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
The voltage map of Bassman25 main board is measured at these status. Please adjust your amp to these status.Because there are the differents between the electronic components, so it is normal that 10% fluctuation of voltage value between your amp with voltage map.
Voltage: DC
Standby ON
Contol: Gain and Master minimum
Biase voltage: -30V
Power tube: Pentode

KLD JCM25 V1,V2 preamp

Voltage map of Bassman25 main board
Start from down to up of board.The power supply filter circuit is on the downest of the board. The rightest is the rectifier bridge consisted by four diodes D1-D4, which provides full-wave rectify for 260 VAC from power transformer. From right to left, through different capacitors filting , the different plate voltages of the tube in amp stages are: B+A: 366VDC, B+B: 339VDC, B+C: 308VDC, B+D:265VDC (V2).
If you could not hear any sound opening amp , please check these voltage normal first, then check the points connected with them in tube sockets.The anodes of 12ax7 are Pin1, 6, the anode of 6L6 is Pin 3.
Then, the uppest line of the main board. Each one has the position code of one letter and two digits . The first letter “V” represent "Valve (Tube) ". First digital means code of tubes, Second digital means code of pin of tube. For example: V2-3 reprent this point connected with Pin3 of second vacuum tube.
Because this main board is universal board of our amps, so the position code printed on board are not totally consistent with real layout drawing of each amp. Please connect wires according to layout drawing of each amp kits.
Bassman 25 has dual independent channels, which are individual amplified by the two amp of the first 12ax7 tube. The anode voltages of two amp are 180VDC all, and cathode voltages are 1.4VDC all.The cathods Pin8/3 connected withV1-8 of the main board,the first amp of Bassman25 is warm bias. The anode Pin6 of V1B connected with V1-6 of the board, the anode Pin1 of V1A connected with V1-1.

JCM25 layout
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.