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Generating Output Tube Distortion


Steve Kennedy (admin)
Posted on Tuesday, May 10, 2005 - 02:09 pm:   

I get questioned about this fairly often, so I thought I would post an explanation about how to achieve the "good" kind of distortion that the standard Music Man models are capable of.

The 65 series and HD130 series of amps are virtually identical in basic circuit topology (the same is true for the later 75/150 series). The HD130/150 has 4 output tubes and slightly heavier duty transformers but the rest of the amp circuitry is essentially the same as their lower-power bretheren within the same model and age brackets.

The older models of the 65/130 family made before 1977/78 had a 12AX7 tube acting as a phase splitter/driver for the output tubes. These models sound warmer at overload than the later models with the solid-state phase splitter circuit.

However, the later models were more reliable as it was found that under certain circumstances, you could wipe-out a set of output tubes and the output transformer if the 12AX7 shorted and the amp would quickly "run-away" thermally and lead to a "melt-down" of the output stage (Think "Chernobyl" or "3-Mile Island").

To get the best overload tone out of a Music Man amp you do NOT want to turn everything up to 10! If you turn the input Gain/Volume control up to 10 you will be generating a lot of solid-state preamp distortion (which isn't as pleasing in my opinion). Turn your Volume control up to "10" and your Master Volume way down and you will hear what the preamp distortion sounds like by itself. What you are looking for is output tube distortion WITHOUT adding much preamp distortion (a little bit will add some "bite").

What you DO want to is first turn the channel volume control down to "0", set all your tone controls to "5" and set your Master volume control to "10". This provides unrestricted drive to the output tubes so that they will overload FIRST before the front-end preamp.

Now, play your guitar (Volume on "10" on the guitar) while increasing the input volume control a little at a time. It will start out clean and somewhere along its rotation (depending on how how much signal you are feeding the amp) you will notice that the signal is starting to sound compressed with just a little distortion. Any increase in input volume just increases the output tube distortion until you reach the level where the preamp starts to overload and starts to introduce a new and potentially unwanted characteristic to the sound.

The trick is KNOW (with your guitar and set-up) just what setting on the volume control you can achieve the start of output tube distortion and then then start of unwanted preamp distortion (a larger number on the knob). A Volume control setting anywhere between these two points will give you ONLY (or mainly) output tube distortion, that "singing" compressed type of distortion you are probably looking for. Of course, once you know what the distortion sounds like, adjust the Tone controls to your taste.

It won't sound crunchy like a high-gain Marshall, but it might remind you of a Fender! If you set the maximum output tube distortion setting while your guitar is set on "10", then you can control the distortion of the amp by using the guitar's Volume control. Turning it all the way up gives you maximum tube distortion (uncolored by preamp distortion) and turning it down will take you all the way to clean or anywhere in between.

Unfortunately, it will be loud. However I like the 65 series BETTER for this because you can get nearly the same sound at about 35 watts by running the amp in Low Power mode. This will be less stressful to the output stage of the amp.

I showed up at an "open mic" night at a club a few years ago with just my Stratocaster and my MM 410-65. I found that I needed the 65 watt setting to cut through the rest of the 5-piece band during solos, but I used no effects and only the guitar and amp to good effect, set up just the way described above.


(Message edited by admin on May 10, 2005)