Channels 2 and 3 Part 3: Mesa Rectifier Design Concepts

Posted: December 8, 2015 in Mesa Boogie Dual Rectifier, Tube Amps
Tags: , , , , , , , , , , , , , , , , , , ,

We’ve looked at the input and Clean, the voicing and Gain control, and V2.  Now, it is time to examine V3, the coupled cathode follower.  I urge you to read about cathode followers if you do not understand how they work and wish to learn more.

The biggest things for a novice to know are:

  • A cathode follower is usually used to lower impedance to drive a tone stack or an effects loop.
  •  The cathode of the second tube follows the voltage on its grid closely (hence “cathode follower”).
  • The first tube provides amplification of the signal and stability to the cathode follower.
  • When coupled (wired together), the two tubes act like one (mostly).
V3a

V3DRThe amplifying section,  V3a, is almost an exact copy of the input stage at a higher voltage.  This is meant to provide relatively low distortion, high gain, and it drives the cathode follower more efficiently than similar circuits at a lower gain or voltage.

Bypassed frequencies are boosted to 74.3 Volts (37.4 dB) compared to unbypassed frequencies with a gain of 41.5 Volts (32.4 dB).  Using a large 220 k anode resistor reduces the difference between bypassed and unbypassed levels, makes the response of the boost flatter, and provides a benefit to the headroom of the cathode follower.  The boost frequencies range from 95 Hz to 6 kHz.  The total effect of V2a is to reproduce the previous stages in a fashion which is more high-fidelity than what is typically found in guitar amplifier circuits prior to the 1980s.  I will explain why this is.

Headroom, Voltage, and Current

If you recall, at 1 Volt from zero grid voltage, the positive going grid signal begins to be overdriven as the current begins to be limited.  It becomes increasingly compressed and overdriven as the grid voltage approaches zero.  Designs prior to the SLO 100, and subsequently the Recto,  usually used 100 k resistors on the anode of the coupled amplifying stage.  This resistance value, along with lower voltages, causes those cathode followers to be biased very close to zero.  There would always be at least a small amount of overdrive and compression on one side of the signal at all times (I wonder why Fender stopped using them?).

What could be seen as an issue is how the positive side of the grid signal for V3b is the half-cycle which received the cold clipping in V2b.  It has already been greatly distorted and compressed by that stage.

Soldano addressed this by changing the anode resistor from 100 k  to 220 k, which increased the gain and headroom.  Mesa went further and increased the voltage for their cathode follower by 55 volts.

V3 Load Line

V3a load line.

The large anode voltage combined with the large resistor have two obvious benefits: they shift the bias of the stage closer to the middle and provide more gain and headroom.  The follower follows the anode of the coupled stage even more closely by reducing the current difference between the two tubes.  The cathode of V3b is only 4 volts away from its grid and the current being stolen from V3a and given to V3b is minimized by the increases, at only around 1 microamp.

V3b

On the load line, the cathode of V3b is so close to the bias point for V3a,  it isn’t even necessary to make a separate mark for it.  The bias point is at about 1.7 Volts on the grid.  This gives the follower close to .7 Volts of “clean” grid headroom along with 1 Volt of increasingly overdriven headroom, until it is at zero volts.  Negative going signal will have close to 3.3 Volts on the grid until cutoff.

V3b provides no gain increase (it actually decreases the signal a little). It’s there to decrease the impedance of the signal before it hits the upcoming tone stack .  The output impedance of the stage is 620Ω – 680 Ω.  This small impedance makes the losses from the tone stack much less (up to 10dB), when compared to a tone stack without impedance bridging (up to 24 to 30 dB).

Effect

The high gain and large headroom make this stage seem stiff by comparison to amplifiers biased with lower anode resistances.  Just as the input stage seems to be designed as a semi-high-fidelity boost, this stage looks like it serves a similar purpose to boost the product of V2b, while also performing the nominal task of bringing down the output impedance.

Initial attack causes v3 to massively distort.  As the signal decays, both tubes can eventually ease out of overdrive and present a relatively clean reproduction of all of the previous stages, depending on the “Gain” setting.

While overdriven, the warm bias of the follower section will change the V2b distortion characteristics imparted on the signal, as it is affecting the same side of the signal as the cold clipping.  The other half will also be affected, but not to the same extent.

I already have an article about the tone stack, if you wish to read it.  Otherwise, thank you for your time.  I will be writing one more article as an overview with observations and effects of the preamp for Channels 2 and 3 as a whole with less nerd-speak.  Until then, take care.

Advertisements
Comments
  1. […] Channels 2 and 3 Part 3: Mesa Rectifier Design Concepts […]

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s