How to Bias a Fixed‑Bias Push‑Pull Tube Power Amp

‍4. Step 1 – Measure OT Primary DC Resistance (Amp Off)

Do this with no power applied.

1. Unplug the amp.

1. Discharge the filter capacitors with a resistor and confirm with your DMM that the B+ node has fallen to a safe voltage.
2. Identify the output transformer primary wires:
   • CT: usually goes from the OT to the first B+ filter cap.
   • Two plate leads: go from the OT to the power tube plates (often pin 3 on octal sockets).

1. Set the DMM to resistance (ohms).

1. Measure from CT to Plate Lead A; call this:

\[R_A\]

6. Measure from CT to Plate Lead B; call this:

\[ R_B \]

A small difference between \( R_A \) and \( R_B \) is normal because the inner and outer halves of the winding use different lengths of wire.

Example values:

\[ R_A = 70 \ \Omega \]

\[ R_B = 72 \ \Omega \]

You will use these resistances when calculating current.

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5. Step 2 – Measure Voltage Drops from CT to Plates (Amp On)

Now you will power the amp and measure the DC voltage drop across each half of the OT primary.

1. Connect a speaker or dummy load.

1. Make sure all power tubes are installed.

1. Power up the amp and let it warm up for at least 5 minutes so the idle current stabilizes.

1. Set the DMM to a suitable DC voltage range (for example, 600 VDC).

Because multimeters are not perfect and the HT voltage can bounce slightly, the most reliable way to get the voltage drop is to measure it directly between the center tap and each plate lead, rather than measuring both to ground and subtracting.

For side A:

• Clip the black probe to the OT center tap (B+).

• Touch the red probe to the primary lead that feeds the plates on side A.

• The meter reads the direct voltage drop across that half of the primary, which we call:

\[ V_{\text{dropA}} \]

For side B:

• Leave the black probe on the CT.

• Move the red probe to the plate lead on side B.

• This reading is the drop across the other half:

\[ V_{\text{dropB}} \]

These are the actual DC drops across each half of the primary at idle, with no subtraction error. This improves accuracy, especially when B+ moves a little as you adjust bias.

It is also useful to note plate voltages to ground for later dissipation calculations:

\[ V_{PA} \]

\[ V_{PB} \]

6. Step 3 – Calculate Current in Each Half (Ohm’s Law)

Ohm’s law in this context is:

\[ I = \frac{V}{R} \]

So the total idle current in each half of the OT primary is:

\[ I_A = \frac{V_{\text{dropA}}}{R_A} \]

\[ I_B = \frac{V_{\text{dropB}}}{R_B} \]

Here:

• \( I_A \) is the total current drawn by all tubes on side A

• \( I_B \) is the total current for all tubes on side B

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Two‑Tube (50 W) Example

For a 2‑tube push‑pull amp (one tube per side), suppose:

\[ R_A = 70 \ \Omega, \quad R_B = 72 \ \Omega \]

and you measure:

\[ V_{\text{dropA}} = 5 \ \text{V}, \quad V_{\text{dropB}} = 4 \ \text{V} \]

Then:

\[ I_A = \frac{5 \ \text{V}}{70 \ \Omega} \approx 0.071 \ \text{A} = 71 \ \text{mA} \]

\[ I_B = \frac{4 \ \text{V}}{72 \ \Omega} \approx 0.056 \ \text{A} = 56 \ \text{mA} \]

In a 2‑tube amp, these are the idle currents of each tube (ignoring small screen current).

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7. Step 4 – Current Per Tube (2, 4, or 6 Tubes)

How you divide that current depends on how many tubes share each half of the primary.

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Case A: Two Tubes (Typical 50 W)

One tube on each side:

\[ I_{\text{tubeA}} = I_A \]

\[ I_{\text{tubeB}} = I_B \]

So each tube’s current is exactly the side current.

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Case B: Four Tubes (Classic 100 W)

Two tubes in parallel on each side.

Assuming the tubes on a side are reasonably matched, you can approximate per‑tube current as:

\[ I_{\text{tubeA}} \approx \frac{I_A}{2} \]

\[ I_{\text{tubeB}} \approx \frac{I_B}{2} \]

Example:

\[ I_A = 120 \ \text{mA}, \quad I_B = 116 \ \text{mA} \]

Then:

\[ I_{\text{tubeA}} \approx \frac{120 \ \text{mA}}{2} = 60 \ \text{mA} \]

\[ I_{\text{tubeB}} \approx \frac{116 \ \text{mA}}{2} = 58 \ \text{mA} \]

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Case C: Six Tubes (Triple‑Tube Output)

Three tubes on each side.

If tubes are reasonably matched:

\[ I_{\text{tubeA}} \approx \frac{I_A}{3} \]

\[ I_{\text{tubeB}} \approx \frac{I_B}{3} \]

A big difference between the per‑tube currents on each side suggests tube mismatch or bias‑balance issues.

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8. Step 5 – Calculate Plate Dissipation

Tube datasheets give maximum plate dissipation \( P_{\text{max}} \) in watts (for example, EL34 ≈ 25 W, 6L6GC ≈ 30 W).

Plate dissipation at idle is:

\[ P = V_{\text{plate}} \times I_{\text{plate}} \]

For biasing, it’s common to approximate plate current as the measured current from this method, knowing that a small portion actually goes to the screen.

For each tube:

\[ P_{\text{tube}} \approx V_{\text{plate}} \times I_{\text{tube}} \]

If side A has plate‑to‑ground voltage \( V_{PA} \):

\[ P_{\text{tubeA}} \approx V_{PA} \times I_{\text{tubeA}} \]

Similarly, for side B with plate voltage \( V_{PB} \):

\[ P_{\text{tubeB}} \approx V_{PB} \times I_{\text{tubeB}} \]

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Choosing a Target Percentage of Max

For fixed‑bias Class AB guitar amps, a common rule is to idle around 60–70% of maximum plate dissipation:

\[ P_{\text{target}} \approx 0.6 \text{ to } 0.7 \times P_{\text{max}} \]

Given \( V_{\text{plate}} \), the corresponding target current is:

\[ I_{\text{target}} = \frac{P_{\text{target}}}{V_{\text{plate}}} \]

Example for a 6L6GC (\( P_{\text{max}} \approx 30 \ \text{W} \)) at 450 V and 70%:

\[ P_{\text{target}} = 0.7 \times 30 \ \text{W} = 21 \ \text{W} \]

\[ I_{\text{target}} = \frac{21 \ \text{W}}{450 \ \text{V}} \approx 0.047 \ \text{A} = 47 \ \text{mA} \]

Biasing cooler (50–60%) increases tube life; biasing hotter (toward 70%) reduces crossover distortion but shortens tube life.

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9. Step 6 – Adjust Bias and Re‑Check HT

Now you adjust the bias pot and iterate measurements.

1. Locate the bias trimmer and confirm its range by measuring the negative grid voltage on the power tube control grids (often between about \(-30 \ \text{V}\) and \(-60 \ \text{V}\), depending on tube type and plate voltage).
2. Turn the bias pot a small amount, then re‑measure:
   • \( V_{\text{dropA}} \) and \( V_{\text{dropB}} \)
   • Recalculate \( I_A \) and \( I_B \)
   • Recalculate per‑tube currents
   • Note the plate voltages \( V_{PA} \) and \( V_{PB} \)
   • Recalculate \( P_{\text{tube}} \)

As you bias hotter (higher idle current), the HT / B+ voltage usually drops because the supply sags under load. When you bias cooler, HT tends to rise.

Because both \( V_{\text{plate}} \) and \( I_{\text{tube}} \) change, you must:

• Re‑measure all relevant voltages after each meaningful adjustment

• Recalculate current and dissipation until you land at your chosen percentage of \( P_{\text{max}} \)

Let the amp idle for several more minutes and confirm that the bias stays in range.

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10. Why This Method Is Accurate and Safer Than Shunt

Accuracy

• It measures real DC current in each half of the OT primary by combining the exact winding resistance with the directly measured DC voltage drop.

• It automatically includes your specific transformer, tubes, and power supply behavior, rather than assuming ideal values.

• In terms of idle‑current accuracy, it is only surpassed by the oscilloscope method, where you set bias under load by watching clipping symmetry and crossover distortion.

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Safety Compared to the Shunt Method

The old “shunt” method puts the meter in series between B+ and a plate lead:

• The meter and probes carry the full plate current at high voltage.

• A slip can short B+ to ground or another node, potentially damaging the amp and risking shock.

• A wrong meter setting (for example, left on volts instead of amps) can be disastrous.

The transformer‑resistance method:

• Uses the meter in voltage mode only when the amp is live.

• Uses ohms mode only when the amp is off and discharged.

• Never requires breaking the plate lead or inserting the meter directly into the high‑voltage current path.

For fixed‑bias guitar amps, this balance of accuracy and safety makes it one of the best everyday bias techniques.

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11. Process Summary

To bias a fixed‑bias push‑pull amp using the output‑transformer resistance method:

1. Measure OT primary resistance CT‑to‑each‑end: \( R_A \), \( R_B \) (amp off).
2. Measure voltage drops CT‑to‑each‑plate: \( V_{\text{dropA}} \), \( V_{\text{dropB}} \) (amp on).
3. Compute side currents:
   \[ I_A = \frac{V_{\text{dropA}}}{R_A}, \quad I_B = \frac{V_{\text{dropB}}}{R_B} \]
4. Derive per‑tube currents for 2, 4, or 6 tubes using division by 1, 2, or 3 as appropriate.
5. Compute plate dissipation:
   \[ P_{\text{tube}} \approx V_{\text{plate}} \times I_{\text{tube}} \]
   and aim for about 60–70% of \( P_{\text{max}} \) at idle for fixed‑bias Class AB.‍
6. Adjust the bias pot, then re‑measure and recalc because HT voltage shifts with idle current, and iterate until you hit your target.
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Hope you enjoyed the ride. Thanks for reading, and stay safe on the bench.‍
Marko, Slightly Technical

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