Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on May 18, 2017, 07:20:06 am
-
I was looking at Mesa Boogie amp schematics this morning & noticed some component in Mesa Boogie amps that I am not familiar with? It's a J175J. Is this a jfet and what purpose does it serve? I am thinking this is a switching device?
http://el34world.com/charts/Schematics/files/mesa_boogie/MesaBoogieMark%20V.pdf (http://el34world.com/charts/Schematics/files/mesa_boogie/MesaBoogieMark%20V.pdf)
A couple of other questions also. What Mesa Boogie is considered a "great sounding amp"? And has anyone on the forum built one?
With respect, Tubenit
-
I was looking at Mesa Boogie amp schematics this morning & noticed some component in Mesa Boogie amps that I am not familiar with? It's a J175J. Is this a jfet and what purpose does it serve? I am thinking this is a switching device?
http://el34world.com/charts/Schematics/files/mesa_boogie/MesaBoogieMark%20V.pdf (http://el34world.com/charts/Schematics/files/mesa_boogie/MesaBoogieMark%20V.pdf)
A couple of other questions also. What Mesa Boogie is considered a "great sounding amp"? And has anyone on the forum built one?
With respect, Tubenit
Yes, the J175 is a jfet, used as a gated switch (think of a gate as an electrically controlled switch contact)
I'm pretty sure that the extra J suffix is just a Mesa labeling, and if you look around you'll see the other J175 have different letter designations based on their function in the amp (i.e. J175(D), J175(E), etc.)
For metal heads, the Dual Rectifier is a great sounding amp.
For early Mesa aficionados, the MarkIIC+ seems to be THE big favorite.
For me, my Solo50 Rectoverb has some great sound in it. It has that cool sustain into feedback thing happening even in the 'pushed' clean channel.
http://el34world.com/charts/Schematics/files/mesa_boogie/boogie_solo50_rectoverb.pdf (http://el34world.com/charts/Schematics/files/mesa_boogie/boogie_solo50_rectoverb.pdf)
I've never tried to build one because of the complexity of sorting out the schematic, but I have borrowed some design concepts from their schematics with success....most notably the FX mix control that I have shared on some of the other series vs. parallel FX loop threads. I like it.
I also like the use of a "SOLO" control, which in this amp is the pot that's switched in to provide an alternate (boosted) output level.
-
You got me thinkin today...so there's this:
http://www.seymourduncan.com/forum/showthread.php?3893-Best-Mesa-Amp (http://www.seymourduncan.com/forum/showthread.php?3893-Best-Mesa-Amp)
-
To expand a little on the JFET useage:
-Pic below is one example of how you can use a J175 as a voltage controlled variable resistor which acts as a boost limiting resistor in series with the bypass cap on that stage. By applying +12V to the gate you are essentially squeezing the current flowing in the circuit and limiting the effect of the bypass cap. With the bright switch in the closed position you will have 0V applied to the gate (because the +12V gets shunted to ground) and therefore allow full current flow, and in this case, full boost from the bypass cap.
Sorry sluckey- had to edit to fix thought for clarity and still not as clearly written as I would like
-
Can you explain why this might be better than simply switching an actual resistor in & out of the circuit?
-
I would guess it will cut down on any switching noise.
-
Lower switching noise and the fact that the switch itself can be anywhere, even in a foot pedal. Only a switching signal needs to go to / come from the switch (essentially a digital signal.) The relatively sensitive analog music signal remains safe.
The one thing I noticed on that schematic is that Mesa evidently patented the tapped primary PT.
https://patents.google.com/patent/US5091700A/en (https://patents.google.com/patent/US5091700A/en)
It amazes me that such a patent ever got granted.
-
JFETs are becoming very popular for switching duties in commercial amps. They cost pennies each, use very little current and are tiny (TO-92 package usually.) All of these are huge benefits to major amp makers (PCBs.) They can also switch signals pretty much silently.
For a great explanation see Merlin 2nd Ed. Chapter 13.
-
I remember carting around all my Marshall backline and the Boogie Mark II were coming out and supposedly were the stuff. They were also hard to get due to backorders and it was almost a badge of honor if you had to wait the longest, sometimes months. All the guys who bought them told me I was a dinosaur and the small amps were the wave of the future and how great they sounded. You know me, I was not a fan. Turned up, I thought they all sounded like a neutered Ted Nugent tone - too much preamp inspired hohum, not enough umpf and grit. I also thought they were pretty much one trick ponys that sounded the same no matter what the settings were. I just did not like that trumpet with a mute tone...
The Dual Rectifiers were the stuff and pretty much defined an era of music much like Marshall did in the 60's and 70's. Not everyone's cup of tea but I always thought they sounded good throttled back clean or wide open in full glorious distortion.
JMHO
Jim
-
Thanks for the SS Switching info!
-
I was once infatuated w/ boogies. I borrowed a friend's mark II (or was it a mark III?) for a month or so. There were too many knobs and switches and shifts and eq's, etc. making it seem more about twiddling controls than playing... then, the Blue Angel came out and I thought this was going to be the perfect amp... Not for me--I wish I'd of kept the Peavy I sold to buy the blue Angel. I like my home brewed amps better.