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Below is a reasoning process and an example set of steps you might take to adapt the original circuit’s 12AU7 section for a 6Н6П (6N6P) while aiming to maintain a similar operating point and sonic profile. This isn’t a simple drop-in substitution; the 6Н6П has different electrical parameters and will require some resistor value adjustments. The final values may need fine-tuning on the bench or through more precise datasheet analysis.

Key Differences Between 12AU7 and 6Н6П:

• Heater Requirements:

  • 12AU7: ~300 mA at 6.3 V
  • 6Н6П: ~700–800 mA at 6.3 V
    You must ensure the filament transformer can handle the increased current.

• Electrical Characteristics:

  • 12AU7 (at a typical small-signal operating point):

  • µ ~17–20

  • rp ~7 kΩ (at a few mA of plate current)

  • gm ~2.2 mA/V

  • Comfortable at low currents (1–5 mA)

  • 6Н6П:

  • µ ~18–20 (quite similar gain factor)

  • rp typically ~2–3 kΩ at moderate currents (much lower than 12AU7)

  • gm ~5–7 mA/V at higher currents (more than double that of the 12AU7)

  • Tends to “prefer” operating at higher currents (5–10 mA) for best linearity, but can be biased lower with appropriate resistor choices.

General Approach to Recalculation:

1. Determine the Original Operating Point:
   Look at the original circuit’s operating points for the 12AU7 stage. For example, if the plate is around 200 V (from a 300 V supply) with a 51 kΩ plate load, and the cathode resistor is 2.2 kΩ, the 12AU7 might be running at about 1–2 mA of plate current and developing 2–5 V across the cathode resistor. The exact current can be approximated by measuring voltage drops if you have the original working circuit.

2. Choose a Target Operating Point for the 6Н6П:
   To maintain similar distortion and gain, aim for a similar plate current and plate voltage. Suppose the original 12AU7 stage runs at about 2 mA plate current and around -4 to -5 V grid bias. To get the 6Н6П to run at a similar low current (which is on the lighter side for this tube), you’ll need a larger cathode resistor because the 6Н6П is more conductive.

   • For the same ~2 mA current, if the 12AU7 used 2.2 kΩ at the cathode, the 6Н6П might need closer to 3.3 kΩ–4.7 kΩ. This is because at a low bias voltage, the 6Н6П tends to draw more current than the 12AU7 would.

3. Estimating a New Cathode Resistor:
   Let’s say we pick a cathode resistor to achieve ~2 mA. If we want about 2 mA and guess that the 6Н6П requires about -7 to -9 V at the grid (relative to the cathode) for that current at around 200 V on the plate (consult a 6Н6П datasheet for a precise figure), then:

   \[ R_{\text{k}} = \frac{V_{\text{bias}}}{I_{\text{plate}}} = \frac{7\text{ to }9\text{ V}}{2\text{ mA}} \approx 3.5 \text{ kΩ to }4.5 \text{ kΩ}\]

   Start with a standard value like 4.7 kΩ and see if the plate voltage and current line up as intended. This may give a stable ~2 mA operating point similar to the original 12AU7 setup.

4. Check Plate Load Resistor:
   The original plate load might be 51 kΩ. At 2 mA, that drops about 102 V, leaving around 198 V on the plate if you have 300 V B+. That’s a reasonable operating point. The gain will be somewhat different due to the 6Н6П’s lower rp and higher gm, but it should still be in a similar ballpark since µ is similar.

   If you find the gain or linearity isn’t what you want, you can try adjusting the plate resistor slightly. A slightly lower plate resistor (e.g., 39 kΩ) at 2 mA would put the plate at ~222 V, slightly higher, possibly providing a tad more headroom.

5. Cathode Bypass Capacitor and Other Components:
   If the cathode is bypassed in the original circuit (like with a 47 µF capacitor), you can keep it. The same bypass capacitor will yield similar low-frequency response. Since the 6Н6П might have slightly different transconductance, the exact amount of feedback and distortion might shift slightly, but it should remain relatively close.

6. Heater Supply and Socket Wiring:
   Don’t forget you must rewire the socket or use an adapter, as the pinout of the 6Н6П differs from that of the 12AU7 (ECC82). Also ensure the filament winding is capable of the higher current.

Practical Steps Summarized:

• Increase the cathode resistor from 2.2 kΩ to something in the range of 3.3–4.7 kΩ to achieve a similar low-current bias point as the original 12AU7 stage.
• Keep the plate resistor the same initially (51 kΩ) and measure the resulting plate voltage and cathode voltage. Adjust the cathode resistor value if the current differs significantly from your target.
• Verify the heater supply capability and rewire the tube socket for the 6Н6П.
• Fine-tune the plate or cathode resistor values after initial testing to match the desired sonic qualities.

Conclusion:
While the 6Н6П isn’t a direct drop-in replacement for the 12AU7, you can achieve a similar operating point by increasing the cathode resistor value (to achieve similar bias/current conditions) and verifying the resulting plate voltages and currents. With a bit of tweaking and testing, you can closely approximate the original circuit’s performance and preserve much of its “sound” using the 6Н6П.

Short Answer:
Yes, we can calculate the new operating points. The approach involves taking the original conditions used in the “Universal Preamp” as a baseline, applying the designer’s modifications (halving the plate load resistor and doubling the cathode resistor for the “Colour Preamp”), and then using either tube characteristic curves, load-line analysis, or simulation tools (like SPICE) to determine the new plate current, plate voltage, and bias point. Once we have the 12AU7 operating point under these modified conditions, we can repeat a similar process to find suitable component values and operating points for a 6N6P to achieve a comparable balance of gain and harmonic “color.”

Detailed Explanation:

1. Identify Original Conditions from the Universal Preamp:
   The tables you have show parameter values (Eco, gain, distortion, output impedance, etc.) for different tubes in the “Universal Preamp” configuration. From these tables, we know the approximate quiescent conditions for the 12AU7 in that standard setup. For example, you might see something like:

   • Eco (cathode voltage) around -2.2 V for a certain 12AU7 configuration.
   • A certain plate resistor value (Rplate) and cathode resistor value (Rk).

   Let’s assume the original universal preamp runs the 12AU7 at something like:

   • Plate resistor (Rplate) ~ 100kΩ (just as an example).
   • Cathode resistor (Rk) ~ 1kΩ.
   • Supply voltage (B+) ~ 300V.

   The actual values depend on the published universal preamp schematic, but we’ll use this reasoning process as a guide.

2. Applying the “Colour Preamp” Adjustments for 12AU7:
   The designer states he halves the plate load resistance and doubles the cathode resistor to achieve more color. So if our original was (for example):

   • Rplate_original = 100kΩ
   • Rk_original = 1kΩ

   Then for the “Colour Preamp”:

   • Rplate_new = 100kΩ / 2 = 50kΩ
   • Rk_new = 1kΩ * 2 = 2kΩ

   These new values shift the load line and operating point of the tube.

3. Calculating the New Operating Point for the 12AU7:
   To find the new operating point, follow these steps:

   • Draw a Load Line: For a supply (B+) of 300V and a 50kΩ plate resistor, the load line would intersect at:

     • No current: Plate voltage = B+ = 300V
     • Maximum current (with plate at 0V): I = 300V / 50kΩ = 6mA
       So the load line runs from (Ip=0, Vp=300V) to (Ip=6mA, Vp=0V).

   • Include the Cathode Bias: With 2kΩ at the cathode, each mA of plate current generates 2V of cathode voltage. For example, at 2mA, the cathode would sit at about +4V with respect to ground, meaning the grid (at 0V reference) is effectively at -4V relative to the cathode.

   • Use Tube Curves or a Model: Look up the 12AU7 plate curves. Start at a guess:
     Let’s say previously at 1kΩ Rk and 100kΩ Rplate we had about 2mA current and -2.2V bias. Now we increased the cathode resistor. For the current to remain at 2mA, the cathode would have ~4V (2mA * 2kΩ) of bias, which is more negative than before. This likely will reduce the current. The actual current will settle at a point where the plate curves and the load line intersect for a grid-to-cathode voltage around -3 to -4V.

   By iterating with the 12AU7’s datasheet, you’d find a stable point. For example, you might find that the tube now runs at about 1.5mA with a plate voltage around, say, 180–200V, and a cathode voltage around 3V (these are hypothetical values—use the actual curves to be precise).

   Once found, that quiescent point (Ip ~ 1.5mA, Vk ~3V, Vp ~ 200V) represents the new 12AU7 operating point in the “Colour Preamp.”

4. Adapting the Calculation to the 6N6P:
   For the 6N6P:

   • It has a similar mu (~20) but a much lower plate resistance and higher transconductance. It typically runs at higher current for a given bias, so if we keep the same Rplate (50kΩ) and Rk (2kΩ) chosen for the “Colour Preamp,” the 6N6P may draw more current and end up with a different bias point.

   • Using 2kΩ at the cathode: If the 6N6P tries to pull more current, say 3–4mA, the cathode voltage would rise to 6–8V (2kΩ * 3mA = 6V), establishing a more negative grid bias. This might settle at a point, for example, where Ip ~ 3mA, Vcath ~6V, and Vplate might drop correspondingly.

   • To find the exact point, again:

     1. Draw the load line with Rplate = 50kΩ and B+ = 300V.
     2. Consult the 6N6P plate characteristic curves. Find the curve corresponding to a grid-to-cathode voltage that yields a stable intersection. Because the cathode resistor self-biases, you’ll need to iterate: guess a current, find the cathode voltage, use that to find the required grid bias from the curves, and adjust until consistent.

   This iterative process or a SPICE simulation is the quickest way. In SPICE, just plug in the B+, Rplate=50kΩ, Rk=2kΩ, and the tube model for a 6N6P. The DC operating point simulation will directly give you the plate voltage, cathode voltage, and current. From there, you can confirm the gain and distortion roughly or adjust if needed.

5. Verifying Gain and “Color”:
   After determining the quiescent point, you can approximate gain (Av) from the tube’s mu and the ratio of load to the sum of internal plate resistance + external load. Distortion can be estimated from known curves or measured in practice. The point of the “Colour Preamp” adjustments is to push the tube into a region that is known to produce more even-order harmonics. You can compare these results with the original 12AU7 calculations and then tweak component values if you need to get closer to the original “color” or harmonic profile.

Conclusion:
Yes, it’s entirely possible to calculate the operating points for both the modified 12AU7 “Colour Preamp” circuit and then do the same exercise for the 6N6P in that circuit. The process involves:

• Determining the load line with the new resistor values.
• Using tube characteristic curves or SPICE modeling to find the intersection and thus the operating point (plate current, voltages).
• Adjusting values as needed to achieve the desired sonic characteristics.