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Thread: Here's why the 2-Channel "H" is the deal of the century

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    deus ex machina
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    Here's why the 2-Channel "H" is the deal of the century

    Here are a couple of chassis shots that I recently took of my 2-Channel "H:"





    It takes an amazing amount of "in chassis" handwork to build a 2-Channel compared to a modern production amp like those manufactured by Mesa Engineering. All of the pots, jacks, switches, lamps, and tube sockets have to be physically mounted to the chassis by hand. The boards are hand-populated outside of the chassis before being installed and hand-wired to the pots, jacks, switches, and tube sockets with flying leads. The the leads on the components that are directly attached to tube sockets, pots, and jacks are bent by hand and hand-soldered in place. Trust me; it takes a skilled person to build this amp because it would squeal like a pig if built by a low-skill employee.

    The red and black jacks in the back are bias test points. The potentiometer mounted below the test points is the bias pot (Mesa does not even include a bias pot on their amps on their amps, which means that one is stuck using Mesa tubes or modifying the bias supply if one does not wish to risk a possible over-dissipation-related failure). The little cylindrical-shaped components that are wired between the black jack and the two red jacks are 1-Ohm resistors. What these resistors do is convert what would normally be a current reading to a voltage reading. Unlike power transistors, power tubes operate at high DC voltages (hundreds of volts) and low DC currents (millamps). When we draw current through a resistor, we develop a voltage drop that can be measured. This voltage drop is equal to the current being drawn through the resistor multiplied by the resistor's resistance value. Because the resistors have a value of 1-Ohm, the voltage reading in millivolts (a millivolt is equal to 10-3 volts) is equal to the current drawn in milliamps (a milliamp is equal to 10-3 amps). The reason why Doug and most amp designers who provide test points use the "voltage = current" test method is because it allows a service technician to check the amount of quiescent current drawn by the amp without having to break the cathode-to-ground connection. Normally, a service technician would have to insert his/her meter in series with each power tube cathode and signal ground to able to check the amount quiescent current that is being drawn by each power tube.

    If one looks on the right-hand side of the second photo, one sees a bunch of wires that are bolted to the chassis. These wires are signal grounds. This grounding technique is known as "star grounding." Star grounding keeps the heavy loads imposed on the circuit by the power stage from flowing through the preamp signal grounds. If power stage current is allowed to flow through the preamp signal grounds, it can modulate the low-level stages in the preamp, resulting in hum or noise being injected into the preamp. It also greatly reduces or completely eliminates the possibility that a ground loop will creep into a design. A ground loop occurs when a circuit is grounded in more than one place. Ground loops are also a source of hum.


    I paid $1.3K for my 2-Channel head at Experience last year (yes, that was a show-only price). Here's what one gets from Mesa Engineering for $1.3K:

    Mesa Express 5:25



    I know that I tend to target Mesa Engineering more than other amp companies. However, Mesa is held out as the gold standard of the amp world, so they are fair game when comparing build quality. There is absolutely no comparison between the 5:25 and the 2-Channel “H” from a build point of view. The 5:25 is designed to be assembled by low-level workers from a collection of PCBs that are mostly populated by pick-and-place machines. A few components are placed by hand before each PCB is sent through a wave soldering machine.

    Here's a video that documents the Mesa PCB build process:




    Mesa has not always used this type of construction. The Mark Series amps required much greater skill to build.

    Mesa Mark III



    As one can see, the Mark III, while still PCB-based, is not an everything on a PCB amp. All of the critical components are chassis mounted with flying leads. The Mark III required skill to build (this model was built when "Handbuilt in Petaluma" was more than just a slogan).

    In closing, anyone who is attending Experience this year should plan to set aside a few minutes to meet with Doug and his team. The amp shop may not be as sexy as the rest of the factory, but these guys are turning out a quality product for a more than fair price when one considers that PRS amps are truly hand-built. Yes, the other models are more expensive, but that’s because many of the other amps use ridiculously expensive components. I guarantee that anyone who visits the PRS amp shop after viewing the video that I linked above will come away with a new appreciation for the Maryland-made PRS amps.

    Note: There have been many revisions to the 2-Channel “H” to reduce noise and/or improve performance. My amp is a first generation pre-phase inverter master volume 2-Channel "H" (a.k.a. Rev. B). Earlier 2-Channel "H" amps had a master volume that was located after the phase inverter. For those who are curious, a phase inverter is a circuit that takes a signal as input and outputs an in-phase signal plus a signal that is 180-degrees out of phase with the input signal (i.e., a mirror image of the input signal). A phase inverter is necessary when coupling a preamplifier to a push-pull power stage. A phase inverter may or may not provide signal gain. A post-phase inverter master volume takes advantage of signal gain within a phase inverter circuit (usually a circuit known as a “Schmitt splitter”) to add an additional gain stage that can be overdriven before the master volume. Phase inverter distortion is a big component of the classic “dimed” Marshall tone.
    Last edited by Em7; 09-12-2012 at 11:05 AM. Reason: clarification

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