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PostPosted: Sun Oct 29, 2017 9:38 am 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
Hi everyone,

I joined the board this morning. I received a Mark IV combo yesterday. I believe it's from 1995. Cosmetically, the amp is pristine. However, there are a few things to note, and I need your expertise to determine how bad things are about to get for me.

The amp produces a constant hum when switching from Standby to On. The hum is very subtle in Pentode mode, but very loud in Triode mode. Also, the Pen/Tri switch on the rear creates a static crackle when switching.

Two of the power tubes glow blue when switching from Standby to On. I pulled each power tube (4 Mesa 6l6), checked contacts, and reinstalled in a different order, taking note of where the "blue tubes" were installed. Sure enough, those tubes glowed blue, even in different sockets. Is blue normal or indicative of a faulty tube?

I also checked the pre-amp tubes (Ruby 12ax7) to ensure a snug connection. All was fine. Maybe I should swap for Mesa?

After the power tube swap, the hum still persisted in both Pentode and Triode. Also, even with all volumes set to 0, there is a loud pop when switching from Standby to On.

I'll call Mesa on Monday to hear what they think. I truly hope this is a minor issue I can work out or simply a tube replacement.

Any thoughts?

Thanks!
John


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PostPosted: Sun Oct 29, 2017 10:55 am 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
UPDATE

I lightly tapped each tube with a pencil eraser with the amp powered on. The blue tubes rang out like a huge glass bowl, while the other two had a soft metallic tinkle.

Does that help you guys?

Thanks!
John


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PostPosted: Mon Oct 30, 2017 12:20 pm 
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Bottle Rocket
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Joined: Fri Jul 21, 2017 10:18 am
Posts: 7
Location: CT
Sounds like you should replace the power tubes to begin with and then go from there.

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PostPosted: Mon Oct 30, 2017 12:28 pm 
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Mark IV
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Joined: Wed Sep 08, 2010 4:12 pm
Posts: 671
Location: Petaluma, California
McFriedy wrote:
Sounds like you should replace the power tubes to begin with and then go from there.


+1.

An increase in hum when switching from Pentode to Triode is somewhat normal - but in excess is a sign of tubes that have drifted apart, and/or old. Always start fresh with new power tubes and go from there...and replace about every 2 years of regular use.

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PostPosted: Mon Oct 30, 2017 5:25 pm 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
Thanks for the advice, guys. I had the pleasure of speaking with Rich at Mesa in CA today, and he advised the same. Are you 'Authorized Boogie' on the forum, Rich? I'll buy two matched sets of Mesa 6L6s at my local dealer in the morning and keep everyone posted.

Cheers,
John


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PostPosted: Thu Nov 02, 2017 1:47 pm 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
Just replaced all 4 power tubes with Mesa 6l6 STR 440. Major improvement to tone, natural ambient sounds of the amp, and the hum in Triode is gone. Now I have to wrestle this amp into tone submission. Going to take a while!

Thanks,
John


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PostPosted: Thu Nov 02, 2017 1:53 pm 
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Mark IV
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Location: Petaluma, California
Glad you got it sorted! (No, I'm not Rich...enjoy the amp!)

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PostPosted: Thu Nov 02, 2017 2:46 pm 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
Oh, I forgot to ask. Should the slightest signal come through the speaker or should it be dead silent when you pull for silent recording? I hear the slightest guitar signal.


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PostPosted: Thu Nov 09, 2017 5:55 pm 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
Unfortunately, a new issue has emerged.

As of yesterday, the amp's tone and volume intermittently fades in and out from bright and clear to muffled while I'm playing. I've tried bypassing my pedal board, different cables, a new 9 volt in my EMGs, and even a patch cable in the FX loop from Send to Right (Mono), per Rich at Mesa Boogie's instructions.

I think I was able to speed up to process by pulling the patch cable one time, switching to standby and swapping instrument cables another time.

Could I need new Pre-amp tubes? Any thoughts whatsoever?

Really bummed out about the state of this amp.


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PostPosted: Tue Nov 14, 2017 6:20 pm 
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Bottle Rocket

Joined: Sun Oct 29, 2017 9:31 am
Posts: 10
In case anyone was interested I think the amp is cured.

I put new Mesa 12AX7 pre-amp tubes in this afternoon and wow, what a difference. The tone is bright and powerful and the low end was rattling my chest. I played on different channels at different volumes and in different modes. No volume or tone fading and everything seems to be checking out.


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