Mark IV - some questions

[msaglien]

Hello everyone,

just recently purchased a used Mark IV. Wondering if anyone could guess what year it was made by the serial number, which is 10996.

Also, when changing tubes, do I have to remove the chassis? If I need to remove the chassis, any tips for me on that?

Thanks for any replies.

-Marius

 

[Markedman]

http://forum.grailtone.com/viewtopic.php?f=27&t=5987

Check that thread out and no, you do not have to take the chassis out to change the tubes but the metal cover on V1 can be tough to get off.

 

[msaglien]

Thanks for the reply :mrgreen:

 

[SquierJVs]

As others have said, you don't have to remove the chassis to change tubes.

A few tips:
- the power tubes are easy to get at and change (I like my fingertips; so I let the tubes cool off).
- preamp tubes are harder to get at. Easier if you remove a power tube or two.
Sometimes, the RCA plugs will block things. Note which one goes where before disconnecting (or take a pic).
- V1 is under an RF shield. Push it and turn it a little. It should pop off. Reverse that action when re-installing
Mesa recommends a SPAX7 for this spot. It's 12AX7 that's been cherry-picked as quieter and has a rubbery wrapping. It's about $5 more than a regular Mesa 12AX7
- Check your amp to see if it's a MarkIV revision A or B (the link above tells you how to identify which is which).
Rev. A uses a 12AT7 for V4. Mine had a 12AX7 (previous owner). Reverb sounded off and the control had no taper. Putting a 12AT7 improved things.

Lastly, I have undone the the chassis bolts and pulled out the chassis a little to access the preamp tubes. If you do the same, push up a little on the chassis to take the weight of the chassis off of the cabinet cleats. When pushing it back, be careful of the front. There is a foam strip on the underside of the top near the front that can get bunched up. Use a plastic ruler (not metal), thin cardboard (like from inside shirt packages) to compress the foam. I have even turned it upside down (a combo). When it's upside down, it's tempting to grab the chassis by putting your fingers under the chassis (and thus inside the chassis). Not a good idea. When upside down, the washers from the shock mounts (the little cups in the top of some combos) may come out. Since they are dark, you may not realize they are there (until they drop out ... if they do).

Good luck.

 

[stokes]

To get at the preamp tubes easier,just remove the front panel.

 

[BlackBoxy]

Mark IV rev a has lead triodes on V3, clean chanel on V1 , and reverb on 12 at7 V4, simplier than the b rev!

best mod to do, change filter caps , and pf tone stack and pres caps with styroflex 630v caps, remove ice pick trebble !


all NOS "pre 80's" tubes USA long plate will get great job, just need to experiment a lil to figure out what's best fit to your ears, best reissue tubes are tung sol 12 ax7 and Sed Winged C power tubes, forget other reissued tubes, they are terrible tone for this hight end, and often badly triodes matched!

Mesa boogie buys lots from manufacturers, Mesa sorts best tubes at gain matched triodes and microphonics "as Groove tube , the same" and sell it as Mesa tubes, so not always best tone, but seirous tubes !