Mesa Mark III Green Stripe (Pics Added)

[Boogie-Nights]

Finally managed to join the forum!

Just aquired a very late (i think) Mark III Green stripe!

Was orignally a combo that was converted to a head, with hardwood head cabinet.

I'm amazed by this amp, its so musical! Both clean and Lead channels are great! Super thick with loads of punch!

Only problem.. It has this hum that increases in volume as you increase the master. Its fine until you get to 1 or more on the master.

Also increase and decreases with gain. So far only on the lead channel but havent had it loud enough to see if it effects clean and crunch channels.

To the point where i dont think you could gig with it, especially not mic'd through a PA...

What could this be, grounding??

Im going to swap pre amp tubes on te weekend as the manual says thats a likely cause... Not sure though

 

[fretout]

Finally managed to join the forum!

Just aquired a very late (i think) Mark III Green stripe!

Was orignally a combo that was converted to a head, with hardwood head cabinet.

I'm amazed by this amp, its so musical! Both clean and Lead channels are great! Super thick with loads of punch!

Only problem.. It has this hum that increases in volume as you increase the master. Its fine until you get to 1 or more on the master.

Also increase and decreases with gain. So far only on the lead channel but havent had it loud enough to see if it effects clean and crunch channels.

To the point where i dont think you could gig with it, especially not mic'd through a PA...

What could this be, grounding??

Im going to swap pre amp tubes on te weekend as the manual says thats a likely cause... Not sure though

Nice score man! Welcome to the Boogie Brotherhood! Maybe some other members can chime in, but your amp is the first Green Stripe I've seen with an Export Trany! Are you positive it's a Green Stripe?

Do you have any pics of the Power Amp Sockets inside the chassis? This will help confirm whether your amp is running in Triode or Pentode (like all Mark III Green Stripes), and will also show us what's going on inside the amp to see if there's anything less obvious that could be causing your amp to hum.

My money is on the fact that you probably have a bad or microphonic preamp tube, most likely V1. This is a fairly common cause of unwanted humming and squealing. You will also want to make sure your transformer's setting (the red box on the back of the chassis) is set to the correct power rating of the electrical sockets you're plugging into. You'll earn bonus points if you make sure you are plugging into a surge protected power source!

 

[McBarry]

G'Day,
if it's def isolated to the lead channel, that's V3.
Swap it with known good tube and if problem persists it's time for exploratory surgery..

Also though, I wonder if it's related to gain, and so if clean ch was driving hard enough and it occurred there also, that points away from V3 and back to starting off troubleshoot again..

BTW, gorgeous timber cab..
I wonder if that's aftermarket as the reverb isn't mounted in a recess, there's non-mesa liike reverb cable(s), and the front baffle board looks nice but I've not seen a factory one in that style - anyone else seen a similar style?

Certainly looks in good nik.
Dave

 

[Boogie-Nights]

Okay I’ve done some investigating...

I tried swapping out pre amp tubes one by one with some Marshall (not great I know) tubes lying around. None of them made any different to the hum. The only one that made things slightly better was the swapping the 3rd tube from the left... Other than the hum was still there. What i did notice is that the hum goes away when the amp is in standby...

It is apparent across all three channels. I tried tapping the tubes also and that had no effect.
I took some picks to help clear up what Mark III I have. Looks like its dated 8/89. There is a faint green line on the back. Initialled MB perhaps??

As far as the circuit board goes it doesn’t look to me like any components are burnt out or have gone bad.
I was however surprised about the tubes...

Here’s what is in there:

For the preamp, L-R: Mesa 12X7-A, ECC83M, Mesa 12X7-A, Mesa 12X7-A, ECC83

Power tubes are: TADEL34B-SRT, 6l6GC USA.

I was surprised to find some no-name tubes in there.

Either way looks to me like a fully loaded Mark III: reverb, GEQ, Simul-Class, Reverb...
What do you think?

Im in the UK so i guess im right in saying it woudl have export trannies, voltage is 240V over here!

As for the cabinet it is non-Mesa. The guy had it made to covert the amp from combo to head. It looks great it person, gonna get the front pannel/baffel change to a wickor grill one to get a more "classic" Mesa look. Joinery is great through!

http://www.flickr.com/photos/92760238@N05/8433652642/in/photostream/lightbox/