'Standby' pre(quad) + 'on' power(strat. 400). Problem?

[dupedd]

I was playing my bandmates quad+strat.400 rig and had trouble for a second...I had both on but the standby switch was on on the quad. I didn't play through it. It was just as stated for a second. Fast forward, later at band practice he's having trouble with it. He's getting a nasty feedback(squeal) at idle plugged in with his normal guitar. He says this never happens.
Could I have damaged the rig?

 

[4Draw]

I also seem to have a very nasty squeal in LD2 when not playing through the guitar.

Funny thing, it dissapears after upping MV to close to maximum (10)...Might this just be the right volume setting for the given output setting, to minimize sound of going into the poweramp?

 

[dupedd]

I'm not sure. I think the squeal was worse the louder the amp got, I assume the MV on the strat400.
Anyone else have an idea?

 

[Nick_cor]

I also get a squeel when i play through my preamp, :x it only happens sometimes, is your friends unit a new one? :?: because my units are old, and they tend to squeel on room level volumes but it does go stop once i turn everything off then back on again.

 

[ak47]

I very much doubt that having the standby switch off caused any problem...

I would start by swapping the tubes around. Squealing is most often associated with a 12ax7 going 'microphonic'. Very noticable with high gain channels!

A simple test is to open up your quad, power up and tap each tube for each channel lightly with a pencil. You will hear this issue quite clearly if this is the problem.

 

[ak47]

And...

Could also be a 'microphonic' guitar pickup.... try with different guitars to see if the problem is still there. These pickups can usually be fixed with a dip in some wax... you might also want to check the earthing of the guitar while you're at it