Build an AB763 Blackface Amplifier
By Rob Robinette
The July '63 (763) Fender amplifier design is considered by most as the high water mark of Fender amplifier design.
Which AB763 to Build?
Take a look at the AB763 Model Differences page to get a feel for which amp to build. My personal favorite is the Vibroverb because it has three preamp gain stages, reverb, high preamp voltage to drive the power tubes harder, a soft tube rectifier for more playing dynamics, a match-more-external-speakers 8 ohm output, smaller output transformer for more compression and sustain and the bold tone of a 15 inch speaker. It's also in high demand in the used and clone amp market.
You can make your build a lot simpler if you go with one of the non-reverb models such as the Deluxe, Vibrolux, Band-Master or Showman.
You also have the option of building a head cab from any of these models--how sexy would a Vibroverb head be? Keep in mind the reverb amps need a taller head cab to give enough separation between the reverb pan and power transformer.
AB763 Model Overview
Notes: Amps with reverb have 3 preamp gain stages in the Vibrato channel with slightly more gain and preamp distortion. The Super Reverb has a higher rated output transformer than the other 2 x 6L6 amps. *The 5AR4 rectifier is a GZ34 equivalent. *2017 inflation adjusted prices were calculated using the Bureau of Labor Statistics CPI Inflation Calculator.
The circuit board differences between the models are actually very limited. The Deluxe and Deluxe Reverb's doghouse cap value, power resistor and wiring differ from the rest of the AB763 amps. The tone stack mid cap is .022uF in the Super Reverb and Concert, and .047uF in all the other amps. Some silverface amps use a .033uF which is a nice compromise value. A smaller mid cap slightly reduces the mid scoop for an audible gain boost and also shifts the scoop a little higher in frequency.
The multi-speaker amps, the Tremolux, Band-Master, Concert, Super Reverb, Dual Showman and Twin Reverb use a "heavy" negative feedback (NFB) circuit compared to the other AB763 amps' "light" circuit. These multi-speaker amps used 2 and 4 ohm output transformer secondaries so the "heavy" NFB circuit compensated for the lower secondary voltages. The feedback circuit is made up of two NFB resistors that form a voltage divider. The "heavy" NFB circuit uses an 820 / 100 ohm divider while the others use a "light" 820 / 47 ohm circuit. The 820 / 100 "heavy" NFB circuit attenuates the output transformer secondary signal voltage by 89%. The 820 / 47 "light" NFB circuit cuts the signal by 95%. I'm a fan of lighter NFB so I typically use a 47 ohm NFB Tail resistor.
The head cab AB763's, the Tremolux, Band-Master and Showman, use 50% smaller 500pF coupling caps leading to the phase inverter compared to the other amps' 1000pF (.001uF). This was done to reduce bass frequencies into the Fender closed back extension cabs which exhibit extended bass response compared to open back combo cabs. The smaller cap also tends to tighten up the overdrive tone.
Some models have Mid tone controls in one or both channels and Bright switches. In models without Bright switches a Bright cap is placed on the Vibrato channel Volume pot.
Some models use a solid state rectifier versus a tube rectifier. The power and output transformers also differ between the AB763 models.
Deviations From the Original 1963 Layout
You must use a 3-prong power cord. The power cord safety ground wire should be bolted to the chassis using a dedicated bolt with some type of lock nut to keep it secure. Leave extra length of this wire so if the power cord is accidentally pulled out of the chassis it should be the last wire to disconnect.
For safety reasons you should delete the ground switch. To delete the switch just disconnect the hot and neutral wires. If you are dead set on keeping the Ground Switch and "death cap" then replace the cap with a Class Y safety capacitor that is designed to run from the mains to ground. This $1 Kemet 900 Series .01uF 400VDC 250VAC Class Y safety capacitor is what you want for mains voltage up to 250 volts AC. The Ground Switch itself is a 3-way ON-OFF-ON SPDT. If you do keep the Ground Switch then power it from the fuse out terminal. This will add fuse protection for a death cap short.
I recommend using a double post single throw (DPST) switch for the power switch as shown in the layout above so you can switch both the hot and neutral mains lines. This Carling DPST is perfect for this. This switching method meets the current required worldwide safety standard for power switches.
Fender used a complicated split power/preamp ground scheme with many ground connections from the circuit board to a brass plate mounted between the chassis and control panel. I recommend the split ground bus scheme shown in the above layout.
The power cord hot (black) wire should run to the AC Receptacle first, then to the Fuse center terminal. You don't want to power the AC Receptacle with fused power because you could blow the amp's main fuse when plugging in something that uses more than two amps of power. Some people chose to leave the AC Receptacle disconnected. You can replace the USA 2-prong receptacle with a 3-prong 160-2-N receptacle to allow the use of modern accessories with a 3-prong power cord. See the above layout diagram for power cord wiring details.
3-Prong 160-2-N Receptacle
This is an easy bolt-in replacement available here.
The layout above has higher watt rated resistors for the power tube screens and doghouse voltage dropping resistors (they were all 1 watt originally). This has no effect on tone but will help with longevity.
I also highly recommend using 2 watt resistors for all plate resistors. I took a thermal image of a 5F6A Bassman chassis and all the 100k 1/2 watt plate resistors glowed very hot. Again this will have no effect on tone but will reduce resistor hiss noise and extend longevity.
Consider using metal film resistors throughout the amp except for the doghouse and power tube screen resistors. Metal film resistors generate 1/10th the resistor hiss of traditional carbon composition resistors. You can keep the amp quiet but add some mojo by using carbon comp resistors just for the phase inverter plates.
Upgrade the first two filter caps in the doghouse from 70uF 350 volt to 100uF 350v (or higher volt rating). This will reduce hum and slightly tighten up the bottom end.
Upgrade the bias filter cap from 25uF to 100uF 100v. This will not affect the amp's tone but it may decrease hum and increase longevity.
The AB763 Concert, Pro, Vibrolux and Vibroverb use the 125A7A 4k:8 ohm transformer. I recommend this transformer for most AB763 builds with one speaker.
In many silverface amps Fender used shielded cable for the runs from the input jacks to the tubes and from the volume pots to the tubes to reduce noise and hum. RG174 is my preferred cable for amp use.
Consider this list of my favorite AB763 mods:
robrob Normal Channel Reverb Mod to add reverb to the normal channel. This mod also puts the two channels in phase so you can jumper them for a thicker tone.
Tremolo Cut Switch Mod Add a lot of preamp gain with this simple and very useful mod.
Slow the Tremolo Slow is good.
Reduce Reverb Noise & Oscillation Simple mod to clean up the reverb.
Browse through other AB763 mods.
For more info on actually building an amp see How to Build a Tube Amp.
Fender Modern Hand Wired 68 Custom Deluxe Reverb
This is how Fender does a modern hand wired AB763 amp. Note the use of quiet metal film resistors except the V3 2.2k cathode resistor. The small board at far left adds a soft start thermistor and Class X RFI noise cap to the mains line. Artificial 6.3v heater center tap resistors are connected to the pilot light at far upper left. The Power switch switches both the mains hot and neutral lines. The Ground Switch and AC Receptacle have been deleted.
Very Clean Original AB763
Note the brass grounding plate at top with multiple bare ground wires running between it and the circuit board, bundled and twisted wires running from the control panel and doghouse and the multitude of chassis grounds. There are also chassis grounds on the rear panel Ground Switch and on the Reverb Out jack.
Just a Few of the Chassis Grounds
Original Fender Super Reverb Schematic
The Super Reverb and Concert are the only AB763s with .022uF middle tone caps. Note the Middle tone control on the Vibrato Channel (highlighted center-left).
Original Fender Super Reverb Layout
Annotated AB763 Schematic With Component Function
Deluxe Reverb Layout
Note the differences in the doghouse board at upper left. The first two filter caps (B+1) in the Deluxe Reverb are wired in parallel. The Deluxe has only one 16uF B+1 cap. All the other AB763 amps connect the first two filter caps (B+1) in series. The Voltage Dropping resistors are also different. Click the image for the hi-res layout. Download the pdf here and download the DIYLC file here.
By Rob Robinette
RCA Corporation, RCA Receiving Tube Manual, RC30.
Merlin Blencowe, Designing Tube Preamps for Guitar and Bass, 2nd Edition. This is my personal favorite tube amp book.
Merlin Blencowe, Designing High-Fidelity Tube Preamps
Morgan Jones, Valve Amplifiers, 4th Edition.
Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Preamps, 2nd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Power Amps
Robert C. Megantz, Design and Construction of Tube Guitar Amplifiers
Neumann & Irving, Guitar Amplifier Overdrive, A Visual Tour It's fairly technical but it's the only book written specifically about guitar amplifier overdrive. It includes many graphs to help make the material easier to understand.