and it all goes away - and sounds fantastic as it should. I didn't notice this before I changed the head cab (although I can't remember if I had the amp on with out the guitar cable plugged in :confused:).

    I'm assuming this is some sort of oscillation of some sort? Maybe I bumped a wire when installing the chassis? Bad ground on the input jack? Looking for some things to check. I wouldn't suspect tubes as it sounds fine with the guitar and cable plugged in. I'd like to think I know my way around amps a bit and it's a simple fix but, I just hadn't seen this before.

    Thanks!
     
  2. JES1680

    JES1680 Silver Supporting Member

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    I had a Chieftan that would do this occasionally, not sure what caused it, I'd like to know as well.
     
  3. slider313

    slider313 Silver Supporting Member

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    Maybe a sprung input ? Sounds like it open.
     
  4. tlpruitt

    tlpruitt Member

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    I agree about it being a 'sprung' input jack or more likely dirty 'switch' contacts on the input jack. The input jack is supposed to short to ground when nothing is plugged in. If the tabs on the jack that make contact when nothing is plugged in get dirty or are stretched so they do not make contact then the input to the first preamp tube does not get shorted to ground when nothing is plugged in. This is a common source of mysterious noises in amps with nothing plugged in.

    If the switching tabs on the jack are dirty you can clean them with contact cleaner and/or fold a small piece of 400 grit sandpaper so it is gritty on both sides and slide it back and forth between the contacts on the side of the input jack.

    If the switching tabs are 'sprung' or stretched so they do not make contact when nothing is plugged in then your best bet is to replace the input jack.

    -Tim
     
  5. plexi67

    plexi67 Member

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    It does sound like what others have posted about the jack. I was wondering,did the old head cab have any shielding in it..and does the new head cab have shielding?
     
  6. sickboy79

    sickboy79 Member

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    Thanks for the replies so far gents. I appreciate it. I'll take a look at the jack tonight and see what's what.

    Plexi67 - yes, the old head cab had shielding in it, as does the new one.

    Thanks again.
     
  7. mark norwine

    mark norwine Member

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    Hi Tim,

    I would only offer: I'd personally never use sandpaper. Sandpaper is silica, silica is an insulator. It's real easy to embed silica into the contact and make the situation worse.

    Hijack: are you gigging at xroads again this year with Billy & Tom & my sister? I look forward to seeing you!

    mn
     
  8. tlpruitt

    tlpruitt Member

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    Hi Mark.

    Good point, Mark. Thanks.

    Hijack continued:
    I have a gig in Northern VA that night so unfortunately I won't be able to play 'Billy Bash' at xroads this year. I am pretty bummed about it. They need a guitar player. You want to do it? This year's album is "Who's Next".

    I almost called you a few months ago when I was having some issues with a 5e5a Pro I put together. I ended up getting things sorted out on my own. I still need to find the right speaker for it. The baffle is cut for a 12" speaker and I was thinking about a RWB or a CRex.

    -Tim
     
  9. mark norwine

    mark norwine Member

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    So who's playing guitar for the gig? Is Billy recruiting a local guy?
     
  10. tlpruitt

    tlpruitt Member

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    PM sent.
     
  11. sickboy79

    sickboy79 Member

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    Update - looks like I got the problem solved - in a way that puzzles me! I pulled the chassis out again tonight and poked around. Everything looked good - no bad solder joints, loose connections, or bad grounds that I could tell. So, I decided to start trying tubes to see if MAYBE that would fix it. The V4 slot - Reverb Recovery effected it. I tried a bunch of 12AX7s - NOS and new production. No dice. So, for kicks, I pulled out a tube that is labeled a 12AU7, but looks exactly like a 12AX7. Not like other 12AU7s I've seen (granted, I don't claim to be a tube expert). Well, that solved it! No more squeel. Quiet as a mouse! Wierd huh? Well, she sounds as good as ever. I've got NOS RCA Greyplates in the crucial tone spots - V1 and V2. I don't really notice any loss of punch or gain. Seems about the same from what I can tell. If anyone has any insight, I'd like to hear it! Thanks again gents!
     
  12. slider313

    slider313 Silver Supporting Member

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    The cathode bypass cap or cathode resistor for that tube socket may be out of spec. and causing the tube to run hot. Being a 12au7 has much less gain it has solved your problem for now.
     

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