Little help needed with minor issue

[Skyless7620]

My red stripe Mark III has this intermittent noise problem that has me a little worried.

Observations:

- Intermittent white noise (constant and too loud to be considered background).
- The more gain the louder the noise, faint but present on the clean channel as well.
- Seems to go away instantly when I flick the switch from Class A to Simul Class. (this one is wierd o.0)
- Occasional swell of increased volume (not a lot, but noticeable).

I changed out V1, didn't do squat. Physically the power tubes look fine, and when I tap them a little when the amp is on to see if any one of them is the source of the problem, nothing happens. They all pass the test.

There isn't a big difference in the overall tone. Hard to tell, but it's possible that it has slightly decreased overall volume lately.

Anyone got any ideas? Thanks!

 

[boogiemon]

sounds like a power tube problem to me.

 

[>Photi G<]

Are we talking about a hiss, or a humming noise? If it's a hiss, it's the power tubes. If it's a hum, then it's the filter caps.

 

[Skyless7620]

more like a shhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

dunno if that counts as a hiss or not, but I wouldn't say its a hum no.

If it was the filter caps, it would do it regardless of whether it was in class A or simul class no?

 

[boogiemon]

sounds like wind blowing through the amp? ...power tubes.

more like a shhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh

dunno if that counts as a hiss or not, but I wouldn't say its a hum no.

If it was the filter caps, it would do it regardless of whether it was in class A or simul class no?

 

[Skyless7620]

Minor update. Put new tubes in it, and while the hissing seems to have dropped, the volume swells are just as prevalent as before, if not even more.

Another thing I noticed: I changed the outside pair of tubes from EL34s to 6L6s. Are the 6L6s naturally supposed to be hotter? Played same things, same volume, same warm up time, and the 6L6s were easily twice to three times as hot to the touch.

 

[SonicProvocateur]

Maybe the screen grid resistors getting ready to go. When my power tubes went that's exactly what mine did. Dunno if that's pre-symptomatic though

 

[Skyless7620]

Maybe the screen grid resistors getting ready to go. When my power tubes went that's exactly what mine did. Dunno if that's pre-symptomatic though

Is that possible given the fact that the amp was supposed to have been serviced less than a year ago? (I say supposed because that's what the seller told me, and he's reliable).

EDIT: No more volume intermittence, just overall lower volume now...

 

[SonicProvocateur]

Yeah, sounds like something's getting ready to go; either a power tube (fireworks show), or a resistor.

Edit; I made the mistake of increasing my master volume to compensate for the volume drop in the band mix when it blew. I would send it to Mike B.

 

[Skyless7620]

I gave it to my (mesa certified) tech, and since we work in the same place it shouldn't cost me much, even with a re-tube.

It's not very feasible for me to send it to Mike B. since I'm in Canada. But if everything fails I'kll have no choice I guess... =\

 

[SonicProvocateur]

When mine grenaded I upgraded my screen grid resistors to Xicon 5 watts instead of the very low load ones. Sure, now I can run any high powered 6L6 without fear, but I think it changed the tone. I would say if I had to do it again, I might just go for the OEM ones instead of the big boys. Just some food for thought - tone vs. reliability.

 

[Skyless7620]

just an update for the sake of those looking at these threads in the search function later on:

It turns out that the problem was the contact points inside the tube sockets! They were loose.

 

[>Photi G<]

When mine grenaded I upgraded my screen grid resistors to Xicon 5 watts instead of the very low load ones. Sure, now I can run any high powered 6L6 without fear, but I think it changed the tone. I would say if I had to do it again, I might just go for the OEM ones instead of the big boys. Just some food for thought - tone vs. reliability.

Yep, it happens. Carbon Comp resistors sound great only in certain positions, mainly those with a lot of voltage going through them. Ceramics sound brittle and trebly. The 1 and 2 watt resistors that Mesa uses for high load positions are always carbon comp for this reason. The reason the tone changed could also be that cc's aren't very reliable, and their tolerance drifts over time. The Xicons were probably delivering what your amp sounded like a while back.

 

[DaveP]

just an update for the sake of those looking at these threads in the search function later on:

It turns out that the problem was the contact points inside the tube sockets! They were loose.

What causes this? I know the newer STR440's have fat pins. Just wondering if a fat pinned STR440 then switching to a tube with thinner pins would cause this to happen.
Did it just stretch the sockets out? The older sockets were not made for fat pins. Maybe it's a dumb question but just wondering.

 

[McBarry]

Dave,
I've noticed it with 2 or my Marks.
My personal theory is metal fatigue.
The pin sockets start out as flat THIN bar, get pressed into a semi-circle, then get heated/cooled repeatedly.. For years.
I'm no metallurgist, but it seems very logical why the sockets would expand over time.
Add thicker pins followed by thinnner pins.. and wiggled pins as tubes are inserted/removed..
.02 worth only..