FX Loop problem live- squeal on Channel 3

[babow2]

I used my Mark V live last night for the first time, although I have rehearsed with it about 4 times in the studio. I use the exact same set up in the rehearsal studio as I do live, so everything is the same except overall volume is slightly higher live (not hugely higher, as we mic up the guitar).

I run a Lexicon MPX500 through the FX loop. In rehearsal I had no problems with the loop and seemed to find a nice balance of the input on the Lexicon and the mix level on the Loop and no issues.

Live, however, I had a nasty squeal when I used the FX loop on leads on Channel 3. I run Channel 3 on MarkIV, bright, 45 watts or 90 watts and no extreme settings on gain or tone, just a pretty straightforward lead tone. Gain- 1:30, Master 10:30, presence 11, T-9, M-9ish, B-10:30 (this works as I have it set on bright) and the preset EQ knob at about 11.

So, given no change in the settings live, why is the FX loop making the lead channel squeal and howl? Any ideas?
Thanks,
Brent

 

[eudaimonia02912]

Could be a bad tube driving the loop. (I forget which tubes control the loop, but you can look in your manual.) Have you tried changing cables or effects in the loop? Does it do it with some effects but not others?

 

[babow2]

Thanks. I will check the tube. However, it only happens on channel 3, which I realize is much higher gain. So, it isn't the cables, as it all works fine in the studio/rehearsal on all three channels and it works fine live on ch1 and 2, but just the lead channel, which is louder and higher gain. I am guessing I am overdriving the input on the FX unit, but it doesn't make sense to me why that would only happen live.

Thanks,
Brent

 

[HeadlessAxeman]

Exact same "setback" I'm facing.

Got the GT10 hooked up in the fx loop of the head. With this in place, turn the knob to Hard Bypass and switch on Channel 3 ...tremendous loud squeal.

My fix: ..roll the gain back from 2:00 to 12:00...and works fine with hardbypass on/off now, although, if the FX loop is engaged on the Mark V, I do freely play aroudn with the gain on Channel 3.

Hope that helps

 

[Barry]

I had a very similar problem with my Mark V recently . It turned out to be a microphonic 12AX7 . Im almost sure thats your problem as well

 

[Barry]

I went back and read your message again . My broblem was also with channel 3 . I cant remember but it was V1 or V3 that was the problem for me . It would really squeel when I kicked on the BB preamp . Your tubes have a warranty if they are under 6 months old .

 

[babow2]

Thanks for the tip. I have a couple sets of extra power and preamp tubes from Mesa. One of the perks of living in Petaluma and knowing folks there....... I will check the manual and try a swap.

 

[babow2]

I replaced the V6 12ax7. V6 handles gain stage 4 of Channel 3 and FX send. With the new preamp tube, Channel 3 with FX loop engaged and all kinds of different effects.... no squeal at tremendous volume. I think that solved it and thanks. While I had it open, I decided to put in a pair of SED winged C 6L6's in the middle position.... We'll see how they sound.

Thanks again for the tip.

 

[Barry]

Glad that got you up and running .I could tell your problem sounded alot like the one I had . I dont live in petaluma but have few friends there at Mesa as Ive been using their gear for 30 years now . Tim McKee is a good friend as is Steve mueller & Trent Blake . Met Randy Smith a few years ago .