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TRACE ELLIOT
SERVICE MANUAL NO. SM00025 ISSUE 1
Date: Product Code : Model No : Technical File No : January 6, 1997 T3455/3456 Velocette 12R / Alnico TE00025

Issued by: Trace Elliot Limited. Blackwater Trading Estate The Causeway, Maldon Essex CM4 4GG.

C32-PCB-PC00064x3.
VELOCETTE 12R

ISSUE 3 14/2/97 PS Description PCB RESISTORS 0 ohm link 2R7 1/4W 10R 1/4W 56R 1/4W 82R 1/4W 1K0 1/4W 1K2 1/4W 1K5 1/4W 4K7 1/4W 10K 1/4W 10K 1W 15K 1/4W 27K 1/4W 47K 1/4W 68K 1/4W 82K 1/4W 100K 1/4W 100K 1W 180K 1/4W 220K 1/4W 220K 1W 330K 1/4W 470K 1/4W 820K 1/4W 1M0 1/4W 100R 4W 120R 6W 1K0 6W 3K3 4W SEMICONDUCTORS 1N4002 1N4007 GP02-40 (4KV) 9V1 ZENER 15V ZENER 2N3904

Part Code PC00064 issue 1

Qty 1

Where Used

72-RCZERO 72-RM2R7 72-RM10R 72-RM56R 72-RM82R 72-RM1K 72-RM1K2 72-RM1K5 72-RM4K7 72-RM10K 72-RM10K-1WATT 72-RM15K 72-RM27K 72-RM47K 72-RM68K 72-RM82K 72-RM100K 72-RM100K-1WATT 72-RM180K 72-RM220K 72-RM220K-1WATT 72-RM330K 72-RM470K 72-RM820K 72-RM1M 72-RWW100R-4W 72-RWW120R-6W 72-RWW1K-6W 72-RWW3K3-4W

1 2 2 1 1 4 1 5 1 4 1 1 2 1 2 1 3 3 1 5 2 1 2 1 6 2 1 1 1

R37 R80 R48 R55 R47 R27 R14 R2 R46 R33 R31 R59 R13 R19 R5 R35 R11 R8 R34 R20 R1 R58 R6 R28 R3 R25 R24 R32 R30

R81 R49

R50 R51 R57 R7 R10 R22 R23

R36 R40 R54

R67 R39 R41 R42 R15 R16 R21 R43 R52 R53 R29 R12 R4 R9 R26 R17 R18 R44

72-D-IN4002 72-D-IN4007 72-D-GP02-40 72-D-BZX55C9V1 72-D-BZX55C15V 72-T2N3904 1

4 4 2 2 1 3

D6 D1 D5 Z1 Z3 TR9

D7 D8 D2 D3 D10 Z2

D9 D4

TR10 TR11

2N3906 J175 BD647 RC4558 CAPACITORS 47p 100p 220p 470p 1n0 4n7 33p 100p 560p 2n2 6n8 22n 100n 330n 22n 47n 100n 1u5 2u2 22u 47u 100u 100u 220u 4700u 500V ceramic 1KV ceramic 1KV ceramic 1KV ceramic 1KV ceramic 1KV ceramic 100V 100V 100V 100V 100V 100V 100V axial axial axial axial axial axial axial

72-T2N3906 72-FET-J-175 72-TBD647 72-IC-RC4558P

2 1 1 2

TR1 TR2 TR6 TR7 IC1 IC2

72-C47P-500VCD 72-C100P-1KVCD 72-C220P-1KVCD 72-C470P-1KVCD 72-C1000P-1KVCD 72-C4700P-1KVCD 72-C33P-100VCA 72-C100P-100VCA 72-C560P-100VCA 72-C2N2-100VCA 72-C6N8-100VCA 72-C22N-100VCA 72-C100N-100VCA 72-C330N-50VCA 72-C22N-400VP 72-C47N-400VP 72-C100N-250VP 72-C1.5-35VT 72-C2.2-35VT 72-C22-450VER 72-C47-63VER 72-C100-16VER 72-CAP-100400V 72-C220-35VER 72-CAP-470035V

1 1 2 1 1 4 2 1 1 1 1 2 9 1 3 1 1 1 1 2 2 4 2 2 1

C29 C10 C7 C12 C6 C9 C30 C31 C32 C33 C41 C43 C4 C3 C46 C15 C2 C50 C39 C42

C47 C18 C21 C27 C28 C36 C60 C61

50V axial 400V poly box 400V poly box 250V poly box 35V 35V 450V 63V 16V 400V 35V 35V tant tant elect rad elect rad elect rad elect rad elect rad elect rad

C5 C16 C17 C14 C13 C1 C8 C25 C11 C51 C22 C19 C48

C26 C64 C52 C53 C62 C23 C40

CONNECTORS 3way 0.1" CRIMP CONNECTORS SOCKETS 1/4" MONO JACK SKT SWITCHES 72-SKT-JCKBNBG 4 SK1 SK2 SK3 SK4 72-HEAD-3W-2 72-CRIMP-PCB-TAB 3 16 HTR0 HTR1 HTR2 TX1 - 14 LS1 (x2)

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Large slide DPDT horiz Mini Toggle SPDT vert POTENTIOMETERS 1M0 250K LIN DUAL GANG 50K VALVE BASES B9A PCB valve base FUSE HOLDERS TEST PIN TO220 HEAT SINK FLYING LEADS ETC Cathode heater lead Reverb input lead Reverb output lead Grommet

73-SWT-SLIDER-DP 73-SWT-M-TGL-PCB

1 1

SW1 SW2

73-POT-A1M 73-POT-B250K-DG 73-POT-50KB

1 1 1

RV1 RV2 RV3

73-VAL-SOCKET 72-FUS-HLD-PCB-2 73-PIN-TERM 71-HS-PF752

4 4 1 1

V1

V2

V3

V4

FS1 FS2 FS3 FS4 TP0 TR7

C00-LEAD-VEL12HTR C00-FLY-TRAMP-R C00-FLY-V12R 45-GROMM-2

1 1 1 1

insert into HTR0, HTR1 & HTR2 REVERB IN REVERB OUT put over reverb leads before soldering into PCB

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VELOCETTE 12R CIRCUIT DISCRIPTION
Please refer to circuit diagram for DC voltages and other information INPUT SECTION AND PREAMP SK1 and SK3 are the LO and HI sensitivity inputs respectively. When the LO socket is used R5, R39 and R3 act as a potential divider to reduce the input level to the preamp. C3 has two purposes, firstly to block any DC from the input that may unintentionally be present, this would otherwise change the bias point of the first valve stage, secondly the value of the capacitor has been chosen so that there is a slight roll off of lower frequencies, this prevents the sound from getting too muddy. V1a is the first gain stage and is configured as a cathode bias, common cathode, voltage amplifier with bypassed cathode resistor for increased gain. R6 and C6 give a slight presence lift and the frequency of the Bright effect is set by C7, which, when switched in, is across pins 2 and 3 of RV1 (Volume). Obviously connected like this the amount of brightness added will decrease as RV1 is turned up. V1b is the second gain stage configured similar to before, C9 is added across the anode resistor R8 to smooth out the top end. The Tone network is passive and controlled by RV2. This is a dual ganged potentiometer, one part of which effectively controls the mids (RV2B) while the other part inversely controls the treble (RV2A). R28, R34 and R35 act as a potential divider to lower the signal sent to the reverb circuitry. REVERB SECTION The signal from the preamp is fed into IC1A which is configured as a boot strapped voltage follower. Z1 and Z2 have been added to prevent any nasty spikes from damaging the opamp, this should in practise never happen. C4 and R44 roll off a lot of the lower frequencies before the signal gets sent to IC1B and IC2A. These opamps are configured for current gain, the actual gain being dependant on the impedance/frequency curve of the reverb tray. Because of this it is crucial to the correct operation of the reverb that the right reverb tray is used. This should be an Accutronics 8CA3B1B. The output of the reverb tray goes into IC2B. This is configured as a differential amplifier as a way of reducing any hum that may be picked up on the sensitive reverb return leads. RV3 controls the level of the reverb. Across pins 1 and 2 is TR6, a J175 FET. When SK4 is shorted out, by a footswitch, TR6 is also effectively shorted which results in the reverb being turned off. 1

The reverb signal is then mixed with the dry signal, via R58, before going into the phase splitter. POWER STAGE The phase splitter (V2A and V2B) is a differential input splitter which produces the two anti phase signals necessary to drive the push pull output stage. V3 and V4 are the two EL84 output valves connected as a push pull, cathode biased class A power amplifier. The quiescent current is set by R24, which is bypassed by C19 for extra gain. SW1A and SW1B is an electrically robust slider switch used to switch the power valves from Pentode to Triode operation. For Pentode operation the screen grid (grid 2, pin 9) is connected, via the screen grid current limiting resistors R25 and R26, to the highly smoothed screen grid supply, G2 - positive pin of C23. For Triode operation the screen grid of each valve is connected to the corresponding anode. Triode operation basically reduces the power output to a bit less than half and also reduces the high frequencies in the overall sound. D5 and D10 have been added to give protection to the output transformer should a fault arise. OUTPUT TRANSFORMER AND SPEAKER CONNECTIONS The output transformer has secondary taps for 16, 8 and 5.3 . The 16 tap is used to drive the External Speaker Output, SK2. When a jack plug is inserted into SK2 the internal speaker is disconnected. The three LINK positions are provided on the PCB so that different impedance internal speakers can be used in production. Depending on whether the internal speaker is 16, 8 or 5.3 the correct LINK should be fitted. This has been done purely so that different impedance speakers can be used if there are any problems with supply. POWER SUPPLIES All three supplies, HT, ac heater, and DC supplies have secondary fusing on the PCB. This is to protect the mains transformer and for approvals. The HT supply is a very simple bridge rectifier diode network, with 4n7 1KV capacitors across each diode for EMC reasons, which is then smoothed by C22, to supply the centre tap of the output transformer. This is then further smoothed by R32/C23, R30/C25 and R31/C26 to supply the screen grids, phase splitter and preamp respectively. R29 is added to discharge the high voltage capacitors when then unit is turned off. The ac heater supply is simply connected via a twisted pair connecting lead to V3 and V4 after first going through the secondary fuses

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The 25.2V DC supply is highly regulated supply using a BD647 (TR7) as the main regulating device. (Please ensure that the small clip on heat sink is attached to TR7) TR1 and TR2 provide a constant current source for Z3 the 15 volt zener. The output voltage is set by the ratio of R59 and R67 which provide the feedback to TR7 via TR9 and TR10 to stabilise the whole circuit, and TR11 with R80 and R81 form a current limiter. This allows the supply voltage to ramp up at switch on when the heater filaments of the preamp valves draw considerably more current while cold. R27 and R57 halve the supply voltage to provide the opamp bias voltage, and as shown on the circuit diagram the DC supply is routed first through the two filaments in V2 in series and then through the two filaments in V1. This will result in each filament having the nominal 6.3 volts across them. Paul Stevens 2 January 1997

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