The pull function on a Mark III's Bass knob...

[benjamin801]

...what exactly does it do? The manual doesn't say, which is odd, because all the other pull knob functions are explained.

I've owned two Mark IIIs and put in thousands of hours on them, and all I know for sure is that I prefer it pushed in. :lol:

 

[Neptical]

In my Mark III manual it says:

Bass- This is a wide ranging control that is very easy to use. Most players prefer to operate the Bass in the 0 to 4 range, "pulled out for single coil and pushed in for humbucking pickups."

 

[gummx97]

I pull my bass knob and also pull the master volume to engage the DEEP function. I use a combo so that bass response is not as comparable to a sealed cab...

 

[chipaudette]

The pull switch on the Bass knob adds an additional bypass capacitor to the cathode of the first tube stage. The effect is to change the amount of bass passing through that first stage. Pulled gives more bass, Pushed gives less. The first tube stage is shared by all 3 channels, so it will affect the sound of all three channels.

With the knob pulled out, there is a total of about 15 uF of capacitance on the cathode. This allows all audible frequencies (highpass -3 dB cutoff is ~15 Hz) to pass through with similar gain. By comparison, with the knob pushed in, there is only 0.5 uF of capacitance on the cathode. As a result, the highpass cutoff (-3 dB) moves up to around 350 Hz, with progressively more attenuation down to -6 dB by the time you reach the low E string at 82 Hz.

So, with the knob pushed in, the first stage of your amp attenuates the lower frequencies of your guitar by 0 to 6 dB. This helps to keep your amp sounding "tight", even with high gain. With the knob pulled out, it brings those bass frequencies back (no attenuation), which gives you the perception that you've just added a lot of bass. This can sound really good and full when playing clean, but can made the high-gain channel sound muddy and loose.

For comparison, your classic Fender tube amp (from which the Mark series derives much of its low gain circuitry) uses a large cap like 15 uF in the first gain stage. It has no switch -- it is always there. As a result, the classic Fenders have that reputation for a great full clean sound. Your classic Marshall tube amp has a reputation for sounding thin when clean. The JCM800 (2204) used a 0.68 uF cap on the first stage, which (among many other circuit differences) thinned the sound. This made it OK for clean, but let it have a nice and tight crunch sound that became a classic.

The Mark series amps, because of the pull shift knob on the bass, gives the user the choice of how they want the amp to sound and feel in this regard.

Chip

 

[benjamin801]

Thanks to all who replied, particularly Chip; I have the manual, but was hoping for an electronic explanation of what happens when the knob is pulled. That was perfect.

And since I play a humbucker-equipped guitar, and live on the lead channel 99% of the time in my current band, it's no surprise I like it pushed in... :idea:

 

[chipaudette]

An interesting thing to do with humbucker guitars is to pull the bass knob and then turn the lead channel gain way way down. Like 2-3 on the lead drive. With my guitar and my amp (IIC+, not III), I'm talking settings like:

Vol1: 6-7 ("pull bright" is to taste)
Treb: 6-7 pulled
Bass: 2-3 pulled
Mid: 6...you need plenty of mids for this experiment
Master: dial to taste...pull deep (or not) is also to taste...I vote no.
Lead Drive: 2-3...yes, keep it really low for this experiment
Lead Master: to taste...as for "pull bright", I'd probably probably vote yes, though try it both ways

Switch to neck humbucker and start playing some sweet, spongey, soul-full leads. Mmm. Good.

With the bass and gain settings like this, you get a really different quality of saturation out of the lead channel. Sure, the gain knob is really low, but with the bass pulled and with the mids turned up, you've got lots of signal making it through the early part of the pre-amp. As a result (on my guitar at least) you get this really nice touch-sensitive saturation with way less of the fizzy buzz. It's a really cool feeling.

Now I'm not saying that it should become your main lead tone or anything. No, no. It's just a different flavor that's worth tasting. In contrast, a lot of people think that using the pull-Bass is a no-no with the lead channel. I'm just here saying that there is some cool stuff to be found if you turn down the gain so that you give that deep bass some space to work. Sweet lovin' can result.

Chip

 

[benjamin801]

I will definitely monkey around with that! I'm a big fan of hitting the neck pickup for leads, anyway. And if I find a great, fat, buttery lead tone there, I can always dial it up in the studio for a change of pace, since it wouldn't mesh with my live rhythm settings. (I tend to think of, and treat, the Mark III as the world's most versatile one channel amp.)

 

[Rhoads]

Thanks Chip for the explanation of the bass capacitor ! but i use with the knob pulled out with my Humbucker ! it's not really muddy to my ears. But, i use a lot presence in my sound.

 

[Neptical]

Thanks for the tip, Chip. 8)

That's an interesting setting, no doubt. Wouldn't work well with my live settings, but for a change up in the studio, that would work pretty good. There's so many different variations on settings with these amps it's nuts. Adding in the delays,chorus,tremolo, and flange in the loop just takes it to a whole other level.

 

[jpdennis]

As always, thanks Chip. I am bookmarking this so I can come back to it next week when I get through this week's gigs.

Dennis

 

[gts]

Pretty funny coincidence Chip. Especially considering we have the same amp.
These have pretty much been my settings for a few years on my 60/100 C+ (and like you I use a humbucker equipped guitar).
But I had no idea why I like them other than listening. Your explanation puts some light on the "why".

An interesting thing to do with humbucker guitars is to pull the bass knob and then turn the lead channel gain way way down. Like 2-3 on the lead drive. With my guitar and my amp (IIC+, not III), I'm talking settings like:

Vol1: 6-7 ("pull bright" is to taste)
Treb: 6-7 pulled
Bass: 2-3 pulled
Mid: 6...you need plenty of mids for this experiment
Master: dial to taste...pull deep (or not) is also to taste...I vote no.
Lead Drive: 2-3...yes, keep it really low for this experiment
Lead Master: to taste...as for "pull bright", I'd probably probably vote yes, though try it both ways

Switch to neck humbucker and start playing some sweet, spongey, soul-full leads. Mmm. Good.

With the bass and gain settings like this, you get a really different quality of saturation out of the lead channel. Sure, the gain knob is really low, but with the bass pulled and with the mids turned up, you've got lots of signal making it through the early part of the pre-amp. As a result (on my guitar at least) you get this really nice touch-sensitive saturation with way less of the fizzy buzz. It's a really cool feeling.

Now I'm not saying that it should become your main lead tone or anything. No, no. It's just a different flavor that's worth tasting. In contrast, a lot of people think that using the pull-Bass is a no-no with the lead channel. I'm just here saying that there is some cool stuff to be found if you turn down the gain so that you give that deep bass some space to work. Sweet lovin' can result.

Chip