mark iv strange high end variation

[bsic]

Hi all,

I bought a used 2006 mk iv combo about a month ago from GC. after playing for a few weeks, i notice a very wierd issue- i have noticed it several times in rhy1, rhy2, and i *think* in lead as well.

When playing chords heavily distorted, sometimes the sound "shifts", as if someone just turned the treble all the way down. I keep playing, and within a minute it comes back. And when it comes/goes its very noticeable, but i cannot consistently reproduce it. Very odd--- wondering if i should return this, or get it repaired...? I only have 30 days for return and that time is rapidly approaching.

Any advice greatly appreciated!!

 

[Boogiebabies]

Try a new V1 preamp tube. It may be acting up and would be the only preamp tube to effect all three channels.

 

[bsic]

Thanks- so it is possible for frequency shifts with a bad tube? I was curious because normally you hear of tube noise when you have a bad tube... not necessarily EQ shift type of issues.

 

[Hailstorm350]

noise, you're right, but also a bad tube can experience variations in grid voltage which can cause a whole variety of effects depending on where in the amp circuit its used, in your case, the first preamp tube is dropping treble when it flakes out...

someone correct me if Im wrong

 

[OverDriven]

I would bet you money it's a bad preamp tube.