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 DC voltage map of main board of each tube guitar amps, could help hobbyists to understand tube amp working principle and find assembly problems.
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 DC voltage map of Bassman25 main board is measured at these status. Please adjust your amp to these status.Because there are a little differents between the electronic components,the 10% fluctuation between voltage values measured from your amp and datas 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 main board
Filter circuit of power
Let us see the circuit board from the side which installed components. Down to up ,the power supply filter circuit is on the bottom 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 electrolytic 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 after turning on amp, please make sure these voltage normal first, then check the pins connected with them in tube sockets.The plates of 12ax7 are Pin1, 6, the plate of 6L6 is Pin 3.
Position code
The uppest line of the main board. Each holes has the position code of one letter and two digits . The first letter "V" represent "Valve (Tube)". First digit means code of tube, second digit means code of pin of tube. For example: V2-3 represent this hole connected with Pin3 of second tube.Because this main board is universal board of our amps, so the position codes printed on board are not totally consistent with the real layout drawing of the each amp. Please connect wires according to layout drawing of each amp kits.
First preamp
Bassman 25 has dual independent channels, which are individual amplified by the two amp of the first 12ax7. The plate voltages of two amp are 180VDC all, and cathode voltages are 1.4VDC all.The cathods Pin8 and 3 connect with V1-8 of the main board，the first amp of Bassman25 is warm bias. The plate Pin6 of V1B connects with V1-6 of the board, the plate Pin1 of V1A connect with V1-1.
V2B is the second stage preamp, V2A is high voltage cathode follower which mainly adjust the impendance of amp. Pin7 is the grid of V2B, connects with V2-7 of the board, the voltage is 0VDC. Pin8 is the cathode of V2B, connects with V2-8 of the board, the voltage is 1VDC. The cathode of V2A (Pin 3) connects 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 connect with V3-2 and V3-7 of the board. The voltages of these two holes are 22VDC all. The plates(Pin1,6) of V3 connect 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, Pin3 and 8, connect with one hole of the board V3-3/8, the voltage is 35VDC.
Class AB power amp
The power tube of Bassman25 use octal tube such as 6L6, EL34 etc . The values of DC voltage map are measured utilizing 6L6 tube. Pin2 and 7 of 6L6 are pins of filament. Pin3 is plate. Pin 4 is grid2(Screen), Pin5 is grid1. Pin8 is cathode and beam plate.
Bassman25 is Class AB amp: two paired 6L6 tubes V4 and V5. Because V4 and V5 are paired, so all voltage values of these two tubes are same. The pins of these two 6L6 connect with the main board include: Pin2s(Filament)of of V4 and V5, connect with board 6.3VAC; Pin4s(Grid 2) of V4 and V5, connect with V4-9,V5-9 of the board, the voltage is 335VDC all; Pin5s(Grid 1), connect with V5-2 and V4-2 of the board, the voltage is -30VDC all; Pin8s (Cathode) connect with V4/5-3 and R24C of the board, the voltage is 30 mVDC all.
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 resoanace in circuit.