How to use DC voltage map of UT18 main board

KLDguitar UT18 is 18w hand wired dual independent channels guitar amp based on VOX AC15, which was one of the first to define the British sound and continues to be used on stages and in studios around the world. 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 voltage map of UT18 main board is measured in 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

UT18 main board voltage map

Voltage map of UT18 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: 360VDC, B+B: 347VDC, B+C: 288VDC, B+D:241VDC (V2). B+E:230VDC (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

First preamp
UT 18 has dual independent channels, which signal need be controlled by separately. V1B amplifies signal of clean channel, the gain is controlled by VR2.V1A amplifies signal of lead channel,the gain is controlled by VR1.The four holes on the main board are relationship with first preamp: V2-7,V2-8, V2-3, V2-6.
V2-7 connect with Pin1 of VR1, its voltage is 0 VDC. V2-8 connect with Pin3 and 8 of first tube. Pin3 and 8 are cathodes of two amp of 12ax7 , which the bias DC voltage of cathode is 1.6VDC all. V2-3 and V2-6 connect with Pin 6 and 1 of V1 individual which are two plates(anode) of 12ax7, the DC voltages of plates are 174VDC and 179VDC individual.

UT18 layout
V6-2 on the main board connect with Pin1 of VR1, the voltage is 0VDC. V6-6 on the main board connect with Pin3 of tube V2,which is the cathode of V2A. V2A is high voltage cathode follower which mainly adjust the impendance of amp, its the DC bias cathode voltage is 150VDC. V6-8 connects with the Pin8 of V2, it is the cathode of V2B, the bias cathode voltage is 1.2VDC.
V6-1 on the main board connect with Pin2 of the Treble control ,its voltage is 0VDC. V3-7 connect with The Pin7 of V3, it is grid of V2B , the voltage of grid of V2B is 37VDC. V3-2 connects with the Pin2 of V3, it is grid of V3A, the voltage is 36VDC. V3-3/8 connects with the Pin3,8 of V3, which are cathodes of V3A and V3B. The Voltage is 54 VDC all. V3-6 on the main board connects with the Pin6 of V3, it is plate of V3B, the voltage is 232VDC. V3-1 connects with the Pin1 of V3, it is plate of V3A. The voltage is 225VDC.

Class AB power amp
UT18 is Class AB amp, used two matched EL 84 tubes. The grid 1of EL84 is Pin2. Through R40, V5-2 hole of the main board connects to the grid1 of tube V4 , V4-2 hole connects to the grid 1 of tube V5 through R41, the grid 1 DC voltage of V4 and V5 is 0VDC all. The cathode of EL84, Pin3s of V4 and V5 tube connect with V4,5-3 holes of the board, individual. The cathode bias voltages of two EL84 are 12.6VDC all.Pin9 of EL84 is grid2. Hole V4-9 connected with Pin 9 of V4, hole V5-9 connected with Pin 9 of V5, the grid2 DC voltages of V4 and V5 are all 350VDC.

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.