How to use DC voltage map of Bassman25P

The DC voltage map of main boards of amp can show the normal static operating points of each tubes. Comparing it with the datas measured from your amp, it will help you find almost all troubles in your amp only by a simple multimeter. It is also the better way to understand the working principle of tube amp.Below is the instruction of DC voltage map of Bassman 25P main board.

Please pay attention: The interior of a tube guitar amp contains fatal high voltage. If you do not have sufficient skill and experience with high-voltage circuits, do not attempt to test it.
Measure condition of voltage map
The voltage map of Bassman25P is measured in these status. Please adjust your amp to these status.There are the a little differences between the electronic components, the 10% fluctuation between voltage value measured from your amp and data in this voltage map is normal.
Voltage: DC
Standby ON
Contol: Gain and Master minimum
Biase voltage: -30V
Power tube: Pentode
DC voltage map of Bassman25 P
The 12AX7 is a Noval dual-triode tube. The plate pins of the tube are 1 and 6, which require a high voltage to attract electrons from the cathode. The cathode pins of the 12AX7 are 3 and 8, and their voltages must be lower than those of the plates. The grids of 12ax7 are Pin 2 and 7
V2A is a high-voltage cathode follower, primarily used to adjust the impedance of the amp. The DC voltage of its cathode (Pin 3 of V2) is high (145VDC). The grid pins of the 12AX7 are 2 and 7. The DC voltages of the grids in tube preamps are typically close to zero, except for the follower amp V2A. This means that tube preamps generally operate with a DC grid voltage near zero.
We used two 6L6 tubes as the power amp, which are Octal tubes. If you use a different type of Octal tube, the voltage values may vary.
  • Pin 2 and Pin 7 are the filament connections.
  • Pin 3 is the plate.
  • Pin 4 is Grid 2 (screen grid).
  • Pin 5 is Grid 1 (control grid).
  • Pin 8 is the cathode and beam plate.
    • The Bassman25 is a Class AB amp, where the two power tubes must be matched during operation. This ensures that the parameters of the upper and lower power tubes remain the same.

    Bassman 25 SCH
    Conclusion
    The static operating points is important parameters describing status of amp, it is useful tool to troubleshoot. Abnormal operating points always means not only connection in mistake, but also possibility of resonance in the circuit.