weird note-specific buzzing and feedback issue with 2000s

[THE COPS]

I know I didn't post this as a question here, but thought this might help other owners experiencing a similar problem.  Playing my 2000s through a newer (fender reissue) sunn 412 (and every other cabinet i had access to) I was experiencing a very loud, abrasive distortion and humming on certain notes.  Everything else sounded at least decent, but it was very apparent on certain locations on the neck.  Even "sticking" from time to time - where I would play a not - the note distorts - then hangs around, reverberating through the speaker even after muting the strings.   Anyways, after trying every guitar and cab combination I have - trying different tunings, etc...  it was still doing it.  I tested the outlet I was using, and the polarity was reversed!  Tried a different outlet and the amp sounds amazing. 

Just a tip if anyone has a similar issue.  Very weird.  Anyone know a technical explanation of why this happens?  I know that newer amps have circuitry built in to combat this issue (hence why my other amps were unaffected) and a 1969 2000s does not, but I am not sure WHY this happens.

[noel]

Disable that polarity switch for the love of god!  Put a grounded IEC socket in and you don't have to worry about this.  When the polarity is wrong you'll be out of phase...hence all the weirdness you experiences but it can also be dangerous.  Can't stress that enough, make these amps safe before you start using them...they're just to powerful and can hurt you.

[THE COPS]

the polarity switch has been disabled and added a grounded cord.    no iec socket, but that shouldnt make a difference if the cord is grounded correctly (which i am sure it is - according to my tech), right?

[noel]

Yeah a grounded cords is the same thing.  The ground is what is important.  I personally prefer to do IEC sockets on all my stuff.  I just don't like the hard wired cords because they're more prone to damage.

[THE COPS]

So check this out.  Went back down to my space tonight - same problem.  I did not have my outlet tester with me (good move) this time, but using the same outlet that was producing the goodness last night (after trying several outlets and cabinets) now had the same weird buzzing/distortion on certain notes.  I guess that means I spoke too soon when i thought this issue was fixed. 

No idea what is happening here.  I will go back down with the outlet tester tomorrow afternoon - but for now, I'm stumped.  This amp plays fine at the techs house (only difference there is playing through a 100w 212 vs. my 300w 412).  Does this make sense?  Could it be my power transformer taking a dump?

[george]

are you using the same guitar when you test the amp at the tech's house? How old are the power supply capacitors in your amp?

[THE COPS]

I have tried different guitars - and cabs.  same issue.  also, none of the other amps i play have this problem - just the 2000s.  :( 

I almost have to wonder if it is this location.  It sounded perfect when I first went to purchase it - then I had the bias supply rebuilt, a new volume potentiometer, and a switch installed to switch the 8ohm outlet between 8ohm and 4ohm.  Was told at that time I should have a recap when I get time & money - but it should be fine for a bit.  I just wanted to play it a little bit that night at practice.  Noticed this noise, shut it down and took it back to the tech the next day. 

He did a complete recap, and replaced the preamp tubes.  Took it to the space, and it still had this problem and a weird oscillation type of noise after warming up, like a helicopter taking off.  Very weird.  Basically, everything at that point had been fixed, checked, or replaced aside from transformers & tubes.  Gave that a shot (i had one mismatched anyways, but they all tested ok) - New EH kt88's, and went ahead and replaced the solid state recs with JJ gz34's.  Both times it had been tested for about an hour at the tech's house with no issues.  Got it down to the space yesterday after the new tubes - same distorted, farting, clipping sound problem (oscillation was gone).  I tried a different cab (at a different outlet) and sounded great.  Tried yet another outlet and it sounded great (with both cabs) - but by this time another band had showed up to use the space and I had to get out of there.  but I was pretty sure it was working great.  One of the dudes from the band had an outlet tester and said checked the  first outlet I had been using.  Said hot and neutral were reversed.  Went down this afternoon to record some stuff, and it sounds like crap (same issue) with any outlet I use.  Haven't even told the tech yet - I think he is getting pissed off at me.

the weird part is it didn't do this when trying this particular outlet yesterday - or if it did, it wasn't nearly this apparent in the short time i was able to play.

Also, it appears that the power transformer has been replaced at some point.  Looks non-original.  No specs or info on it that I could see.

any ideas?  going to pick up an outlet tester tomorrow and head back down to conduct some tests.  Next step will be to load up the cab and the head and bring them to my house to test.  This is going to be a pain in the ass for me.

thanks -

[mckinnon audio]

  Hi there,have you tried cleaning and retensioning the tube sockets.Also clean the tube pins,don't use anything abrasive,I use an old tooth brush and some De-Oxit.I'm sure your tech looked for some broken solder connections,but I'd look again.Good luck,Mel.

[THE COPS]

yes, he cleaned up the tube sockets - and resoldered the connections.  New tubes, too.  Basically  everything in the amp has been verified as good aside from the power transformer.  I will be bringing the amp and cab home to try at my house today.  Hopefully it is just some polarity anomaly with the outlets at my space.

[loudthud]

Many of the ground connections in the amp are made with aluminum pop rivets. These tend to corrode and can go open or intermittent. It's best to solder these to the chassis but it takes a really big soldering iron.

[THE COPS]

Cool - thanks for the tip.  I will have him check those out.  I have to believe it has to do with the outlets at my rented space - since it has performed perfectly at his house for over an hour each time we tested it.  And it happens immediately at my space - every time.

would a failing power transformer have this effect? 

[loudthud]

Are you plugging the amp directly into a wall outlet? Try using an outlet strip. I'm thinking possibly a poor connection of the power cord ground pin. Do you get any kind of mild shock when you touch your guitar strings and a microphone or another guitar player? Try touching one of the tuning keys of your guitar to a microphone or another guitar when your amp is making the noise. Does that make the noise go away?

[THE COPS]

i have tried both power strip and direct into the outlet.  Haven't felt any shock touching anything else either.  i will try touching a key to a microphone when it's making the noise.  Heading down there in a couple hours.  I only get this reaction on certain notes (frequencies) - even the same note on different strings.  Even weirder, is the last time it was different notes than this most recent time.  when i am not playing these notes, it sounds great.

[pombagira]

I know this may be of no help at all...but what you are describing sounds very similar to a problem I had with my selmer treble and bass 100 watt. Some notes sounded great others sounded like there was a really ugly distortion. I had the thing recapped, soldered the lot and still it was there. My amp tech checked the output transformer and that seemed to be operating fine, but the power transformer was found to be playing up. When we took out two of the four valves it sounded amazing, when the two valves were placed back in it sounded awful. We assumed from this that over the years the power transformer had lost the ability to handle the draw of power to the valves. Anyway, that may be of some help, but it also may not be. I am not sure on the 2000s if you can halve its capacity by taking out two valves..but that's what I would do and then see if the sound improved. It would seem like a simple exercise to undertake.