How to use voltage map of Bassman25 main board
KLDguitar Bassman25 25 w two independent channels hand wired amp based on the late Fender 50s 5F6A circuit which is known for bold and pristine clean sounds as well as classic unmistakeable dirt. The voltage map of main board of each tube guitar amps, could help hobbyists to understand tube amp principle and find assembly questions. 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 |
![]() |
Voltage map of Bassman25 main board |
![]() |
One of amp in the second tube V2B is the preamp, V2A is high voltage cathode follower which mainly adjust the impendance of amp. The Pin7 is the grid of V2B, connected with V2-7 of the board, the voltage is 0VDC. The Pin8 is the cathode of V2B, connected with V2-8 of the board, the voltage is 1VDC. The cathode of V2A (Pin 3) connected with V2-3 of board, the voltage is 145VDC. |
![]() |
The third tube is Long Tail Pair phase inverter of Bassman25 amp.The grids (Pin2,7) of V3 connected with V3-2 and V3-7 of the board. The voltages of these two points are 22VDC all. The anodes(Pin1,6) of V3 connected with V3-1 and V3-6 of the board, the voltage of V3-1 is 193VDC and V3-6 is 203VDC. The cathodes of V3 are Pin3 and 8 of 12ax7, connected with one point of the board V3-3/8, the voltage is 35VDC. The power tube of Bassman25 can use 6L6, EL34 etc octal pins power tubes. We used 6l6 being power tube in the test. Pin2 and 7 of 6L6 are pins of filament. Pin3 is anode(Plate) . The pin 4 is grid2(Screen, Pin5 is grid1. Pin8 is cathode and beam plate. The power amp of Bassman25 is class AB : V4 and V5. The pins of these two 6L6 connected with the main board include : Pin2s(Filament)of of V4 and V5, connected with board 6.3VAC; Pin4s(Grid 2) of V4 and V5 connected with V4-9,V5-9 of the board, the voltages are 335VDC all; Pin5s(Grid 1) connected with V5-2 and V4-2 of the board, all are -30VDC; Pin8s (Cathode) connected with V4/5-3 and R24C of the board, all are 30 mVDC. |