KLD PVA18 is hi-gain guitar amp based on Marshall TMB18 which power amp are two EL84s. It has dual PRESENCE controls: "SKY" controls high freq tone , "Earth" controls low freq tone. Below introduce 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
The DC voltage map of PVA18 main board is measured at these status. Please adjust your amp to these status.Because there are the 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
DC voltage map of PVA18 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 presence capacitors filting, the different plate voltages of the tube in amp stages are : 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 EL84 is Pin 3
Position code
The uppest line of the main board. Each hole 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 the pin of the tube. For example: V2-3 represent this hole will be connected with the 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
PVA 18 has three preamp stages, V1B, V1A, V2B. The voltages of first 12ax7 tube are below: The plate voltage of V1B is 120VDC (V1-6 of board), the plate voltage of V1A is 190VDC(V1-1 of board). The cathode voltage of V1B is 0.9VDC (V1-8) and V1A is 1.8VDC (V1-3), the second amplify stage(V1A) is clod cathode bias.Make sure your tubes working around these quiescent point(q-point). If their working point were far away with these q-points, the periphery circuits of tube must have problems.
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.
Long Tail Pair phase inverter
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.
Class AB power amp
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.
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 possibility of resonance in the circuit.