The 5751 has a mu of 70 whereas the 12AX7A has a mu of 100. A 12AT7 has a mu of 60, but it can source more current than a 12AX7A. That is why is one see's 12AT7's used in driver circuits. A 12AT7 is not as affected by the loading effects of the circuitry that follows it because it can source more current.
For those who do understand the word transconductance, a transconductance device uses a voltage to control current flow in much the same way that valve controls water flow through a faucet. That is why the Brits call vacuum tubes valves. A tube is biased with direct current, which sets the base amount current that flows through the tube at idle. The signal coming off of a guitar is AC, not DC, which is why using DC resistance as a measure of pickup output is not very accurate. The DC bias point is zero volts for the AC signal. The AC signal from one's guitar is fed through the control grid and adds to or subtracts from the bias voltage. This action causes the current flow from the tube to oscillate with respect to the incoming AC voltage, which, in turn, works against the plate resistor, to cause an oscillating voltage increase or voltage drop. You have probably heard the term "voicing" used when referring to amp circuits. That is because the different stages of an amp are AC coupled using RC (resistor/capacitor circuits). Capacitors block DC, but allow AC to flow. What this action does is strip away the DC voltage from a gain stage output while letting the original AC circuit pass through only larger and 180 degree out of phase. The gain of a tube circuit sets how much larger it can make the incoming AC signal. The RC time constant sets the frequency above which the AC signal is allow to flow unrestricted.
A little understood major difference between amps that are derived from the 5F6-A Tweed Bassman circuit (Marshall and basically every modern hard rock and metal tube amp) and the AB763 circuit that underlies the Blackface Deluxe Reverb, Twin Reverb, and Super Reverb and the amps that are derived from this circuit (Mesa's Mark Series, Mark Series derivatives, and Dumble amps as well as many modern boutique amps and clones) is that the the 5F6-A has a gain stage known as a cathode follower. A cathode follower is responsible for much of the crunch tone in the 5F6-A as well as its derivatives. A cathode follower is a gain stage with a gain of 1 (a.k.a. unity gain). What it does is buffer (firewall) the gain generated in the previous stages from the loading effects of the tone stack (a.k.a. the tone controls) because it is a DC-coupled current source, not voltage source. That is why any 5F6-A-based amp has a more ballsy crunch than any AB763-based amp for any given number of gain stages. The tone stack loads the heck out of the first gain stage in an AB763-based amp because a 12AX7A was not designed to drive a high impedance tone stack directly. That is part of the Blackface sound. It is why even high gain circuits like the Mesa Marks and their derivatives do not crunch like a Marshall. Marshall owes the crunch that is so familiar with their amps to the cathode follower found in the 5F6-A circuit and so does the Soldano, the 5150, the Recto (and all of the copycats of these amps). The latter amps have higher gain preamps than the 5F6-A. However, they place the master volume post-cathode follower in order to take advantage of overdriving the cathode follower at lower volumes like the 2203 and 2204, which are basically a four-hole 100W and 50W Marshalls with the two gain stages (channels) wired in series instead of parallel. The 50W Marshall is just a modified 5F6-A Tweed Bassman and the 100W Marshall is just a modified 5F6-A Tweed Bassman with double the amount of power tubes. The JTM 45 is basically a 5F6-A Tweed Bassman built with British versions of the 5881; namely, the KT66 and a cheap output transformer design that Jim Marshall and company picked up on the surplus market. That is not a slant against Marshall. Fender was very cheap as well. Both companies were in business to make money.
