Here is something not often seen in a guitar amp: a fourtriode phase inverter, as found in the Guild 100J.
Each of the two triodes shown are actually a pair of triodes in parallel, connected platetoplate, gridtogrid, and cathodetocathode.
The 150kΩ plate load resistors carry the plate current of two triodes, so the equivalent value for one triode is double: 300kΩ. The 1.2kΩ and 68kΩ resistors carry the plate current of four triodes, so the equivalent value for one triode is quadruple: 4.8kΩ and 272kΩ, respectively. Here is the equivalent DC circuit for one triode.
If the plate current is 0mA, the platetocathode voltage is equal to the plate supply voltage, so one endpoint of the DC load line is at 360V, 0mA. If the platetocathode voltage is 0V, then according to Ohm's Law the plate current is
360V / (300kΩ + 4.8kΩ + 272kΩ) = 0.62mA
The other endpoint for the load line is therefore at 0V, 0.62mA, as shown here in red.
If the gridtocathode voltage is 1V, 1.5V, or 2V, then according to Ohm's Law the current through the 4.8kΩ resistor is
1V / 4.8kΩ = 0.21mA
1.5V / 4.8kΩ = 0.31mA
2V / 4.8kΩ = 0.42mA
These points define the blue line segments. They intersect the load line at the DC operating point: 1.7V gridtocathode, 155V platetocathode, and 0.36mA plate current. A quick check of Ohm's Law for each of the three resistors validates the voltages shown here, which are for an average 12AX7.
Guild's measured voltages are lower: 220V at the plate and 80V at the cathode.
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In response to an audio signal, when the plate current increases for the inverted phase it decreases for the noninverted phase, so the 95V across the tail ideally stays constant. The AC plate load is the 150kΩ plate load resistor in parallel with the 220kΩ gridleak resistor: 89kΩ. This value is doubled to get the equivalent AC load for one tube: 178kΩ. The AC load line has this slope and passes through the DC operating point, as shown here in green.
Maximum output voltage swing is from 20V to 319V, representing 135V to +164V relative to 155V at the DC operating point. This is some serious swing! The power tubes will be in overdrive long before these limits are reached.
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The gain of the LTP is approximately half the gain of a common cathode voltage amplifier because gridtocathode voltage swing is only about half the input gridtoground voltage swing.^{1} Two triodes in parallel are the equivalent of one triode with double the transconductance and half the plate resistance. The amplification factor is the same. For two 12AX7 triodes in parallel and a typical DC operating point, the equivalent single triode has a transconductance g_{m} = 3.2mS, a plate resistance r_{p} = 31kΩ, and an amplication factor μ = 100.
For plate load resistor R_{L} = 150kΩ and gridleak resistor R_{G} = 220kΩ, the voltage gain when loaded by the grid leak is
The output impedance is
R_{L}r_{p} = 150kΩ31kΩ = 26kΩ
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Let's design a traditional 2triode LTP for the same constraints and compare the results. We assume the plate supply voltage is 360V and the desired voltage across the tail is 95V. The tail voltage is a tradeoff between two performance metrics: a higher tail voltage creates greater balance between phases but less output voltage swing.
The equivalent plate supply voltage for a common cathode voltage amplifier is the voltage difference between the actual plate supply and the tail voltage: 265V. According to the 12AX7 calculator if we set the bias to a warm 1.05V, the cathode resistor value for one triode is 1.235kΩ.
The value for two triodes is half: 620Ω. The DC plate current is 0.85mA for one triode, double for the current through the tail resistor: 1.7mA. According to Ohm's Law, the tail resistor value for a desired voltage drop of 95V is
95V / 1.7mA = 56kΩ
Here is our traditional design for a 360V plate supply and a 95V tail.
The green AC load line indicates that maximum output swing is from 71V to 210V, representing 65 to +74 relative to 136V at the DC operating point.
This is quite a bit less swing than for the 4triode, parallel design. For one triode, the plate resistance is 62kΩ. This makes the voltage gain 29.5 (29.4dB). Output impedance is 44kΩ.
Here is a performance summary.
Guild 100J 
Traditional Design 

output, peaktopeak  299V  139V 
gain  31dB  29dB 
output impedance  26kΩ  44kΩ 
The 2dB increase in gain is not worth an extra tube. The pair of triodes can instead be configured as typical voltage amplifiers for a total loaded gain of 71dB.
The decrease in output impedance impacts the dynamics of overdrive, because the Guild design can drive more grid current. Combined with the 100J's lack of global negative feedback, this should smooth the power amp's transition into overdrive.
The main difference in the two designs, however, is the top line: a substantially greater output voltage swing. Doubling the number of triodes rotates the AC load line counterclockwise to extend the limits of output swing at both ends. The 100J phase inverter drives a pair of 6L6 power tubes cathodebiased at 33V. The LTP plate voltage needs to swing at least 66V peaktopeak to drive the power amp to full power. For that, a traditional design using one dual triode is quite sufficient.
^{1}Richard Kuehnel, Guitar Amplifier Electronics: Basic Theory, (Seattle: Amp Books, 2018), p. 139.
^{2}Richard Kuehnel, Fundamentals of Guitar Amplifier System Design, (Seattle: Amp Books, 2019), pp. 131133.
^{3}Richard Kuehnel, Guitar Amplifier Electronics: Circuit Simulation, (Seattle: Amp Books, 2019), pp. 3337.
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