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 inner of tube guitar amp has fatal high voltage. If you have not enough skill and experience in high voltage circuit, Do not try 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

12ax7 is Noval dual-triode tube. The plates pins of tube are 1 and 6, which need a high voltage attracting electron from cathode. The cathode pins of 12ax7 are 3 and 8, which voltages must be lower than the plates of the tube. V2A is high voltage cathode follower which mainly adjust the impendance of amp, the DC voltage of its cathode(Pin3 of V2)is high (145VDC). The grids of 12ax7 are Pin 2 and 7. The DC voltages of the grids of tube preamps commonly are about zero, except the follower amp V2A, which means that tubes preamp commonly worked about DC zero.The DC voltages of plate, cathode and grid of tube are keys to understand working status of tubes, you need pay more attention during checking amp.
We used two 6L6 being power amp, which is Octal tube. If you use other type of Octal tube, the voltage value maybe different. Pin 2 and 7 are pins of the filament of tube. Pin3 is plate. Pin 4 is grid 2(screen), Pin 5 is grid1. Pin 8 is cathod and beam plate. JCM25 is Class AB amp , which two power tubes need matched during working, so the parameters of up and down power tubes are same.