Mesa Boogie Studio 22+

[CJ]

let's get under the hood of a Mesa-B 6BQ5 amp, what say?

 

[CJ]

4 screws to get chassis out, which is nice,

this amp gets mixed reviews, some like it, some say it runs hot and has the PC board above the tubes which heats the board into stress fractures,

also a few comments on the smallish OPT but i like core saturation better than overdrive sizzle, 

 

[CJ]

has pots on chassis and input jack on chassis, 2 points for that,

some folks don't like tube sockets mounted to the PC board, but amp makers have so much comp nowadays that they struggle to meet a reasonable price point,

 

[CJ]

running the US made EL84s at 364 B+ and 285 screen which helps the tubes survive the excessive plate voltage,


fixed bias at minus 12 which allows 60 ma per tube, which is right at the limit of the tube manual specs,

would change to cathode bias to get a really warm amp, but Mesa has a rep for large pwr from small amps, so they take it to the limit,

microphonic foam on the tube cage thingy,

 

[CJ]

back plate options, good spk jack labeling,

wish amp companies would dump the effects loops, everybody sticks the box in front of the amp, loops cause problems as the signal is routed thru pc board jacks, so when the jacks get funky,  the amp quits working,

 

[CJ]

schematics here>

http://www.classictubeamps.com/SchemMesa.html

 

[served]

Hi CJ

I have the same Amp at my workshop.
It probably has a failed PT. As it is a 110VAC unit but I have 230VAC mains grid, so someone reined it.
At the moments its unclear what the issue is.

I am going to change the Tubesockets as the ones that are in there are bad. Socket legs are not solderable for some reason (I don't have a good enough soldering paste probably).

There are some additional bad soldering points on the board. Looks like a bad build for me.

One thing I noticed is that I don't have that Metal profile to put on tubes for some reason.

Second thing, That Direct  output just sounds so bad.

 

[CJ]

this amp cuts in and out, looks like another return jack problem,

weird  circuit, clean channel input tube has 3.3meg/10 pf  pad to make foot switching better, adds hiss when the amp is all the way up, this is the same pad you see after V2 on a lot of blackface amps,

best thing to do with this amp would be to rip out the pc board and do a point to point job on some new sockets, but that would take a lot of time and solder,

nice sounding amp when it works,

 

[CJ]

clean channel sample

Epiphone SG

 

[CJ]

overdrive channel>

 

[mjrippe]

Hi CJ,

So was it just the return jack or does the amp still cut out?  I have a .50 Caliber + in that runs great til it gets warm, then cuts out.  I am suspicious of the vactrol switching as some folks have had them fail.  Saw that method was used in the Peavey you posted as well.  Will get it back on the bench soon...

Mike

 

[CJ]

getting the amp to fail so you can fix it is the biggest hassle, sometimes they start working while you are taking voltage readings,  :-X

if you want to determine if the effects loop is screwing up the signal, wiat for the sound to drop then plug a patch cord into the effects loop,

pwr tube sockets transfer a lot of heat to the pc board so maybe touch those up with the soldering iron,