Bob Latino vta-120

No worries.

The OP now has a stack of learned tomes taller and wider and deeper than the amplifier.

In no time he'll know the difference between common cathode, common grid, and common plate, and how to calculate Rk, Rp, Rg, and all the rest, and why mu and Gm matter.
 
Yeah, that's more or less true, but gain is very often irrelevant as the issue is rarely solely about gain. Many times the required gain is low enough, typically about 50 in most circuits, that either tube could be used if the issue was purely about gain, and, again, it rarely is this simple. The maximum gain is an obvious difference between the tubes, true, but that often little matters in actual circuits where we have more than enough gain to go around.

As a general rule, the 12AX7 is generally used a high-gain voltage amplifier while the 12AT7 and 12AU7 are generally used as high-gain current amplifiers. The input and output impedance are different, as is the transconductance, and the plate current. The tubes, in short, are different.

A current-amplifier is often as a unity-gain voltage device (ok, it isn't quite unitary, but this is written for pedagogical purposes) to improve current drive. This is why tubes are often seen with grounded grids where current gain is the desired effect.

Just because the 12AX7 has a particular reputation among those who don't understand tube circuitry does not meant that dropping it in willy-nilly improves the circuit. Usually quite the opposite.

Swapping tubes of different types is generally doomed to fail because the tube characteristics are never quite the same and the circuit, including passive component values, was designed for a particular device's properties. Unless you understand the circuit at least as well as the original designer it is dubious that swapping tubes will improve the sound. The same goes for opamps and transistors. Substitutions randomly made without understanding the ramifications can result (at best) a degradation in performance and (at worse) oscillation or damage to the equipment.

It's not about the size of your gain, it's about what you do with it.
Thanks for the clarification. I figured the gain had something to do with it, but there is often more to the story. At the least, the OP should be aware that using a 12AX7 in place of a 12AU7 or 12AT7 isn't going to blow anything up, but it probably won't sound terribly good, either. Cranking the gain to 11 can work for guitar amplifiers, but rarely (if ever) for hi-fi gear. :)
-Adam
 
Thanks for the clarification. I figured the gain had something to do with it, but there is often more to the story. At the least, the OP should be aware that using a 12AX7 in place of a 12AU7 or 12AT7 isn't going to blow anything up, but it probably won't sound terribly good, either.

That is true here, but not necessarily true for all circuits. Many times swapping tubes, transistors, or opamps will destabilize an amplifier and cause oscillation.

In the case of transistors or opamps it's about higher bandwidth, so the amplifier can now easily amplify signals in the high kHz to low MHz range. This is why the homotaxial transistors generally cannot be replaced with faster devices without reducing bandwidth and Q. Slow parts replaced with faster parts often causes problems.

In the case of tubes, the Miller capacitance changes can add or remove filters, the Q may be higher so oscillation or ringing results, or the grid leak may be inadequate so the tube self-biases itself into Valhalla.

Ya gotta know whatcha changin', s'all I'm sayin'.
 
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