Tube Choice Depends On Its Application, But What If All Things Are Equal

I had nothing to do Friday, so I started investigating based on an old discussion with an audiophile about tubes and why specific designs are upheld over others. The debate started and ended with the old trope, "My ears are better than your equipment." Since I refuse to argue subjective points, I let the conversation drop. This has bothered me for years; therefore, I will share the results of some testing I have been doing using math and science versus hearing since most adults lose their high-end sensitivity after 30 years of age (science, not voodoo). Here are some Spice Scripts used to simulate electronic components when tested by programs like PSpice, Spice, and Multisim (I will be using Multisim).

The following is the 6n16b (Russian made Tube):

The following is the American-made 12AU7

While laying out these spice scripts, I chose to use typical values to avoid the possibility of "juking" the stats; as some people in some places tell engineers, this would be called "adjusting parameters to produce a desired result instead of an accurate result."

I used a standard tube design separating a Dual Triode (as both of these are) into a Single Triode. This way, you can quickly tell what is going on, and I can explain the circuit without going into interstage dynamics. Because there are none, there is no need for a suppressor grid.

Let's talk about what these values mean:

.subckt starts the simulation of the Vacuum Tube

All tubes have interelement capacitance, so these values are essential in the design of a tube.

CCG is the capacitance between the Cathode and the Grid.

CGP is the capacitance between the Grid and the Plate (Anode)

CCP is the capacitance between the Cathode and the Plate (Anode)

These next set of items are constants unique to the tube's makeup based on the materials used and the placement of the elements.

RGI is the equivalent resistance of the grid to the flow of current in the tube

Mu is the amplification factor of the tube.

EX is the Exponential factor (in math, this is the factor by which things are multiplied) (found on the datasheet)

KG1 is the cathode to grid constant (found on the datasheet)

KP is the cathode to plate constant (found on the datasheet)

KVB is the voltage-related constant (found on the datasheet)

These next items are the equations and components used to set tube parameters:

E1 Defines the voltage source; it calculates the voltage between the grid and anode based on the anode-cathode voltage and the grid-cathode voltage

RE1 Defines the resistance attributed to leakage and is viewed as parasitic

G1 Defines the voltage-controlled current source of the tube and is used in the modeling of the tube's transconductance

C1, C2, and C3 are the interelement capacitance variables

D3 is a diode function caused by the interaction of the heater and the cathode. It represents the cathode and describes the effect of such element interaction.

R1 represents the internal resistance of the grid (variable of RGI)

Next are your model definitions:

.Model DX this defines the diode “D3” it sets values like “Saturation Current = IS, Series Resistance=RS, Junction Capacitance= CJO, and Transit Time=TT)

, ENDS closes the script.

Now you see why I used the same model for both tubes. Instead of using a different model and explaining stuff that does not matter to the tests, I would plug in the proper values and be off and running.

I installed the spice scripts into Multisim and built the tubes using the component wizard. The test runs well and equally within the individual tube manufacturers' parameters. There are some warnings because of using log() instead of int(); the problem was solved by replacing LOG() with LN()which converted log() into int()

The circuit I have chosen is a basic Shunt (series) Regulated Push-Pull Amplifier with Regenerative Feedback, Bypass Capacitors, and Stabilization Resistors. This provides a stable environment for both tubes.

These are the screenshots of the results:

Input and output values:

6n16b:

Filament voltage 12v

Input: 5vP-P

Frequency: 4Khz

Output: 71.9vP-P (avg)

Gain: 14.38

12AU7:

Filament voltage 24v

Input: 5vP-P

Frequency: 4Khz

Output: 94.65vP-P (avg)

Gain: 18.93

The American Tube is about $82.00 max (can get cheaper depending on the seller)

The Russian Tube is about $24.00 max

What does this tell us?

The American tube has a higher gain but costs more. This is expected; however, I have noticed that the circuit's design has more to do with worth than anything else. The 12AU7s are used in one of the most expensive amplifiers I have repaired. The SRPP circuit used was subpar; the circuit was close to an example model of the same circuit. That was the reason I designed my circuit and ran these tests. As far as gain the American tube wins hands down, as far as cost and efficiency the Russian Tube wins hands down.

From what I have seen in these tests, the circuit design makes the difference more so than the tube used because the circuit I designed allows both tubes to be used depending on my needs, with higher gain versus higher efficacy. Also, this shows some credence to noise but nothing to justify spending $90.00 on a foot of connector cable and expensive tube brands that buy you nothing in the long run.

OK, I'll bring this to an end.