How to use DC voltage map of PVA18 main board
KLD PVA18 is hi gain guitar amp based on Marshall TMB18 which power amp are two EL84. It has dual PRESENCE controls: "SKY" controls high freq tone , "Earth" controls low freq tone. Below is introduction of the DC voltage map of main board of PVA18, which could help hobbyists to troubleshoot after assembly and understand tube amp working principles more deeply.
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
DC voltage map of PVA18 main board
|The second tube V2B is the third stage preamp, V2A is high voltage cathode follower which mainly adjust the impendance of amp. Pin7 is grid of V2B, it connected with V2-7 of the board, voltage is 0VDC. Pin8 is the cathode of V2B, it connects with V2-8 of the board, the voltage is 1.4VDC. Pin3 is the cathode of V2A, It connected with V2-3 of the board, the voltage is 170VDC.|
|The third tube is long tail phase(LPT) inverter of PVA18 amp.The grids ( Pin2, 7 ) of V3 connect with V3-2 and V3-7 of the board, the voltages are 23 VDC all. The plates ( Pin1,6) of V3 connect with V3-1 and V3-6 of the board, the voltage of V3-1 is 240 VDC and V3-6 is 250 VDC. The cathodes of V3 are Pin3 and 8 of the tube, they connect with V3-3/8 of the board , the voltages are 35VDC all.
Tips :Please pay attention to the wire connecting from SKY control with SKY, it is feedback of SKY control, its shield net is connected with ground. If it was short with inner wire , the voltage of V3-2 and V3-7 would become 14 V( 25-11V), the PRESENCE control could not work.
EL84 has nine pins. Pin4 and 5 are pins of filament. Pin7 is plate(Anode). The pin 9 is grid 2(Screen grid）, Pin2 is grid 1( Control grid). Pin 3 is cathode or grid 3(Suppress grid), and three NC pins ,Pin 1,Pin6, Pin8. The type of power amp stage of PVA18 is Class AB: V4 and V5. The pins of these two tubes connect with the main board include: Two Pin 5s(Filament) connect with board 6.3VAC.Two Pin9s(Grid 2) connect with V4-9,V5-9 of the board individual, the voltages are 360VDC all.The Pin 2 of two tubes (Grid1) connect with V5-2 and V4-2 of the board, the voltages of them are 0VDC all. The Pin3 of V4 and V5(Cathode) connect with V4/5-3 of the board, the voltage is 16VDC.
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.