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SERVICE MANUAL NO. SM00025
Date: January 6, 1997
Product Code : T3455/3456
Model No : Velocette 12R / Alnico
Technical File No : TE00025
Trace Elliot Limited.
Blackwater Trading Estate
The Causeway, Maldon
Essex CM4 4GG.
ISSUE 3 14/2/97 PS
Description Part Code Qty Where Used
PCB PC00064 issue 1 1
0 ohm link 72-RCZERO 1 R37
2R7 1/4W 72-RM2R7 2 R80 R81
10R 1/4W 72-RM10R 2 R48 R49
56R 1/4W 72-RM56R 1 R55
82R 1/4W 72-RM82R 1 R47
1K0 1/4W 72-RM1K 4 R27 R50 R51 R57
1K2 1/4W 72-RM1K2 1 R14
1K5 1/4W 72-RM1K5 5 R2 R7 R10 R22 R23
4K7 1/4W 72-RM4K7 1 R46
10K 1/4W 72-RM10K 4 R33 R36 R40 R54
10K 1W 72-RM10K-1WATT 1 R31
15K 1/4W 72-RM15K 1 R59
27K 1/4W 72-RM27K 2 R13 R67
47K 1/4W 72-RM47K 1 R19
68K 1/4W 72-RM68K 2 R5 R39
82K 1/4W 72-RM82K 1 R35
100K 1/4W 72-RM100K 3 R11 R41 R42
100K 1W 72-RM100K-1WATT 3 R8 R15 R16
180K 1/4W 72-RM180K 1 R34
220K 1/4W 72-RM220K 5 R20 R21 R43 R52 R53
220K 1W 72-RM220K-1WATT 2 R1 R29
330K 1/4W 72-RM330K 1 R58
470K 1/4W 72-RM470K 2 R6 R12
820K 1/4W 72-RM820K 1 R28
1M0 1/4W 72-RM1M 6 R3 R4 R9 R17 R18 R44
100R 4W 72-RWW100R-4W 2 R25 R26
120R 6W 72-RWW120R-6W 1 R24
1K0 6W 72-RWW1K-6W 1 R32
3K3 4W 72-RWW3K3-4W 1 R30
1N4002 72-D-IN4002 4 D6 D7 D8 D9
1N4007 72-D-IN4007 4 D1 D2 D3 D4
GP02-40 (4KV) 72-D-GP02-40 2 D5 D10
9V1 ZENER 72-D-BZX55C9V1 2 Z1 Z2
15V ZENER 72-D-BZX55C15V 1 Z3
2N3904 72-T2N3904 3 TR9 TR10 TR11
2N3906 72-T2N3906 2 TR1 TR2
J175 72-FET-J-175 1 TR6
BD647 72-TBD647 1 TR7
RC4558 72-IC-RC4558P 2 IC1 IC2
47p 500V ceramic 72-C47P-500VCD 1 C29
100p 1KV ceramic 72-C100P-1KVCD 1 C10
220p 1KV ceramic 72-C220P-1KVCD 2 C7 C12
470p 1KV ceramic 72-C470P-1KVCD 1 C6
1n0 1KV ceramic 72-C1000P-1KVCD 1 C9
4n7 1KV ceramic 72-C4700P-1KVCD 4 C30 C31 C32 C33
33p 100V axial 72-C33P-100VCA 2 C41 C42
100p 100V axial 72-C100P-100VCA 1 C43
560p 100V axial 72-C560P-100VCA 1 C4
2n2 100V axial 72-C2N2-100VCA 1 C3
6n8 100V axial 72-C6N8-100VCA 1 C46
22n 100V axial 72-C22N-100VCA 2 C15 C47
100n 100V axial 72-C100N-100VCA 9 C2 C18 C21 C27 C28 C36
C50 C60 C61
330n 50V axial 72-C330N-50VCA 1 C39
22n 400V poly box 72-C22N-400VP 3 C5 C16 C17
47n 400V poly box 72-C47N-400VP 1 C14
100n 250V poly box 72-C100N-250VP 1 C13
1u5 35V tant 72-C1.5-35VT 1 C1
2u2 35V tant 72-C2.2-35VT 1 C8
22u 450V elect rad 72-C22-450VER 2 C25 C26
47u 63V elect rad 72-C47-63VER 2 C11 C64
100u 16V elect rad 72-C100-16VER 4 C51 C52 C53 C62
100u 400V elect rad 72-CAP-100400V 2 C22 C23
220u 35V elect rad 72-C220-35VER 2 C19 C40
4700u 35V elect rad 72-CAP-470035V 1 C48
3way 0.1" 72-HEAD-3W-2 3 HTR0 HTR1 HTR2
CRIMP CONNECTORS 72-CRIMP-PCB-TAB 16 TX1 - 14 LS1 (x2)
1/4" MONO JACK SKT 72-SKT-JCKBNBG 4 SK1 SK2 SK3 SK4
Large slide DPDT horiz 73-SWT-SLIDER-DP 1 SW1
Mini Toggle SPDT vert 73-SWT-M-TGL-PCB 1 SW2
1M0 73-POT-A1M 1 RV1
250K LIN DUAL GANG 73-POT-B250K-DG 1 RV2
50K 73-POT-50KB 1 RV3
B9A PCB valve base 73-VAL-SOCKET 4 V1 V2 V3 V4
FUSE HOLDERS 72-FUS-HLD-PCB-2 4 FS1 FS2 FS3 FS4
TEST PIN 73-PIN-TERM 1 TP0
TO220 HEAT SINK 71-HS-PF752 1 TR7
FLYING LEADS ETC
Cathode heater lead C00-LEAD-VEL12- 1 insert into HTR0, HTR1 & HTR2
Reverb input lead C00-FLY-TRAMP-R 1 REVERB IN
Reverb output lead C00-FLY-V12R 1 REVERB OUT
Grommet 45-GROMM-2 1 put over reverb leads before
soldering into PCB
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
R28, R34 and R35 act as a potential divider to lower the signal sent to the reverb circuitry.
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
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
The reverb signal is then mixed with the dry signal, via R58, before going into the phase
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
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.
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
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.
2 January 1997
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