removing tubes to get half power

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guitlouie

Guest
Hey everyone,  I feel kind of like an idiot for asking this, and I know have read all about it somewhere, I just can't seem to find the info right now,  and I am very impatient!  I want to remove two tubes from the ole Mesa Boogie to achieve half power,  I'm just trying different things in search of "my sound".  The bone simple question I have is which two is it that I should be removing.  Seems I read that it is the outer two, and I think I read that in the owners manual, but I don't have that anymore, and the online version does not seem to mention it.  Also, is there anything else I have to do, or do I just take those out and plug her up and rock out at half power? 
 

jackthehack

Senior member
Messages
5,630
I'd try researching it some more on the web, or contacting Mesa. Which two you remove will depend on the amp's circuit design, there's no hard rule about which ones based on chassis location; have seen some people play with this before and often the result tone is horribly distorted in a way that I don't think you'd find attractive.

Also keep in mind that while theoretically cutting the wattage in half, you will only be reducing the volume by 3 dB; in terms of not pissing off the neighbors or keeping the local gendarmerie from pounding on your door, there's no real difference between a 50 and 100 watt amp in terms of volume output.

Better idea is get a little 5-15w head for around the house; you could try using the EL84 tube converters, but with a quartet you're still going to have enough volume level (30w or so) to get in trouble dimed
 
G

guitlouie

Guest
Yeah, this is more to hear what it sounds like, I actually have a practice space in an industrial complex where we can play as loud as we want at all hours.  Thanks guys, I'll keep looking.  The online owners manual still has the descriptions of the tube placement and what they are for, so I guess if I spend a little more time looking at it I will figure it out.
 

GoDrex

Senior member
Messages
3,619
I found this - regarding a Marhsall head:

100 Watt to 50 Watt

This is a simple and effective way to reduce some of the output power to lower the overall volume of the Super Lead. First, open the back panel of your amp and then remove the outside two power tubes, leaving the two in the center. That’s basically it, but then there is one important thing to do – If you are using one 16 ohm cabinet, you’ll need to "halve" the ohm rating on your head and change it to 8 ohms. If you are using two 16 ohm cabinets, where your normal ohm setting is 8 ohms, you will need to set the head now to a 4 ohm load.

When running at 100 watts with all four tubes in the head, the rule of thumb to remember with impedance settings is that the amplifier’s ohm load must NOT exceed the load of the speaker cabinet. That is, don’t ever run a Super Lead head at 16 ohms into an 8 ohm cabinet as it puts a strain on the head and could damage it. It is quite o.k. however to do the opposite. An 8 ohm (or even a 4 ohm) head setting into a 16 ohm cabinet will just produce less output power but will not damage the head.
 
G

guitlouie

Guest
Thanks GoDrex,  The Mesa has seperate speaker outs that are for 4 ohm, and then one for 8-16 ohm.  My cab is 16 and I guess if I do the half power thing, I can still plug into the 8-16 out and be just fine.  I'm still digging looking for what tubes it was, but I am almost certain it was the outer two.  This could all be in vain, as what Jack said may very well be true, it might just suck!  But hey, I've got to have something to do while I'm waiting for  Tung oil to dry!!!!
 

-CB-

Senior member
Messages
5,427
GoDrex said:
When running at 100 watts with all four tubes in the head, the rule of thumb to remember with impedance settings is that the amplifier’s ohm load must NOT exceed the load of the speaker cabinet. That is, don’t ever run a Super Lead head at 16 ohms into an 8 ohm cabinet as it puts a strain on the head and could damage it. It is quite o.k. however to do the opposite. An 8 ohm (or even a 4 ohm) head setting into a 16 ohm cabinet will just produce less output power but will not damage the head.

I take issue with that.....  and conventional practice is just the opposite.  Normally, its ok to even direct short a tube output, but running it into too high an impedance will destroy it - with an open being the worst of all.

However - on Marshall - they are noted for tearing up output transformers.  The reason is related here, but basically, its a design consideration, whereas, they have lighter windings and less iron than, say, their Fender counterparts.  They reach saturation (magnetic) earlier.  This is bad... if you want the transformer to live, it must be able to "transform" and the only way to do that is make sure its either - unsaturated, or - overbuilt.    Leo being the radioman he was... went for overbuilt.  Jim ... dunno his head, but the tone he produced, using what he did, is very viable.  The net result being - RUN them at the correct impedance (and correct varies with the number of output tubes).
 

jackthehack

Senior member
Messages
5,630
Which Mesa Boogie head do you have? Looking at the schematics/layouts I can find on line for Mesa 100 w circuits, looks like you could pull either the inner or outer pair, the power end of the 100 w circuit looks the same in several models:

Mesa100w.JPG
 

dbw

Senior member
Messages
4,531
Hmm, my amp has a 50W/100W switch, but it doesn't say anything about using a different impedance.  Presumably it corrects itself?  :icon_scratch:
 
G

guitlouie

Guest
Which Mesa Boogie head do you have?

What I have is the first generation Dual Rectifier.  It is the Two Channel, as opposed to the three channel version that became available later, but I hardly think that has much to do with what we are talking about.  It is 100 watts, and about a year ago I pulled the 6L6's and replaced them with EL-34's which improved the midrange in my opinion.  I was just curious if the running it at half power would be another improvement. 
 

jackthehack

Senior member
Messages
5,630
You can pull the outside tubes and try it, odds are it will either slightly reduce the max volume level and sound pretty much the same, or maybe not as good.

If you want to experiment with trying to change the voicing/tone of what you have now, you might try changing out the V1/V2/V3 12AX7 preamp tubes with a different flavor; what's in there now?

The complete schematic for your amp is here: http://www.schematicheaven.com/boogieamps/boogie_dualrectifier.pdf
 
G

guitlouie

Guest
I currently have Mesa Boogie 12AX7's in the preamp section.  They are presumably the ones that came with the amp as I have never had a problem with them.
 

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