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Pulling two tubes from a 1959 circuit, such as in a Marshall 1959 Super Lead or similar vintage tube amplifier, is a common modification to reduce output power, typically halving it (e.g., from 100 watts to 50 watts). Below, I’ll outline the **method**, **safety concerns**, and **technical details** based on the amplifier’s design and general principles of tube circuits, with specific reference to the Marshall 1959 and related discussions.
---
### **Method for Pulling Two Tubes from a 1959 Circuit**
1. **Identify the Tubes to Remove**:
- The Marshall 1959 Super Lead typically uses four power tubes (e.g., EL34s) in a push-pull configuration, with two tubes per side of the output transformer (OT).
- To reduce power, remove one pair of tubes (one from each side of the OT) to maintain balance. For a quad of tubes (V1, V2, V3, V4), remove either V1 and V3 or V2 and V4, ensuring one tube remains on each half of the OT. Consult the amplifier’s schematic to confirm which tubes are paired.
2. **Adjust Speaker Impedance**:
- When removing two tubes, the impedance seen by the remaining tubes doubles because the output tubes’ impedance is effectively halved. For example:
- If the amplifier is set to a 16-ohm speaker load with four tubes, switch to an 8-ohm speaker load after removing two tubes to match the new impedance.
- This adjustment is critical to prevent damage to the output transformer and ensure efficient power transfer.
3. **Rebias the Amplifier**:
- After removing tubes, the bias voltage may need adjustment to prevent overcurrent or thermal runaway in the remaining tubes. For a Marshall 1959, the recommended bias current for EL34 tubes is typically 33–35 mA per tube.[](https://www.tubeampdoctor.com/en/tubeset-for-marshall-1959)
- Use a bias probe or consult a technician to measure and set the bias correctly.
4. **Power Down and Discharge**:
- Turn off and unplug the amplifier. Discharge the high-voltage capacitors to avoid electric shock, as tube amps can retain lethal voltages (up to 500–700 VDC).[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
- Use a multimeter to confirm capacitors are discharged before handling tubes.
5. **Remove Tubes**:
- Gently remove the selected pair of tubes from their sockets, ensuring not to damage the pins or sockets. Store the tubes safely to prevent breakage.
6. **Test the Amplifier**:
- Connect the amplifier to the appropriately adjusted speaker load (e.g., 8 ohms instead of 16 ohms).
- Power on and test at low volume to ensure stability and sound quality. Monitor for excessive heat or unusual sounds.
---
### **Safety Concerns**
1. **High Voltage Hazard**:
- Tube amplifiers like the Marshall 1959 operate at high voltages (500–700 VDC and 230–300 VAC). Touching live components can cause severe injury or death. Always unplug the amp and discharge capacitors before working on it.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
- Use one hand when probing a powered amp to avoid current passing through the heart.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
2. **Incorrect Impedance Matching**:
- Failing to adjust the speaker impedance after removing tubes can overload the output transformer, leading to overheating, arcing, or permanent damage. Always double the speaker impedance (e.g., 16 ohms to 8 ohms) when halving the number of tubes.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
3. **Biasing Risks**:
- Incorrect bias settings can cause the remaining tubes to overheat, reducing their lifespan or damaging the amplifier. In fixed-bias amps like the 1959, the bias voltage often self-corrects to some extent, but manual adjustment is recommended to ensure safety.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
4. **Thermal and Component Stress**:
- Removing tubes reduces current draw on the power transformer (PT), which may cause less voltage sag but could increase the high-voltage (B+) supply slightly, stressing other components. Ensure the remaining tubes and OT are rated for the new operating conditions.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
5. **Mechanical Damage**:
- Mishandling tubes during removal or installation can damage the tubes or sockets. Ensure tubes are cool before removal to avoid burns or glass breakage.
---
### **Technical Details**
1. **Circuit Configuration**:
- The Marshall 1959 Super Lead uses a push-pull output stage with four EL34 tubes, two per side of the output transformer, wired in parallel. Each pair handles one half of the audio signal’s waveform. Removing one tube per side maintains the push-pull balance but halves the power output (e.g., from 100W to 50W).[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
- The tubes on each side of the OT are wired in parallel, so the amp “sees” the remaining tubes as sufficient load, provided impedance is adjusted.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
2. **Impedance and Power Transfer**:
- With four tubes, the OT expects a certain load (e.g., 2k ohms plate-to-plate for a 16-ohm speaker). Removing two tubes doubles the effective tube impedance (e.g., to 4k ohms), requiring the speaker load to be halved (e.g., to 8 ohms) to match.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- Mismatched impedance reduces efficiency and can cause distortion or damage to the OT due to excessive voltage or current.
3. **Bias and Current**:
- In a fixed-bias amp like the 1959, the bias voltage is derived from the power transformer’s high-voltage (HT) tap. Reducing tubes decreases current draw, which may slightly increase the HT voltage, but the bias typically self-adjusts to prevent thermal runaway.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- Recommended bias for EL34s is 33–35 mA per tube at idle. After pulling tubes, verify bias to avoid over-dissipation (typically 25W per EL34).[](https://www.tubeampdoctor.com/en/tubeset-for-marshall-1959)
4. **Power and Volume Reduction**:
- Halving the tubes reduces power by approximately 50% (e.g., 100W to 50W). However, volume reduction is minimal, as a 3 dB decrease (halving power) is barely perceptible to the human ear.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- This mod is more effective for reducing heat and stress on downstream components (e.g., speakers or attenuators) than for significantly lowering volume.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
5. **Sound Quality**:
- Pulling tubes may alter tone due to changes in power supply sag and output transformer loading. Some report that high-gain circuits (like the 1959) may not sound as good with fewer tubes compared to a Plexi-style circuit.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
- The mod won’t drastically change distortion characteristics or achieve “early breakup” at low volumes.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
6. **Component Longevity**:
- Reducing tubes lowers current draw, which may extend the life of the power transformer and reduce heat in the amp. However, improper impedance matching or biasing can negate these benefits by stressing the OT or remaining tubes.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
---
### **Additional Considerations**
- **Not a Significant Volume Fix**: If the goal is to lower volume for home use, consider an attenuator or a lower-wattage amp, as pulling tubes offers minimal volume reduction.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- **Professional Assistance**: If you’re not experienced with high-voltage circuits or biasing, consult a qualified technician to avoid safety risks or damage.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)[](https://forum.pedalpcb.com/threads/tubes-102-intro-to-power-supplies-for-tube-circuits.10652/)
- **Alternative Mods**: Some amps (not necessarily the 1959) have switches to disable tube pairs safely, maintaining proper impedance and bias. Check if your amp supports this feature.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
---
### **Summary**
Pulling two tubes from a Marshall 1959 circuit is generally safe if done correctly, requiring:
- Removal of one tube per side of the output transformer.
- Doubling the speaker impedance (e.g., 16 ohms to 8 ohms).
- Rebiasing the amp to 33–35 mA per EL34.
- Strict adherence to high-voltage safety protocols.
**Safety concerns** include lethal voltages, improper impedance matching, and incorrect biasing, which can damage components or cause injury. **Technically**, the mod halves power but has minimal impact on volume, may alter tone, and requires careful impedance and bias adjustments to protect the amp. If unsure, consult a professional technician to ensure safety and optimal performance.
**Sources**:,,,,[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
Pulling two tubes from a 1959 circuit, such as in a Marshall 1959 Super Lead or similar vintage tube amplifier, is a common modification to reduce output power, typically halving it (e.g., from 100 watts to 50 watts). Below, I’ll outline the **method**, **safety concerns**, and **technical details** based on the amplifier’s design and general principles of tube circuits, with specific reference to the Marshall 1959 and related discussions.
---
### **Method for Pulling Two Tubes from a 1959 Circuit**
1. **Identify the Tubes to Remove**:
- The Marshall 1959 Super Lead typically uses four power tubes (e.g., EL34s) in a push-pull configuration, with two tubes per side of the output transformer (OT).
- To reduce power, remove one pair of tubes (one from each side of the OT) to maintain balance. For a quad of tubes (V1, V2, V3, V4), remove either V1 and V3 or V2 and V4, ensuring one tube remains on each half of the OT. Consult the amplifier’s schematic to confirm which tubes are paired.
2. **Adjust Speaker Impedance**:
- When removing two tubes, the impedance seen by the remaining tubes doubles because the output tubes’ impedance is effectively halved. For example:
- If the amplifier is set to a 16-ohm speaker load with four tubes, switch to an 8-ohm speaker load after removing two tubes to match the new impedance.
- This adjustment is critical to prevent damage to the output transformer and ensure efficient power transfer.
3. **Rebias the Amplifier**:
- After removing tubes, the bias voltage may need adjustment to prevent overcurrent or thermal runaway in the remaining tubes. For a Marshall 1959, the recommended bias current for EL34 tubes is typically 33–35 mA per tube.[](https://www.tubeampdoctor.com/en/tubeset-for-marshall-1959)
- Use a bias probe or consult a technician to measure and set the bias correctly.
4. **Power Down and Discharge**:
- Turn off and unplug the amplifier. Discharge the high-voltage capacitors to avoid electric shock, as tube amps can retain lethal voltages (up to 500–700 VDC).[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
- Use a multimeter to confirm capacitors are discharged before handling tubes.
5. **Remove Tubes**:
- Gently remove the selected pair of tubes from their sockets, ensuring not to damage the pins or sockets. Store the tubes safely to prevent breakage.
6. **Test the Amplifier**:
- Connect the amplifier to the appropriately adjusted speaker load (e.g., 8 ohms instead of 16 ohms).
- Power on and test at low volume to ensure stability and sound quality. Monitor for excessive heat or unusual sounds.
---
### **Safety Concerns**
1. **High Voltage Hazard**:
- Tube amplifiers like the Marshall 1959 operate at high voltages (500–700 VDC and 230–300 VAC). Touching live components can cause severe injury or death. Always unplug the amp and discharge capacitors before working on it.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
- Use one hand when probing a powered amp to avoid current passing through the heart.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)
2. **Incorrect Impedance Matching**:
- Failing to adjust the speaker impedance after removing tubes can overload the output transformer, leading to overheating, arcing, or permanent damage. Always double the speaker impedance (e.g., 16 ohms to 8 ohms) when halving the number of tubes.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
3. **Biasing Risks**:
- Incorrect bias settings can cause the remaining tubes to overheat, reducing their lifespan or damaging the amplifier. In fixed-bias amps like the 1959, the bias voltage often self-corrects to some extent, but manual adjustment is recommended to ensure safety.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
4. **Thermal and Component Stress**:
- Removing tubes reduces current draw on the power transformer (PT), which may cause less voltage sag but could increase the high-voltage (B+) supply slightly, stressing other components. Ensure the remaining tubes and OT are rated for the new operating conditions.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
5. **Mechanical Damage**:
- Mishandling tubes during removal or installation can damage the tubes or sockets. Ensure tubes are cool before removal to avoid burns or glass breakage.
---
### **Technical Details**
1. **Circuit Configuration**:
- The Marshall 1959 Super Lead uses a push-pull output stage with four EL34 tubes, two per side of the output transformer, wired in parallel. Each pair handles one half of the audio signal’s waveform. Removing one tube per side maintains the push-pull balance but halves the power output (e.g., from 100W to 50W).[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
- The tubes on each side of the OT are wired in parallel, so the amp “sees” the remaining tubes as sufficient load, provided impedance is adjusted.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
2. **Impedance and Power Transfer**:
- With four tubes, the OT expects a certain load (e.g., 2k ohms plate-to-plate for a 16-ohm speaker). Removing two tubes doubles the effective tube impedance (e.g., to 4k ohms), requiring the speaker load to be halved (e.g., to 8 ohms) to match.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- Mismatched impedance reduces efficiency and can cause distortion or damage to the OT due to excessive voltage or current.
3. **Bias and Current**:
- In a fixed-bias amp like the 1959, the bias voltage is derived from the power transformer’s high-voltage (HT) tap. Reducing tubes decreases current draw, which may slightly increase the HT voltage, but the bias typically self-adjusts to prevent thermal runaway.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- Recommended bias for EL34s is 33–35 mA per tube at idle. After pulling tubes, verify bias to avoid over-dissipation (typically 25W per EL34).[](https://www.tubeampdoctor.com/en/tubeset-for-marshall-1959)
4. **Power and Volume Reduction**:
- Halving the tubes reduces power by approximately 50% (e.g., 100W to 50W). However, volume reduction is minimal, as a 3 dB decrease (halving power) is barely perceptible to the human ear.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- This mod is more effective for reducing heat and stress on downstream components (e.g., speakers or attenuators) than for significantly lowering volume.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
5. **Sound Quality**:
- Pulling tubes may alter tone due to changes in power supply sag and output transformer loading. Some report that high-gain circuits (like the 1959) may not sound as good with fewer tubes compared to a Plexi-style circuit.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
- The mod won’t drastically change distortion characteristics or achieve “early breakup” at low volumes.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
6. **Component Longevity**:
- Reducing tubes lowers current draw, which may extend the life of the power transformer and reduce heat in the amp. However, improper impedance matching or biasing can negate these benefits by stressing the OT or remaining tubes.[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)
---
### **Additional Considerations**
- **Not a Significant Volume Fix**: If the goal is to lower volume for home use, consider an attenuator or a lower-wattage amp, as pulling tubes offers minimal volume reduction.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
- **Professional Assistance**: If you’re not experienced with high-voltage circuits or biasing, consult a qualified technician to avoid safety risks or damage.[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)[](https://forum.pedalpcb.com/threads/tubes-102-intro-to-power-supplies-for-tube-circuits.10652/)
- **Alternative Mods**: Some amps (not necessarily the 1959) have switches to disable tube pairs safely, maintaining proper impedance and bias. Check if your amp supports this feature.[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
---
### **Summary**
Pulling two tubes from a Marshall 1959 circuit is generally safe if done correctly, requiring:
- Removal of one tube per side of the output transformer.
- Doubling the speaker impedance (e.g., 16 ohms to 8 ohms).
- Rebiasing the amp to 33–35 mA per EL34.
- Strict adherence to high-voltage safety protocols.
**Safety concerns** include lethal voltages, improper impedance matching, and incorrect biasing, which can damage components or cause injury. **Technically**, the mod halves power but has minimal impact on volume, may alter tone, and requires careful impedance and bias adjustments to protect the amp. If unsure, consult a professional technician to ensure safety and optimal performance.
**Sources**:,,,,[](https://marshallforum.com/threads/safe-to-pull-two-output-tubes.109019/)[](https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm)[](https://www.tdpri.com/threads/pulling-2-tubes-from-twin-reverb-or-any-amp-is-it-safe.1060928/)
