LOUD Mark IV pop on start-up (with sound clip)

[guitardude05]

Warmed up properly. This is my 2nd Mark IV, this one makes a LOUD pop when flipping from standby to ON. The louder the output level, the louder the pop. On only lead 0 and Master volume 3, it is chest thumping and quite scary! I can't imagine normal volumes.

Here is the recorded noise flipping from standby to ON (first pop is at master 1 lead 0, 2nd is master 3 lead 0):

http://dl.dropbox.com/u/3331450/Mark%20IV%20Power%20On%20Pop.mp3

What could be the problem? This amp is in mint condition, an early B. It sounds fine, and if anything, it sounds a bit beefier than my last Mark IV. All tubes appear to be in good condition, lighting up properly. I traded away my late 90's Mark IV combo for this one and I'm kind of regretting it now on top of the fact that I just miss the combo anyways. I really hope that somebody knows what this is... Please help!

 

[zodiac272]

hey,

I'd check the tubes, then the tube sockets, then the capacitors. it may be a little early for the caps to be failing, unless the amp sat unused for years. They could be bleeding signal and creating the pop. It seems like signal is moving when it should be static. Sounds like caps to me, but I'm far from expert


scott

 

[guitardude05]

I swapped out the 2 outer 6l6's for el34's. It's not making this noise anymore. It only makes a very slight low end noise now which is hardly noticeable, and is probably normal power up noise.

These 6l6's must be bad, but they were in my last mark iv and never caused problems. They are odd russian tubes with no branding. Forgot the name. The other tubes in there are JJ's and they light up much brighter than these 6l6's, but I really thought these tubes were good still. Bummer, but I am so thankful it's only a tube issue!!!!!! Relief.

 

[McBarry]

GD05, I'd try the questionable tubes in another amp - they "should" exhibit same symptom there, thus proving their demise.
I'd also try another set of 6l6's in outer sockets, to eliminate the sound still being there but being attenuated by the EL34's.
They were nasty thumps on your audio file.
It's almost like a DC spike is getting through before being filtered out.
The experienced tech guys here I'm sure will have an informed opinion.
Don't want to be a pessimist, but whatever it is/was, I'd get someone inside to take a peek. Might be good insurance..