Mesa Express,

jestaudio · Post by **jestaudio** » Sat Dec 05, 2015 7:08 pm

One for the more serious tech heads here, I have a express 5 50 plus, a reasonable amp but a very weedy clean, I,ve finally got a scematic, perhaps some of you could take a look see and suggest possible mods that could thicken the clean up a little

TUBEDUDE · Post by **TUBEDUDE** » Sun Dec 06, 2015 3:54 am

I'd recommend selling it. Sorry, I'd like to help but unfortunately the diagram you posted is not a schematic, it's.just a block diagram. If you can post a.schemaic,recommendations still might be hard to implement, due to the construction of the amp. Have you opened it up? Is it the usual printed circuit board amp with limited space?

YouTube · Post by **xtian** » Sun Dec 06, 2015 4:00 am

TUBEDUDE--the PDF contains multiple pages, including the schematics.

Component values look OK--don't know why it would sound "weedy." Have you tried a different speaker?

bal704 · Post by **bal704** » Sun Dec 06, 2015 4:31 am

I've been inside a couple of Mesa amps, and built a clone. They are tight PCB boards, several of them, connected by ribbon cables etc.

If you've only worked on eyelet/turret type builds, prepare to be dazzled. Lots of guts stuffed inside that chassis.

SilverFox · Post by **SilverFox** » Sun Dec 06, 2015 7:55 am

Don't do this unless there is a consensus from others but- How about making V1a look like V1b regarding the cathode resistors and change C17 to .033. These would be relatively simple changes that would not require much in the way of hacking. You could also change R3 and C3 to higher values to change the gain and frequency response of the first stage.

I haven't seen the inside of your model amp but it could be akin to brain surgery and your rework skills should be very good before you start cutting traces and changing components on a PC board.

Survey says?

silverfox, just enough knowledge to be dangerous...

dorrisant · Post by **dorrisant** » Sun Dec 06, 2015 8:27 am

For a Mesa... relatively easy enough to pull the front panel controls and jacks to be able to get to the solder side of the board.

Picture is a 5 25 but exactly the same layout except for the different power tube board.

Edit: I noticed the OP stated the "plus" model... So there is also the EQ on the front panel. IIRC, the EQ doesn't have to be removed. It stays put with very little to disconnect. Don't let it discourage doing mods.

jestaudio · Post by **jestaudio** » Sun Dec 06, 2015 9:52 am

xtian wrote:TUBEDUDE--the PDF contains multiple pages, including the schematics.

Component values look OK--don't know why it would sound "weedy." Have you tried a different speaker?

Yep. C90, V30 celestion and a EV12l, essentially it seems to lack lower mids on the clean which from my experience of having many boogies over the years is unusual, almost every Boogie I have owned had a cracking clean, this sounds fine in isolation but in a band situation it sounds toppy and very thin.
All the tubes have been changed out to no avail, if its a inherent fault then I get rid off it which would be a pity as its a pretty decent go to amp.
The first stage is v2b for the drive and v2a for clean so I was considering upping the cathode bypass to match the other stages but am slightly puzzled by the cathode arrangement on v2a

TUBEDUDE · Post by **TUBEDUDE** » Sun Dec 06, 2015 1:34 pm

Ah! Thanx Xtian, after a phone reboot and download i have all the pages now. The cathode circuit for V2a is using a FET as a constant current source to bias the stage. For V2b increase the value of C3, as mentioned above, slope resistor R13 reduced a little further, and possibly C13 and C17 a little larger.
it does look a little easier than working on a Classic 30. If you're methodical and precise, i'm sure you will be successful. Let us know what you decided on and the results. Good luck.

JazzGuitarGimp · Post by **JazzGuitarGimp** » Sun Dec 06, 2015 4:10 pm

Goodness, that circuit is more relays than anything else! I have to wonder if the problem may be a failing relay contact...

Isn't Mesa known for using vactrols instead of relays?

TUBEDUDE · Post by **TUBEDUDE** » Sun Dec 06, 2015 6:42 pm

There are quite a few relays, I like these better than sandy state switches though. There are a gaggle of PN junctions in this amp also. Circuit boards, ribbon cables...many of the things I like to avoid in an amp. Especially a combo. Hopefully this is a head. At least this has chassis mounted tube sockets, right?

dorrisant · Post by **dorrisant** » Tue Dec 08, 2015 1:52 am

TUBEDUDE wrote:. At least this has chassis mounted tube sockets, right?

You are joking, right? It is a Mesa...

Stevem · Post by **Stevem** » Tue Dec 08, 2015 11:42 am

That so called Mesa midrange tone on the early amps was in large part due to how they chose to deal with the oscillation problem those amps had due to the small chassis!

cbass · Post by **cbass** » Tue Dec 08, 2015 12:10 pm

Gut it build a good sounding amp. Never mind mesas trannies tend to suck pawn it off on some other sap. Build a good sounding amp.
This is an amp building forum after all

Phil_S · Post by **Phil_S** » Tue Dec 08, 2015 3:34 pm

No love for those Boogies here, huh? Once you strip the components out of those green boards, they make great chassis patches. I like to cut them and use them to close up the holes I'm not going to use when I recycle a chassis. They work great and they look great. You can even drill them out to mount a new noval socket in an octal hole. Just my 2¢ tip on recycling your junk.

dorrisant · Post by **dorrisant** » Tue Dec 08, 2015 4:00 pm

Phil_S wrote:No love for those Boogies here... Just my 2¢ tip on recycling your junk.

You said it, man!!

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