Tube Amplifier Repair

Tube Amplifier Repair

Ken's 降龍十八掌 - after learning this kungfu, you should be able to tackle most tube amplifier problems.

Tube amplifier repair is not difficult if one knows how to relate symptoms to the problematic area. This requires experience and know-how plus some luck too. Here are some of the simple diagnoses towards tube amplifier repair.

Hum

• Improper grounding – ground loop
• Poor filament transformer grounding
• Poor filament termination
• Plate and filament winding short-circuit
• B+ voltage filter capacitor – insufficient or capacitor malfunction
• Bias supply filtering – insufficient or capacitor malfunction
• AC filament – voltage too high for AC operation, imbalanced AC feed, DC filament filtering – insufficient or capacitor malfunction

Distortion

• Push-pull topology – imbalance operation on driver or power stage (one tube is aged or damaged), causing imbalance gain = distortion
• Push-pull topology – output transformer winding damaged (open or short-circuit)
• Push-pull topology with pentode – grid resistor damaged
• Coupling capacitor damaged / leak / change in value / short circuit
• Bias runaway
• Input stage headroom is lower than input signal level
• Input signal level causing grid current (power stage not designed for A2 grid current operation)
• Insufficient drive strength or aged driver tube
• Plate load resistor, cathode resistor, and grid resistor value resistor change (damage) causing operating point to shift
• B+ voltage runaway – damaged choke  / filter / regulator / transformer

Tube plate becomes red / exceeds dissipation

• Overloading – output tap mismatch with speaker impedance, short circuit, output transformer primary short circuit, or improper operating point design
• Bias runaway – bias supply damaged, cathode bias capacitor or resistor damaged, improper bias setting, tube socket damage (open circuit)
• B+ too high – power supply damaged, high voltage transformer short circuit, bleeder resistor damage, choke short circuit, load current too low causing B+ to rise.
• Pentode grid voltage too high – grid resistor damage causing excess grid voltage
• High frequency oscillations – improper circuit design, component quality, lack of grid resistors
• Push-pull topology – tube aged, causing imbalance and over-plate dissipation, by either push or pull tube, or one of the paralleled tubes.
• Push-pull topology – partial output transformer winding short-circuit
• Excessive input signal, causing overload
• Improper amplifier design

 Parasitic oscillations

• Feedback circuit – component damage or improper feedback design
• Plate or grid resistor damage, or improper material of grid stopper resistor used (inductance in grid stopper resistor will cause oscillations)
• Excessive B+ voltage – working out of normal operating range

Power on or operation fuse blow

• Check all solder joints to all components
• Check components polarity
• Check if output transformer is short circuit – primary itself, or primary to secondary, or secondary itself
• Tube load short circuit or overloaded
• Change to slow blow fuse – overcome turn on surge

Lower output power, output soften

• Low filament voltage
• Wrong tubes operating point
• Tube aged – check plate current versus bias
• Check tube operating point
• Check tube bias
• Check driver tube drive strength / headroom
• Output transformer short circuit
• Check coupling components – capacitors / interstage transformer if damaged
• If parallel tube, check if any tube is weaken / damaged or not properly connected
• Check cathode bypass capacitors / resistors

That covers most of the issues a tube amplifier will have. If you have more tips, you’re very welcome to add it at the comments section. We will put it in accordingly.

Note: multi-meter, oscilloscope, and other measurement equipment might be needed for the above tests.

J&K Audio Design

29/12/2013

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