Mesa Boogie Express 5:50

[CJ]

2 ea. 6L6GC,  two channels,  cool contour circuit,  no eq thanks, 5 watt SE switch,

 

[CJ]

nice sounding 12inch, reverb pan, effects loop, pain in the ass to work on,

 

[CJ]

nice workmanship, pots are hard wired which is good for dependability but bad for working on the board,

more ribbon cables being used which is good for service, good tube sockets, separate board forpwr tubes,

hot lead of B+ cap (right side)bit me a few times, no big, 300 dc,

 

[CJ]

notice the tweaked relays, copper shields soldered to ground for less switching noise, Randall Smith is like that,

unfortunately he likes  fets, J175's to be exact, 

7 or 8 relays, 7 or 8 fet switches, troubleshooting nightmare if you don't know the amp,

 

[CJ]

problem #1 was that the excessive spring pressure from the five tube shields popped the board off the nylon standoffs,
#2 was cold solder joints on tube sockets,
#3 was a bad 12AX7a,
#4 was a leaky 500 volt 250 pf mica,
#5 was a 100 pf stuck in a .001 hole by some other tech,
#6 was a bad fet shorting out the phase inverter in either crunch mode (ch 1) or burn mode (ch 2)
#7 was  a bad fet shorting out the reverb signal
#8 was a bad fet that switched in a bypass cap on one of the preamp  cathodes,
#9 was a disconected connector inside the reverb pan
#10 was the foot sw connector not being engaged all the way into the socket,

other than that,it was a cakewalk, 

couple of coupling caps hidden under the board

 

[PRR]

Three bad FETs??

In basically easy-work?

Might be worth thinking about why they failed. I suspect over-enthusiastic gate drive.

 

[CJ]

we left one fet out that does a subtle change of the feedback circuit but could also shut down the amp during a solo,

either gate drive or somebody not wearing a wrist band, or the stock room guy wearing Nike's,

use to have a problem with Fairchild fets,

they diduse precisionn resistors throughout, 1 watt models for plate resistors,

got some one inductors

 

[CJ]

look at this, Fet gates tied udernneath the inductive kick of the relay,might be a problem area:

 

[CJ]

12 switches in the preamp circuit, labels do not say much, no truth table, so aux drawings must be drawn up for troubleshooting,

that 39K cathode resistor on V2a does sound awesome, we will borrow that forour own amp,

 

[CJ]

here is an aux drawing showing the ch 2 signal flow,

v2a plate signal walks over v2b signal

 

[PRR]

> Fet gates tied udernneath the inductive kick of the relay

Well spotted.

 

[sahib]

12 switches in the preamp circuit, labels do not say much, no truth table, so aux drawings must be drawn up for troubleshooting,

Lousy drawing if you ask me.

NC and NO contacts but drawn as  ON-OFF-ON.

And the FET gates is the icing on the cake.

 

[CJ]

this is a nice sounding part of the circuit, a lot better than the old 5 band found on the Mk 4 series

 

[CJ]

here is the inverter with a feedback tweak and also the 5 watt/50 watt thing,

when J175c went south, it shorted the gate voltage right onto the V4a grid, disabling the whole inverter in the crunch/burn modes of ch 1 and ch 2, so half the amp options were not available,

notice that the OPT sees a DC bias during SE operation,  looks like they limited the core saturation with the 1.6 K resistor, and dropped plate voltage, it does have a nice crunchy sound,


looks like the .02 from plate to cathode is pos f/b to maybe simulate a degen cathode cap

dropping the screen also in SE mode with the 120 v 5 watt zener good for 40 ma,

 

[Whoops]

Nice Thread CJ,
thanks for the info.

Not a big fan of most of Mesa Boogie amps but I actually think the Express Amps sound really good both the 5:50 and the 5:25