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20 W Stereo Audio Amplifier

chassis, or well above the chassis so as to stay as far as possible away from signal-carrying lines

and to rest against the bottom cover plate, which further improves shielding.

Right (Line input) Left

Built in California, USA

Right Channel

Ground 8 Ohms

Left Channel

8 Ohms Ground

Preamp Power Connector

Molex 38720-3202

Neutrix NF2D

0.88-in

0.13-in

Center

Neutrix NF2D

0.88-in

0.13-in

Center

AC connector shell
0.75-in x 1.08-in

Center

0.15-in

TE shell 17

Center

0.15-in

1.21-in

20 W Stereo Amplifier

Figure 1.6

The back panel of the 20 W Stereo Amplifier.

Summary of Contents for J C Whitaker 20 W Stereo Audio Amplifier

Page 1: ...20 W Stereo Amplifier WhitakerAudio 20 W Stereo Audio Amplifier...

Page 2: ...hitakerAudio Specifications subject to change without notice Any trademarks used in the manual are the property of their respective owners Note This document is intended to assist readers in building...

Page 3: ...y of the 20 W Stereo Amplifier PWB 2 Left Channel 65 5 3 Assembly of the Preamplifier Power Supply Module 74 5 4 Assembly of the Power Management Module 78 5 5 Assembly of the Power Management Expansi...

Page 4: ...s 151 6 2 2Feedback Trim Adjustment 152 6 2 3Power Management Expansion Board Adjustments 153 7 Performance Measurement 155 7 1 Power Amplifier Performance Measurement 156 7 1 1IMD Measurement 157 7 1...

Page 5: ...a kit or assembled as a part of the signature series of products This amplifier shares the craftsmanship and impressive industrial design of the other offerings in the WhitakerAudio line This amplifi...

Page 6: ...hum and noise The boards feature a unique mounting technique for the vacuum tubes that keeps heat away from the PWB and minimizes hum in the amplifier The PWBs are designed to reduce off board connec...

Page 7: ...20 W Stereo Amplifier is ideal for use in computer sound applications for driving speakers or headphone listening or both The amplifier generates little heat and for a tube amplifier consumes little...

Page 8: ...t may be used without danger of low frequency instability A low noise 6U8A tube V1 V4 which contains a high gain pentode section and a medium triode section in one envelope fulfills the active compone...

Page 9: ...9 20 W Stereo Audio Amplifier Figure 1 1 The 20 W amplifier a amplifier PWB 1 right channel b next page amplifier PWB 2 left channel...

Page 10: ...10 WhitakerAudio Figure 1 1 b...

Page 11: ...ight channel and capacitors C10 and C11 for the left channel are all 5 tolerance devices Two front panel jacks are provided for connection of one or two headphones to the amplifier for private listeni...

Page 12: ...de good voltage regulation as the plate current of the 6973 tubes vary considerably with signal level The conventional choke input type of power supply used provides the required stability The fixed b...

Page 13: ...ve as an interlock When the umbilical cable is connected from the amplifier to the preamplifier Pins 7 and 8 are shorted which activates relay RYL301 The relay energizes the filament supply and pilot...

Page 14: ...14 WhitakerAudio Figure 1 2 The preamplifier power supply PWB...

Page 15: ...ment circuit board is a supervisory device that adds additional layers of protection to the built in protection already provided on the power amplifier The power management system adds the following f...

Page 16: ...front panel This switch as two positions Auto Off Normal A schematic diagram of the power management system is given in Figure 1 3 Relay based logic is used to latch power to the amplifier on and inte...

Page 17: ...17 20 W Stereo Audio Amplifier Figure 1 3 Power management circuit for the 20 W Stereo Amplifier...

Page 18: ...cuit The values of R214 and R215 are determined by the operating voltage being measured As a general guideline R214 R215 800 voltage being measured The voltage sensing circuit is designed to latch on...

Page 19: ...ses it can cause component damage The expansion board monitors the bias voltage and removes power from the amplifier if the voltage is outside recommended operating limits A chassis interlock is provi...

Page 20: ...20 WhitakerAudio Figure 1 4 Power management expansion circuit for the 40 W Stereo Amplifier...

Page 21: ...operating with no bias Under such a condition circuit breaker CB51 would trip removing power from the amplifier The benefit of the bias detection circuit is that it operates faster than the circuit br...

Page 22: ...handles included on each side of the chassis are for more than just appearance They are quite useful for moving the unit The amplifier also generates heat from the seven tubes and plate transformer N...

Page 23: ...lding Right Line input Left Built in California USA Right Channel Ground 8 Ohms Left Channel 8 Ohms Ground Preamp Power Connector Molex 38720 3202 Molex 38720 3202 Neutrix NF2D 0 88 in 0 13 in Center...

Page 24: ...llied Stock No Other Supplier C1 C7 47 F 450 V electrolytic 2 Illinois Capacitor 476TTA450 M 70111929 C13 C14 20 pF 1 kV ceramic 2 Vishay 561R10TCC Q20 70079497 C2 C4 C5 C8 C10 C11 C305 0 22 F 400 V 5...

Page 25: ...0 V electrolytic radial 1 Cornell Dubilier SEK470M10 0ST 70186512 R1 R18 R307 1 m 0 5 W 3 Ohmite OL1055E 70023092 R11 R12 R28 R29 1 k 0 5 W 1 4 Ohmite ML1001FE 70022827 R13 R30 8 2 k 0 5 W 2 Vishay RN...

Page 26: ...D8250 F 70200291 R401 R407 4 7 k 0 5 W 2 Ohmite OL4725E 70022934 R402 R408 5 k potentiometer trimmer 2 Bourns 3386P 1 502LF 70154094 R404 R410 R413 R414 R419 R420 R423 R425 100 k 0 5 W 8 Ohmite OL1045...

Page 27: ...D402 D409 100 V switching diode 4 Vishay 1N4148 TR 70061726 D1 D2 D202 D204 D205 D206 D303 D403 D406 D407 Rectifier diode 1 A 600 V 10 NTE NTE116 70214755 D203 D404 D405 Zener 25 V 5 W 3 ON Semicondu...

Page 28: ...ener 56 V 5 W 1 ON Semiconductor 1N5370BG Newark 61K2025 V1 V4 6U8A tube 2 JAN Philips GE others 6U8A or 6GH8A Tube Depot V2 V3 V5 V6 6973 tube 4 Electro Harmonix others 6973 Tube Depot V54 6CA4 tube...

Page 29: ...081495 RYL51 5 V dc relay contacts 240 V ac 30 A SPDT 1 Magnecraft W9AS5D52 5 70184949 RYL1 RYL2 RYL201 RYL203 RYL205 RYL301 RYL402 DPDT 10A 24 V dc relay 7 Omron MY2DC24 S 70178955 RYL202 RYL204 DPDT...

Page 30: ...541 Power connector socket 1 Schurter 6100 33 70080620 4 terminal Molex header 0 1 pitch 9 Molex 22 03 2041 70190629 4 terminal Molex housing 0 1 pitch 9 Molex 22 01 2041 70190701 Molex crimp terminal...

Page 31: ...Power Management PWB 1 ExpressPCB custom ExpressP CB Power Management Expansion PWB 1 ExpressPCB custom ExpressP CB Plexiglas trim 1 TAP Plastics custom TAP Plastics Top chassis decals 1 FastSigns cus...

Page 32: ...miniature 22 wire 4 Quick disconnect male 22 wire 4 Butt splice connector 22 wire 6 Ground lug terminal 4 screw 9 Ground lug terminal 6 screw 70 Phillips head screw 4 3 8 inch 30 Phillips head screw...

Page 33: ...ch 10 Tie wrap small 10 ft Rosin core solder 10 Vinyl pads self adhesive 2 Press on heatsink 3 caution stickers 2 Aluminum shield 6U8A region 1 Aluminum shield input RCA jacks acrylic conformal coatin...

Page 34: ...module The power management module Each of these modules are described in the following sections 3 1 20 W Stereo Amplifier Module A printed wiring board PWB implementation is used that incorporates a...

Page 35: ...control GND Ground NC GND Ground NC The following terminals are quick disconnect posts GND GND Black Connects to GND terminal Rectifier PWB BIAS Bias supply input Slate Connects to BIAS terminal on A...

Page 36: ...ry lead from T51 NC Not connected Brown yellow Connects to brown yellow primary lead from T51 NC Not connected Black red Connects to black red primary lead from T51 NC Not connected White black Connec...

Page 37: ...37 20 W Stereo Audio Amplifier Figure 3 1 Component layout for the Amplifier PWB 1 right board...

Page 38: ...rom T51 6VH2 6 V ac heater connection Brown Connects to 6VH2 post on Amplifier PWB 1 right and 2 left 6VHct 6 V ac heater center tap Green yellow Connects to green yellow lead from T51 400V Plate volt...

Page 39: ...39 20 W Stereo Audio Amplifier Figure 3 2 Rectifier PWB component layout...

Page 40: ...k Connects to GND terminal Rectifier PWB BIAS Bias supply output Slate Connects to BIAS terminal Amplifier PWB 1 right B 350 V dc B supply input Red Connects to B terminal on rectifier PWB B Sense B v...

Page 41: ...ue Connects to blue lead from T2 SC1 Screen from V5 Blue yellow Connects to blue yellow lead from T2 PL2 Plate from V6 Brown Connects to brown lead from T2 SC2 Screen from V6 Brown yellow Connects to...

Page 42: ...42 WhitakerAudio Figure 3 3 Component layout for the Amplifier PWB 2 left board...

Page 43: ...ode Notes GND Chassis ground Black Connected to main ground point RYL Relay input voltage 35 V dc Yellow To front panel Speaker switch not used on 40 W amplifiers GND Ground Shield Circuit ground not...

Page 44: ...44 WhitakerAudio Figure 3 4 Layout of the speaker terminal PWB a right channel board b left channel board a b...

Page 45: ...mplifier PWB 2 ground 35Vdc DC power input Violet 40 V dc maximum voltage input Connects to switched V source on Power Management PWB B B high voltage input Red B from Amplifier 1 PWB typically 350 to...

Page 46: ...46 WhitakerAudio Figure 3 5 Layout of the preamplifier power supply PWB...

Page 47: ...ac line filter FL51 hot terminal Switch1 Front panel power switch Black Connects to front panel power switch SW51 Switch2 Front panel power switch Black Connects to front panel power switch SW51 Load...

Page 48: ...Figure 3 6 Enable Auto off enable Yellow Connects to front panel auto off switch SW202 Enable Auto off enable Yellow Connects to front panel auto off switch SW202 Table 3 6 Power Management Module PWB...

Page 49: ...nal on the power management PWB SW203 High temperature thermostat White Connects to SW203 on the chas sis SW203 High temperature thermostat White Connects to SW203 on the chas sis BiasSense Bias input...

Page 50: ...t expansion board is shown in Figure 3 7 Timer art Timer reset arm Yellow Connects to timer reset CONN201 pin 2 on power management PWB Timer NC Timer reset NC Not connected GND Ground Not connected T...

Page 51: ...51 20 W Stereo Audio Amplifier Figure 3 7 Component layout of the power management expansion PWB...

Page 52: ...rement bridge While this is perhaps overkill it is instructive to see the variation in component values which measurement before installation will reveal The resistor color code is shown in Figure 4 1...

Page 53: ...that will adversely impact the noise floor Efficiently making the socket connections to the PWB requires a bit of practice The recommended procedure is detailed in Section 5 Be certain to wear protec...

Page 54: ...t a time as this will reduce the possibility of component mix ups Each section heading that follows marks a logical start stop point in construction of the amplifier Do not rush through the project in...

Page 55: ...al begin populating the circuit boards As the packing envelopes are used set them aside for recycling Before beginning clean the soldering iron tip Install the quick disconnect male tabs using the pad...

Page 56: ...r of this component is typically larger than a common 1 2 watt resistor It may be necessary therefore to mount this component about 1 8 inch above the PWB in order to accommodate the leads Install R5...

Page 57: ...amage due to static electricity Note that the electrostatic protection packaging may not be recyclable Install D51 rectifier 2 A 1 kV This diode goes on the rectifier board Solder in place providing a...

Page 58: ...he 4 40 hardware provided Use a lockwasher on the screw head component side of the board Standoffs are positioned at the four corners of the PWB and on each side of each tube socket For the standoff o...

Page 59: ...he component side through the pad and into the socket soldering tab 2 Crimp in place see Figure 5 2b 3 From the component side pull the wire tight and solder in place 4 Trim the excess lead length 5 S...

Page 60: ...e heater connections Unscrew and remove the standoff adjacent to pins 4 and 5 on each socket This will facilitate access to the pins Open the Heater Circuit Wire package cut one 9 inch piece of 22 gre...

Page 61: ...not dress the wires against the PWB as this may increase hum in the output of the amplifier Trim the wire lengths as needed to comfortably reach V2 As before slip a piece of heat shrink tubing over e...

Page 62: ...ase of the socket Apply a stream of hot air to shrink the tubing over the pins Be careful to not use too much heat Replace the standoff removed from the V3 socket The screws should be finger tight Rem...

Page 63: ...er 0 1 pitch One 4 pin Molex housing header 0 156 pitch Speaker terminals PWB If desired a single light coat of acrylic conformal coating MG Chemicals 419C 340G may be applied to the foil side of the...

Page 64: ...64 WhitakerAudio a b Figure 5 3 Completed 20 W Stereo Amplifier PWB 1 module a amplifier component side b amplifier foil side c rectifier component side d rectifier foil side c d...

Page 65: ...n tip Install the quick disconnect male tabs using the pads provided on the board There are 25 positions in all Solder in place Note that the mounting pins fit tight into the board This is normal Use...

Page 66: ...roviding about 1 4 inch clearance above the board Install R53 270 k 2 W Solder in place providing about 1 4 inch clearance above the board Install R54 R56 47 k 2 W Solder in place providing about 1 4...

Page 67: ...clearance above the board Be certain to observe proper polarity Note that the banded end of the diode connects to the square pad on the PWB Install D53 zener 60 V 5 W Solder in place providing about 1...

Page 68: ...amped on the bottom side of the socket Pin 1 on the PWB is identified by a square pad Place the socket in the proper orientation over the PWB so it rests on the standoffs The top of the socket the sid...

Page 69: ...wire and one 9 inch piece of 22 brown wire Place one piece of heat shrink tubing over one end of each wire Crimp the green wire on pin 4 of V4 Solder in place When soldering make sure the heat shrink...

Page 70: ...p the brown wire through the heat shrink tubing on the lead already connected to pin 5 of V5 Route the wire to the pin and crimp in place Solder keeping the heat shrink tubing away from the hot wires...

Page 71: ...screws should be finger tight Remember to include the lockwasher on the component side of the board The heater leads should straddle the standoff Twist the wires together and route to the 6V heater pa...

Page 72: ...acrylic conformal coating MG Chemicals 419C 340G may be applied to the foil side of the amplifier PWB Use masking tape to cover the face of the tube sockets Assembly of the 20 W Stereo Amplifier PWB 2...

Page 73: ...73 20 W Stereo Audio Amplifier a Figure 5 4 Completed 20 W Stereo Amplifier PWB 2 Left Channel a component side b foil side b...

Page 74: ...ng the pads provided on the board There are 5 positions in all Solder in place Note that the mounting pins fit tight into the board This is normal Use a pair of pliers to hold the tab while inserting...

Page 75: ...dial Solder in place observing proper polarity Clean the soldering iron tip Install D301 D302 Zener 200 V 5 W Solder in place leaving about 1 4 inch clearance above the board Observe proper polarity N...

Page 76: ...ace of the lockwasher Tighten Find relay RYL301 DPDT 10 A 24 V dc Install in the RYL301 socket Find the auxiliary power connector hardware Set this package aside it will be used later to build the pre...

Page 77: ...77 20 W Stereo Audio Amplifier a b Figure 5 5 Completed preamplifier power supply PWBs a component side b foil side...

Page 78: ...ded on the board There are 13 positions in all Solder in place Note that the mounting pins fit tight into the board This is normal Use a pair of pliers to hold the tab while inserting the pins into th...

Page 79: ...sitive terminal is denoted by the square mounting pad Install C202 1000 pF 100 V Solder in place Install C203 0 047 F 100 V Solder in place Install C204 C205 1 000 F 50 V electrolytic Solder in place...

Page 80: ...VR204 Varistor 130 V ac Solder in place Clean the soldering iron tip Install Q201 Q202 2N494 2N491 NTE6400A Solder in place Be certain to properly align the leads of the device Be careful when solderi...

Page 81: ...short bare connecting wires from the terminals to the pads on the PWB Solder in place Tighten the set screw do not over tighten A method that works well for the above step is to first mount the fuse b...

Page 82: ...WhitakerAudio Assembly of the power management PWB has now been completed Double check your work The completed PWB is shown in Figure 5 6 Figure 5 6 Power management PWB a component side b foil side a...

Page 83: ...abs While not strictly necessary it is recommended that the flat side of the tab be oriented in the same direction for all posts Clean the soldering iron tip before moving on to the following steps In...

Page 84: ...te that the anode terminal is denoted by the square mounting pad This convention is used for all diodes Install D403 D406 D407 Rectifier diode 1 A 600 V Solder in place observing proper polarity Insta...

Page 85: ...facing the center portion of the PWB Prepare a two 2 inch 22 yellow wire Insert one end of the wire into an SW401 connection point on the board Solder Secure the other end of the wire to the arm term...

Page 86: ...86 WhitakerAudio Figure 5 7 Power management expansion PWB a component side b foil side a b...

Page 87: ...g the builder to proceed at a measured pace A full size image of the completed chassis bottom view is provided Refer to this image as you complete the steps that follow A few general comments on wirin...

Page 88: ...er provided on the shaft Note that one side of the mounting shaft has a key this key should face downward Insert the switch into the chassis and then fit the front panel onto the shaft Use one of the...

Page 89: ...and secure using the nut provided Do not tighten Note the connection terminals on the switch The center screw terminal should face toward the chassis bottom plate Mount pilot lamp PL51 as follows Thr...

Page 90: ...h Install the 35 mm jack in the available Headphone opening on the front panel Secure using the hardware provided Install knobs on the Volume potentiometers Using a small common screwdriver run the se...

Page 91: ...chassis so they can be used individually Tighten the lock tight nut Install two flat washers on the screw and then the wing nut Tighten the wing nut finger tight The finished installation is shown in...

Page 92: ...iliary power connector These wires should first be threaded through the sleeving and tubing Prepare a 10 inch long 22 black wire Attach a Molex terminal 0 156 pitch on one end and a female socket conn...

Page 93: ...he female socket to position 7 on the auxiliary power connector Leave the other end free it will be connected later Prepare an 11 75 inch long 22 blue wire Attach a Molex terminal 0 156 pitch on one e...

Page 94: ...power supply PWB The auxiliary power connector has now been installed Double check your work The back panel components have now been installed Figure 5 10 shows the back panel and top of the chassis a...

Page 95: ...cable clamp chassis lockwasher standoff chrome spacer flat washer screw head 6 32 x 3 4 The acrylic cover will be installed later Examine FL51 the AC line filter Note there are two chassis mounting t...

Page 96: ...5 in 0 2 in Cable clamp Cable clamp Right Line input Left Built in California USA Right Channel Ground 8 Ohms Left Channel 8 Ohms Ground Preamp Power Connector Molex 38720 3202 Molex 38720 3202 Neutri...

Page 97: ...sition so the wiring will rest against the chassis when secured in place Carefully place the right channel amplifier PWB in position from the underside of the chassis Install the six 4 40 x 3 8 inch m...

Page 98: ...hieve proper alignment After the socket is in position install the four screws for the four standoffs at the corners of the PWB Do not tighten the screws until all screws have been threaded and the so...

Page 99: ...om screw flat washer external tooth lockwasher transformer mounting tab chassis nut with captive lockwasher For the mounting position that includes the ground lug insert the lug between the mounting t...

Page 100: ...Hum Balance Control Mount the device from the top of the chassis and secure with the lockwasher and nut provided Position the device so that when at the center of travel the white dot on the knob poin...

Page 101: ...nd of the sleeving near the base of the transformer where the leads enter the chassis Stretch the sleeving to its full length Place one piece of heat shrink tubing on each end of the sleeving Process...

Page 102: ...ads enter the chassis Process with the heat gun Be careful to focus the heat on the heat shrink tubing not the leads This cable bundle will connect to the rectifier PWB Cut one 3 4 inch piece of large...

Page 103: ...onnect SC1 output transformer screen 1 connection Find the blue yellow wire from the right channel output transformer Install a connector to the end of the wire Attach to the SC1 post on the right cha...

Page 104: ...brown yellow wire from the left channel output transformer Install a connector to the end of the wire Attach to the SC2 post on the left channel PWB Connect T2ct output transformer center tap Find th...

Page 105: ...6VHct post on the rectifier PWB It is unlikely that the 5 V heater leads yellow yellow black yellow center tap will reach their respective terminal posts on the left channel PWB These leads will there...

Page 106: ...lead from the power transformer T51 Install a connector to the wire Attach to the Hot 4 post on the right channel PWB Connect Hot 5 AC hot to T51 Find the black lead from the power transformer T51 In...

Page 107: ...d from L51 check to see if the wire will reach the L51out terminal on the rectifier PWB If so install a connector to the wire If not use the procedure detailed previously to extend the wire Attach to...

Page 108: ...Connect to the B post on the right channel PWB Feed the free end of the red wire to the rectifier PWB through the cable clamps at the rear of the chassis Prepare a 27 inch long 18 black wire Attach a...

Page 109: ...from the B post on the left channel PWB Slip the three wires into a quick disconnect terminal and crimp Attach to the B terminal on the rectifier PWB Cut a 10 inch long 22 white wire Attach a quick di...

Page 110: ...the wire through the cable clamp in the rear portion of the chassis Prepare a 10 inch long 18 black wire Secure one end to the ground lug on the corner of the left channel PWB Solder 2 wires Route the...

Page 111: ...ge Cut a 1 inch piece of small heat shrink tubing Place over the splice point and process heat Observe the rear of the ac power connector There are three terminals The center terminal is ground as mar...

Page 112: ...ctor to each end of the wire Attach one end of the black wire to the line terminal on the ac power connector Attach the other end of the black wire to the available terminal of circuit breaker CB51 Se...

Page 113: ...te wire Attach a quick disconnect terminal to one end of the wire Attach to the N 4 AC neutral to PL51 post on the right channel PWB Route the other end of the wire toward the front panel It will be c...

Page 114: ...um Balance potentiometer will be connected Terminal numbering for the chassis mounted potentiometer is shown in Figure 5 13 Prepare two 16 inch long 22 wires one brown and one green Twist along the le...

Page 115: ...Balance control R55 terminal 2 Solder in place Process heat the tubing 5 6 11 Headphone Speaker Connections In the following steps the rear panel speaker terminals and the front panel headphone outpu...

Page 116: ...adjacent to the right channel PWB It will be connected later Prepare a 19 inch long 22 yellow wire Attach a quick disconnect terminal to one end Attach to the GND post on the left channel PWB that is...

Page 117: ...embly Find the speaker terminal PWBs Two are provided one for the right channel and one for the left channel Each board mounts on the speaker terminals for the respective channel Temporarily fit each...

Page 118: ...correct value Install R14 and R15 4 10 W on the right channel speaker PWB Mount so the devices are about 1 2 inch above the PWB Solder both devices in place Install R31 and R32 4 10 W on the left chan...

Page 119: ...the Conn6 position on the right channel speaker PWB Solder in place Mount the 4 terminal Molex header male 0 156 pitch at the Conn8 position on the left channel speaker PWB Solder in place The complet...

Page 120: ...running along the back panel where the left channel speaker PWB will be mounted Position the wire bundle about 1 2 inch away from the back panel Insert the left channel speaker PWB over the left chan...

Page 121: ...he terminals This can be done by a visual inspection of the socket or with an ohmmeter Note that The sleeve is the common ground connection The ring is the right channel connection The tip is the left...

Page 122: ...me basic tests be performed on the speaker headphone circuits before proceeding Connect a pair of stereo headphones to one of the front panel headphone jacks Place the headphones on your head making s...

Page 123: ...er management PWB on its side in order to make connections and later for maintenance if needed Attach the brackets to the front mounting posts adjacent to the power transformer leads Temporarily mount...

Page 124: ...rminal Molex connector Insert the other wire pin into position 2 on the connector Route the free end of the twisted pair to the power management expansion PWB using the available rear cable clamp Prep...

Page 125: ...nts Solder Twist the wires together Route toward the power management expansion PWB using the available rear cable clamp Attach quick disconnect terminals to the free ends of the wires They will be co...

Page 126: ...ch long 22 brown wires twisted Attach quick disconnect terminals to one end of each wire Connect the twisted pair to the SW203 posts on the power management PWB Route the free end of the wires to the...

Page 127: ...e power management PWB has now been completed Check to be sure that the board mounts flat on the standoffs It may be necessary to reorient the cable bundle under the board to achieve a proper fit 5 6...

Page 128: ...el speaker PWB An additional cable will be installed later on the connector Cut two short pieces of small heat shrink tubing each about 3 4 inch long Place the tubing at each end of the audio cable Pr...

Page 129: ...n the power management expansion PWB Attach a quick disconnect terminal and install on the 35V post Find the 22 black wire from the GND post on the power management PWB Trim the lead length as needed...

Page 130: ...o position 1 and white to position 2 Cut two short pieces of small heat shrink tubing each about 3 4 inch long Place the tubing at each end of the audio cable Process Insert the Molex socket into the...

Page 131: ...e for the following steps can be found in the Molex Housing and Terminals package Prepare a 5 inch long audio cable Attach connector pins to each of the two wires on one end of the audio cable Insert...

Page 132: ...e front panel using the cable clamps along side the preamplifier power supply PWB Connect the white wire of the 28 inch long audio cable to the high terminal 3 of the left channel Volume control Solde...

Page 133: ...operly align with the mounting holes on the chassis loosen and then re tighten the standoff s and try again Note that some of the connection terminals on the preamplifier power supply PWB slide in bet...

Page 134: ...provided for the input pentode triode stages V1 and V4 Installation of the shields is recommended but not strictly required The shields are available in three colors blue red and black Blue is provid...

Page 135: ...135 20 W Stereo Audio Amplifier Figure 5 15 Chassis view of the 20 W Stereo amplifier...

Page 136: ...WhitakerAudio a b c d Figure 5 16 Detail views of the 20 W Stereo Amplifier a preamplifier power supply PWB b power management PWB c right channel and rectifier boards d close up view of the front pa...

Page 137: ...of the ac cord It should be infinity with the power switch off and 4 k with the power switch on 5 Remove the ac power cord from the chassis 6 Using an ohmmeter confirm that the resistance values liste...

Page 138: ...This will permit the V2 V5 6973 1 100 k Varies due to filter capacitors in circuit 2 3 170 k Varies due to filter capacitors in circuit 4 40 k Heater 5 40 k Heater 6 170 k Varies due to filter capacit...

Page 139: ...fied here Long term operation with no tubes present other than V51 can result in component damage For reference the following series of events occurs at power on listed below in order Power from the f...

Page 140: ...eme caution check for proper voltages at the tube connection points listed in Table 6 2 All voltages are measured with respect to ground using a high impedance voltmeter Variations within 10 percent a...

Page 141: ...the output tube control grids as detailed in Table 6 2 and vary the position of the Bias Adjust control A voltage range of approximately 20 V dc to 50 V dc should be observed Set the bias voltage for...

Page 142: ...rtant that an oscilloscope be connected to the output terminals in addition to a speaker and perhaps other test equipment when the amplifier is first powered up It is possible that high frequency osci...

Page 143: ...See the cautions above for the right channel equivalent C3 26 If problems are experienced disconnect power and recheck the installation of all components particularly the input stage V1 of the amplifi...

Page 144: ...on is less than 2 A Table 6 3 Typical Voltmeter Readings readings within 10 can be considered normal Tube Pin No Typical Reading Notes V1 V4 6U8A 1 205 V 2 0 V 3 40 V 4 60 V Heater 5 60 V Heater 6 80...

Page 145: ...ier as detailed in the next section Note The amplifier should be operated only with its rated load applied Operation into an open circuit or short circuit load can result in damage to components 6 1 P...

Page 146: ...our screws from the power management PWB Using the L angle brackets mount the board at a right angle to the chassis deck using the available 6 hardware Be careful to not stress the board while mounted...

Page 147: ...end the auto off time period and shut down the amplifier Set the Delay Adj trimmer potentiometer for center of travel This should result in a time out period of about one hour which is a good starting...

Page 148: ...ished with the power management PWB in place One of the challenges in checking the operation of the auto off circuit is the long time delay needed to determine if the problem has been fixed about one...

Page 149: ...at depending on the gate trigger current tolerance the SCR will not fire after the voltage at the junction of R211 C207 D208 has reached 22 V In this condition the voltage at the gate of Q206 may not...

Page 150: ...The following steps should be performed to confirm proper construction and operation of the amplifier Connect speakers to the right and left channel outputs Remove any input source from the line inpu...

Page 151: ...the amplifier on Measure the dc voltage between pin 4 and ground It should be zero Measure the ac voltage between pins 5 and 6 It should be zero Measure the ac voltage between pins 2 and 3 It should...

Page 152: ...oad to each channel of the amplifier Apply power Connect an audio voltmeter to the right channel output terminals Connect a 1 kHz audio source to the right channel input RCA jack Set the Volume contro...

Page 153: ...me control fully counterclockwise Note that after a short delay the voltage on pin 2 will begin increasing Turn the left channel volume control clockwise to a a listenable level The voltage at pin 2 s...

Page 154: ...otentiometer R418 sets the trip point for the bias sense circuit With the amplifier operating normally at a bias voltage of 30 V slowly turn R418 counterclockwise until the amplifier shuts down Be ver...

Page 155: ...s power output levels e g 1 W 5 W and 10 W the rated output power of each channel It is important to note that each channel of the amplifier is capable of producing more than 10 W output into an 8 loa...

Page 156: ...Measurements taken at 1 W power output 2 8 V rms Input voltage 0 30 V rms Volume control fully clockwise Frequency response 1 dB 10 Hz to 65 kHz THD 0 8 Note 1 IMD at 2 3 V rms output 0 4 4 1 mix rat...

Page 157: ...ill indicate the rms value while responding to the average value of the signal In the case of the complex waveform of a two tone signal the meter will respond to the average value of that signal but t...

Page 158: ...e other cannot try swapping the input tubes If the problem moves with the tube then the device is defective and should be replaced It is possible that lowest noise is found at opposite ends of the Hum...

Page 159: ...it may be measured as follows Connect a suitable resistive load to each channel Set the Volume control fully clockwise Apply a 1 kHz signal at 1 0 V rms to the right channel input This will result in...

Page 160: ...he following observations can be made regarding typical square wave performance at various power levels and frequencies At 5 W power output a A fair amount of low frequency tilt is observed at low fre...

Page 161: ...to the right channel speaker output at minimum a distortion analyzer and oscilloscope Apply power to the amplifier With no audio input applied adjust the Bias supply for approximately 30V f b c d e a...

Page 162: ...that distortion on either side of this point increases albeit only slightly Be careful when adjusting bias since lower settings less negative will increase the idling current For test purposes setting...

Page 163: ...s on a given channel can be caused by a variety of issues in addition to the operating bias point If high distortion figures are encountered first try replacing the output tubes and see if that solves...

Page 164: ...from the amplifier The power bandwidth curves are produced by driving the input signal to each channel at an increasing level until a specified distortion point is reached in this case 1 5 over a fre...

Page 165: ...ure 7 2 Frequency response over the range of 20 Hz to 20 kHz for both channels at rated power 10 W per channel WhitakerAudio 08 11 15 15 50 59 THD N Ratio vs FREQUENCY 0 05 1 0 1 0 15 0 2 0 25 0 3 0 3...

Page 166: ...nction of drive level for both channels The extreme right end of the chart 1 V rms represents the rated output of the amplifier 10 W per channel WhitakerAudio 08 11 15 17 30 52 AMPLIFIER POWER BANDWID...

Page 167: ...y room temperature The power transformer runs warm but not too hot to touch The vacuum tubes are of course quite hot and are capable of inflicting burns on the skin No other areas of the amplifier sho...

Page 168: ...t by creating a chimney effect that collects cool air from the edges of the chassis and exhausts heated air from around the tubes and transformers Do not over tighten the mounting screws If the bottom...

Page 169: ...s that led to the problem Then the cause of the failure not just the more obvious symptoms will be corrected In high voltage power supplies look for signs of loose connections and overheating Do not o...

Page 170: ...or other protective eye wear when building or working on electronic equipment Injuries can result from solder splashes cleaning chemicals and other unexpected events When trimming the leads of a comp...

Page 171: ...act pinout is given in Figure 10 1 In phase splitter circuits the pentode unit should drive the triode unit The 6GH8A is an acceptable substitute Selected tube parameters are listed in Table 10 1 Tabl...

Page 172: ...to be used Plate to cathode heater grid 2 grid 3 internal shield 2 6 pF Direct interelectrode capacitance approx other Triode cathode to heater 3 pF Pentode cathode pentode grid 2 internal shield 3 p...

Page 173: ...tube used in high fidelity audio equipment The device pinout is given in Figure 10 3 Operating characteristics are charted in Figure 10 4 2 2 2 2 2 c cc c g c E E E R P This curve also applies to typ...

Page 174: ...t hottest point 250 C Push Pull Class AB1 Amplifier with Grid 2 of Each Tube Connected to Output Transformer Tap Design Maximum Values Plate and grid 2 supply voltage 410V Plate dissipation 12 W Grid...

Page 175: ...control grid voltage 33 V Peak AF grid 1 to grid 1 voltage 67 V Zero signal cathode current 62 mA Maximum signal cathode current 95 mA Effective load resistance plate to plate 12 5 k Maximum signal o...

Page 176: ...ilter 1 H Typical Operation with Capacitor Input to Filter AC plate to plate supply voltage 700 V rms Filter input capacitor 50 F Total effective plate supply impedance per plate 240 Average output cu...

Page 177: ...covers all parts and labor There are a few limitations which are detailed below 1 Because vacuum tubes have a limited lifetime the warranty covers tubes for the first 2 years following shipment 2 Whit...

Page 178: ...normally run hot Do not touch these devices as serious burns may result Other components may normally run hot notably the power transformer 3 This product is intended for private consumer audio appli...

Page 179: ......

Page 180: ...20 W Stereo Amplifier WhitakerAudio 20 W Stereo Audio Amplifier...

Page 181: ...ed method of cutting the tubing is to use a so called heat knife If such a tool is unavailable scissors may be used Examine the connector body The pin numbers are stamped on both sides of the device B...

Page 182: ...ors into a matching housing as follows Black wire to pin 1 White wire to pin 2 White wire to pin 3 Red wire to pin 4 Green wire to pin 5 Green wire to pin 6 Blue wire to pin 7 Blue wire to pin 8 Black...

Page 183: ...een the connectors is not necessary Insert the female connectors into a matching housing as follows Black wire to pin 1 White wire to pin 2 White wire to pin 3 Red wire to pin 4 Green wire to pin 5 Gr...

Page 184: ...184 WhitakerAudio Figure 1 1 Completed cable assembly...

Page 185: ...ereo Amplifier PWB 2 Left Channel components Preamplifier Power Supply Module components Power Management Module components Power Management Expansion components Chassis components Parts are organized...

Page 186: ...mplifier PWB 1 right R8 3 9 k 2 W Amplifier PWB 1 right Tube socket hookup bare wire heat shrink tie wrap Amplifier PWB 1 right C54 10 F 100 V electrolytic Amplifier PWB 1 right R9 R10 100 k 0 5 W 1 A...

Page 187: ...20 W Stereo Amplifier D1 1 A 600 V diode Amplifier PWB 1 right RYL1 DPDT 10A 24 V dc relay with socket Amplifier PWB 1 right Table 1 The 20 W Stereo Amplifier PWB 1 Right Channel Component Package Lab...

Page 188: ...c Amplifier PWB 2 left R25 R58 3 9 k 2 W Amplifier PWB 2 left inch threaded 4 standoff quantity 10 Amplifier PWB 2 left C58 C59 10 F 100 V electrolytic Amplifier PWB 2 left R26 R27 100 k 0 5 W 1 Ampli...

Page 189: ...er Amplifier PWB 2 left R69 150 0 5 W Amplifier PWB 2 left Conn8 4 terminal Molex connector pkg Amplifier PWB 2 left R68 270 2 W Amplifier PWB 2 left RYL2 DPDT 10A 24 V dc relay with socket Amplifier...

Page 190: ...Power Supply Module R304 1 k 0 5 W Preamp Power Supply Module F301 0 1 A slo blow fuse and fuse holder Preamp Power Supply Module 3 8 inch threaded 4 standoff quantity 7 Preamp Power supply Module R30...

Page 191: ...t Module T201 Transformer PC mount 117 V ac primary 24 V ac secondary Power Management Module R208 1 k 0 5 W Power Management Module RYL201 RYL203 RYL205 DPDT 10A 24 V dc relay with sockets Power Mana...

Page 192: ...erAudio SW203 High temp thermostat Power Management Module R217 180 k 0 5 W Power Management Module Press on heatsink quantity 2 Power Management Module Table 4 Power Management Module Component Packa...

Page 193: ...Power Management Expansion C406 100 F 100 V electrolytic Power Management Expansion R405 R411 820 k 0 5 W Power Management Expansion Q401 Q402 Q403 Q405 Q406 2N3053 NTE128 Power Management Expansion C...

Page 194: ...Power switch SPST pushbutton Chassis Components Front panel 19 in x 3 5 in Chassis Components Large knob quantity 2 Chassis Components SW52 Speaker switch SPST toggle Chassis Components Plexiglas tri...

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