What's with this "fizz" I'm getting on my ol' Plexi?

[zerocharisma]

So I've been recording some clips lately on my '68 bass plexi and when I have the mids cranked it sounds smooth and saturated--perfect for leads. However, when I try to cut back the mids below, say 5, for some AC/DC type crunch I begin to hear a very obvious "fizz", especially while playing chords. I understand there may be a number of factors that could be responsible, i.e. tubes, speakers, mic placement, etc. But I'm using all quality NOS tubes, a quad of broken-in G12M Heritage greenbacks, and placing a SM57 angled at a 45 degree angle off axis about an inch of the grillcloth. I'll have clips up soon, but in the meantime what specifically can be the likeliest contributor of this "fizz" and is there anything I can do to reduce it?

[scottpic]

FWIW,

I got a new Zvex Box of rock the other day and couldn't wait to plug it in.
When the pedal was off (the amp was set clean) I heard this slight buzz or fizz noise.

It turned out to be a bad cord!!!
This may not be your problem, But I have all hi quality cord that I take care of. I was really surprised to find it was the cord.

Anyway, starting with the basics never hurts.
Good luck.

[zerocharisma]

Thanks for the reply scott. That is a very interesting suggestion that no one else has pointed me to. Looking at my setup now, I have some crappy cables that I got for free from GC (oh the shame!). I was thinking of picking up some Vovox cables in the near future. But for anyone else who might wanna chime in here's a soundclip I made of the noise today:

http://www.soundclick.com/bands/default.cfm?bandID=848763

It sounds like chords are surrounded by this persistent fizz. What's frustrating is that it could be anything. Oh well, I'll keep tweaking a few things here and there to see what I can come up with. Any other suggestions would be appreciated!

[Lyle Caldwell]

Could be a lot of things.

First, swap out each preamp tube.

Then make sure all the jacks and pot connections are tight.

Then make sure all the tube sockets are tight and clean.

Then make sure the jacks (including the speaker jacks) have clean contacts.

Sometimes the pins on a tube need to be very very slightly splayed out to ensure good contact with the sockets.

Change guitar cables. Change speaker cables.

If none of the above do the trick, you might well have a bad plate resistor or a bad solder joint within the circuit itself.

Methodical testing is the way to go.

[VikingAmps]

Kinda sounds like a speaker issue to me. But it could be a lot of things.