Mesa DC-2

[Watershed]

So I gave the 'ol Mesa DC-2 (Studio Caliber) some new tubes over the weekend and thought I'd share some shots of the insides for those interested in amps.

This is a 1993 Mesa Boogie Studio Caliber that I bought new in 1994 or so.
The preamp tube locations are written right on the board.  This is the V3 tube.  To the right is the V2.

It has been a very reliable amp over the years, and if memory serves me, the preamp tubes are original.

Having spent a fair amount of time reading up on amp design and construction I'm looking at the internals in a whole different way.
I have a slightly better idea of what I'm looking at.

The Studio Caliber was one of the first Mesa's to have "Dyna Watt Power."
Looking this over, I see what appear to be 4 rather large filter caps, in addition to a choke.

If nothing else, the current is probably pretty well smoothed out after the rectifier.

I was a bit surprised to see the Sprague caps in there.

I was never really a fan of the lead channel.  I always thought it had too much gain and sounded washed out.
I replaced the V2 tube (lead drive) with a 5751, to tone things down a bit.
The 5751 is a replacement for a 12AX7 with about 30% less gain.

I also thought I would try a "matched" 12AX7 tube from Torres Engineering in the phase inverter position.
The idea here is that a 12AX7 is a dual triode, and the "matched" tube is supposed to have a matched A and B side.
This would be useful in the phase inverter position, where one half of the 12AX7 is feeding one power tube, and the other half of the 12AX7 is feeding the other power tube.
No point in matched power tubes, if they aren't getting a matched signal to begin with.
I get the theory.

I also picked up a new set of Ruby Tubes EL84's, which are just rebranded JJ's.
Here's what I got:

I do have a bit of corrosion where the tube sockets attach to the board.

Aside from being ugly, I don't think it has really presented a problem.

So the result?
Well the 5751 definitely tamed down the lead channel.  It sounds less washed out and compressed.
Still plenty of gain in my opinion, but now with more character.

It's hard to quantify the benefits of the "matched" 12AX7, but in conjunction with the new EL84's, everything sounds very nice.
"Blossoming" overtones and all sorts of other guitar tone adjectives.
It sounds very nice, hard to describe though.
Everything sounds very "full."

I guess I should swap out the other 4 preamp tubes as well now.

James

 

[Superlizard]

"Matched" preamp t00bs = buncha bull.

"Matched" powert00bs = some truth and the only worthy concern for the gigging geetarist

Only people who go "matched" all-around are those anal-retentive "audiots/audiophools"... AKA audiophiles
with their fancy-schmancy hi-end t00b stereo rigs replete with $500 wooden knobs caked with "tone-enhancing"
lacquer.

 

[Patrick from Davis]

A set of matched triodes is a very good idea for the phase inverter.  That way the push and the pull signals coming out of the PI are equal, or as equal as they were entering.  Matching other preamps tubes, might have a nice effect, but from what you described, I doubt it.  Matching power tubes helps with biasing properly.

If you play inside the amp, make sure you discharge the caps first.  In the picture with the 220 uF/ 300V caps (the ones on the left in the big shot) right above the wire assembly that goes into the board, it looks like there are three diodes.  I am assuming that there is a fourth and it is hidden by the wire assembly.  That appears to be the rectifier for the high voltage side of things.  Try to figure out what kinds of diodes those are.  My guess is 1n4007's.  You can upgrade those to UF4007 diodes (or UF XXXX if it is a different diode make) for about 10 cents a piece and it will bring the fizzy-ness of an amp down.  Some amps power sections have some extra filtering caps that can help with this, but the UF diodes will make it even less noticeable.  Make sure you discharge the caps first before you go playing in there.
Patrick

 

[Watershed]

Thanks for the tip on the diodes Patrick.
Yes, I definitely discharged the caps, but thanks for the potentially life saving advice.
:toothy11:

Also to clarify, although a 12AX7, I guess it's not really accurate to call what I'm talking about a "preamp tube", since it's not really part of the preamp.
If you look up a schematic, the V6 tube (phase inverter in the case of the DC-2) is in the power amp drawing.

Matching the triodes of the phase inverter tube just assures that you get the best benefit from the matched power tubes you paid a premium for.
I'm not going to tell anyone that this is a life changing event, but if you buy into the theory of matching power tubes and can part with an extra $15.00, then give it a shot and decide for yourself.

I don't think anyone would argue that a matched preamp tube, in the preamp section of the amp, would make any difference at all.  There are no parallel circuits to "match", so what's the point?

James

 

[Patrick from Davis]

Well, in a guitar amp people do not match the separate tubes for any good reason.  For a stereo amp it make more theoretical sense to have the right and left at the same gain level.  But, I am not sure it makes any practical sense.

The phase inverter tube is a duo triode, that was the part I didn't clarify earlier, and having the triodes balanced does make sense.  Since both halves of the tube are used for the phase inverter function, send the same level of signal to the push and the pull for best results.  I am pretty sure you knew this, but for completeness sake, and I am a bit bored at work, I thought I'd mention it.
Patrick

 

[Watershed]

I completely understand...on both counts.
:laughing8:

So, for nothing else than the sake of further clarification, I'll throw out that all 12AX7's are dual triode.  When used as a phase inverter, the two triodes divide the signal, with one triode output being "in phase", and the other triode output being "out of phase."  The reason being, when the two respective signals are run back into the output transformer, from the power tubes, the are coming into opposite ends of the coil, which ends up putting them out of phase with each other.  So if one signal is already out of phase, the phase issue cancels itself out and everything is in phase with each other.  And life is grand...

p. 2
http://www.schematicheaven.com/boogieamps/boogie_studio_caliber_dc2.pdf

It's Friday.....

 

[Superlizard]

A set of matched triodes is a very good idea for the phase inverter.  That way the push and the pull signals coming out of the PI are equal, or as equal as they were entering.

On paper yes, and makes perfect logic.  But in real-world terms, barring any given PI t00b was manufactured completely off-kilter between its 2 triodes (highly unlikely), doubtful there'd be an audible difference if the PI t00b didn't have precisely matched triodes.

As an aside, and interestingly enough, there are some out there who claim that non-matched powert00bs/PI t00bs are more desirable than matched; due to adding more even/odd harmonics to the tone.  But there is no conclusive proof to this.

Ultimately, I am of the firm opinion that the "audiophool" world has inadvertently seeped into the guitar amp world... leading to many misconceptions.  They are of course similar genres, but with one huge difference:  one strives for the lowest signal distortion possible (thus very specific and low tolerances for error), the other relishes distortion.

 

[Watershed]

Good points.  Again, though, "tone" is so subjective.
Do you want to sound like Joe Pass or Steve Vai?
Poly or nitro?  Ceramic disk, or oil filled caps?

Roughly a bazillion variables out there, that people are ready to throw down over.

The best way to debunk marketing/tone myths (and there are many) is to try them out and get the most important opinion, yours.  Not that you should run out and buy into every myth for the sake of science. 

My impression is that the amp sounds better than it did with the previous ten+ year old beaten tubes. 
That I will stand by.

Is that because the old tubes were beat?
Of course

Do these tubes crush the standard replacement Mesa tubes?
I wouldn't say "crush", but it does sound nice.

Am I loosing sleep over $15?
No

Did I get a "happy ending"?
No

:icon_jokercolor: