Technical HELP!

T

take2

New member
Hi all, so I've got a squealing problem (I know its a personal issue) - but here's the deal, I have a Gibson Super Goldtone GA-30RVH (which I love by the way) going into a 4x12, if that matters, and then various pedals running in front and a delay in the loop. If I plug my guitar straight into the amp and bypass all but the delay, which is in the loop, there is no issue. However, if I plug any of the pedals in, or all, it squeals like a bugger. The volume pedal is particularly interesting because it only squeals on settings in between loudest and mute but NOT on loudest or mute. I had to replace the output transformer recently after burning one up with an attenuator (a discussion for another thread I'm sure) - anyway I replaced it with a Mercury (good reputation) and had a Tech do the work. Could a ground problem have developed or is this unrelated.
Any help would be appreciated.
Thanks, Paul
 
Re: Technical HELP!

If you try running some of the pedals with batteries you'll be able to confirm or eliminate whether it is a ground loop or not :)

From what you said about the volume pedal it sounds like it could well be a ground loop problem. If it turns out that it is, all you have to do is get some isolated power supplies, or something like a Trex Fuel Tank Jnr or a Voodoo Labs Pedal Power (which have isolated outputs) and you're golden.
 
Re: Technical HELP!

Thanks Fikealox - just a quick add on question - I currently have all my pedals sitting on the SKB PS-45 pedal board which has power included - both standard outlet type and the 9 volt single pin outlets. Knowing this, would your recommendation be the same or is the pedal board already doing what you suggested?
thanks again
 
Re: Technical HELP!

From what little I can find on the SKB PS-45, it doesn't appear to have isolated outputs, so each pedal will be sharing an earth, so a ground loop's still a possibility.

Then again, your problem might not be a ground loop at all, so try using batteries first to see if it is :)
 
Re: Technical HELP!

This doesn't jive with the pedals part, but squealing is a symptom of an output transformer that has been wired out of phase. If the amp has a negative feedback loop, wiring the OT backwards turns it into positive feeback, leading to instability and oscillation.

If the amp has a presence control, does it affect the squealing?

Ordinarily, I'd say keep looking at the pedals, but since this amp just got a new OT, it's worth checking to make sure it's wired right. Have your tech double-check it.
 
Re: Technical HELP!

Hey Rich, given what you are saying, would it make sense that the issue with an OT wired out of phase would only become a squealing issue when pedals are involved, or would it present as a problem regardless?. My problem happens exclusively when running pedals in front of the amp.
 
Re: Technical HELP!

Normally, an out-of-phase OT will pretty much oscillate all the time, but it could just make it borderline unstable. Though I can't really think of a way the pedals would affect it, it's unlikely to be a coincidence. If an amp develops a squeal just as the OT is changed, the OT is in backwards.

Did it 1) never do this before the OT change, and 2) start doing this immediately after the OT change?
 
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Re: Technical HELP!

Well, it definitely did not squeal before. Just now i rolled the volume back on the effects loop - which I didn't think should matter since the only effect in the loop is the only effect NOT causing the squeal - in doing that, however, I have stopped the squeal but I don't think I have solved the issue since there is still a buzzing - oddly enough both the squealing (prior to turning the loop volume down) and the buzzing get worse when I turn the effects off.
I don't know, I'm taking the whole rig into the tech on Wednesday night to try and figure things out.
It feels like something on the pedal end to me, but I'm not discounting the possibility of the OT.
 
Re: Technical HELP!

By the way Rich, I love the old band photos - a definite 80's flashback. I also see you're a skilled amp guy - wish you were here to putz around with this thing - not that you don't have better things to do with your time :)
 
Re: Technical HELP!

Oh yeah, and it still squeals like a mother if i bypass the effects loop - all i did by turning it down was not let the problem be amplified.
 
Re: Technical HELP!

Thanks for all the input - turns out I had two 12AX7 tubes going microphonic - one drinving the reverb and another driving the efx loop -- Who knew???
I've only ever had a power tube go microphonic - those symptoms I would have recognized.
Anyway, thanks again!
 
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