How to use DC voltage map of BFLEX25 main board

KLD BFLEX25 Bass amp is based on Ampeg PF20T, which power amp is Class AB amp of two 6L6 tubes. Below is introduction of the DC voltage map of main board of BFLEX 25, which could help hobbyists to understand tube amp working principles and troubleshooting after assembly.

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 by self.

Measure condition of voltage map
The DC voltage map of BFLEX25 main board was measured at the status. Please adjust your amp to the status.There are a little differences 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

BFLEX25 voltage map of main board

DC voltage map of BFLEX25 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. Then through different electrolytic capacitors filting, we get different plate voltages of the tube in amp stages: B+A: 390VDC, B+B: 360VDC, B+C: 308VDC, B+D:270VDC (V2). B+E:255VDC (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 Pin 1, 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 the tube, second digit means code of pin of the tube. For example : V2-3 represent this hole will be connected with the Pin3 of the 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
BFLEX25 only has two amplify stages, V1B, V1A. The voltages of V1 are below: The plate voltage of V1B is 149VDC(V1-6 of board). The plate voltage of V1B is 144VDC(V1-1 of board), and the cathode voltage of V1B is 1.6VDC (V1-8) and V1A is 1.3VDC (V2-3).

BFLEX25 layout

Long Tail Pair phase inverter
The second 12ax7 is Long Tail Pair(LTP) phase inverter of BFLEX25 amp.The grids (Pin2,7) of V2 connected with SKY and V2-8 of the board, the voltages of these two holes are 0VDC. The plates( Pin1,6) of V2 connected with V3-1 and V3-6 of the board.The voltage of V3-1 is 211VDC and V3-6 is 190VDC. The cathodes of V2 are Pin 3 and 8 of tube, they connected with one hole of the board V3-3/8, the voltage is 1.6VDC.


Class AB power amp
The power tube of BFLEX25 can use 6L6 or EL34 etc. The datas in the voltage map were measured by 6L6. Pin2 and 7 are pins of filament. Pin3 is Anode(Plate). The Pin 4 is grid2(Screen). Pin5 is grid1.Pin8 is cathode and beam plate. The type of power amp stage of BFLEX25 is Class AB : V3 and V4. The pins of these two 6L6 connect with the main board include: Pin2s(Filament)of V3 and V4 connect with board 6.3VAC; Pin4s(grid 2) of V3 and V4 connect with V4-9,V5-9 of the board, the voltages are 330VDC all; Pin5s(grid 1) connect with V5-2 and V4-2 of the board, one is -43VDC, one is -40VDC; Pin8s (Cathode) connected with V4/5-3 and R24C of the board, all are 80 mVDC.

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 .