What is the Hoffman AB763 reverb combo circuit? |
| Layout diagrams, schematics and BOM (Bill of materials) for the Hoffman AB763 boards are listed on this page I have parts list for many different Hoffman Turret board amps listed on this page Hoffman Turret Boards are listed on this page The Fender AB763 circuit is not just one amplifier. It can be made into several different amplifiers by just using a different transformer set, power tubes and speakers. What is the difference between a Deluxe reverb, Vibrolux reverb, Bandmaster reverb, Pro reverb, Super reverb, Twin reverb and Dual Showman Reverb? About the only difference is the transformers, the power tubes and the cabinet/speaker box. There are a few minor differences on the circuit boards themselves but it is not a different circuit. One model may have a .022 capacitor and another model may have a .047 capacitor in the exact same spot on the circuit board. The differences between any of the models mentioned above are very few if any. The only reverb combo amp that is not the same as the AB763 circuit is a Princeton Reverb. The Princeton Reverb is not the same circuit as the standard AB763 circuit. A Deluxe Reverb has no mid pots, but other amps do. Some amps have bright switches, some don't. The power and output transformer will be different on some of the models. The Hoffman AB763 turret board does have one major difference from the original Fender AB763 circuit. The Hoffman AB763 turret board uses the Tremolo section from the Fender 6G16 Vibroverb. This is not an opto isolator tremolo. This 6G16 tremolo circuit pulses the power tube bias voltage up and down. This tremolo method sounds way better than the opto tremolo to me. It is a very smooth tremolo. You can look at the 6G16 Vibroverb on my schematics library page to see the differences between the 6G16 tremolo and the opto isolator tremolo found on most Fenders. Most people like the 6G16 tremolo way better than the Opto Tremolo. Hoffman AB763 Part list build Notes: Make sure you look at all the documents for the boards on this page Parts shown in Blue on the Hoffman AB763 PDF file vary for each model. Parts shown in Green are not on the AB763 Short Board The AB763 short board is shorter because it has these items removed Bias circuit - Screen grid Resistors - One ohm cathode resistors - 100 ohm heater resistors Use the stand alone AB763 bias board with the AB763 short board The Bias boards has the bias circuit and the two 100 ohm heater CT resistors The screen grid resistors get mounted to the power tubes like in Vintage Fenders The short board may be better if you have a short chassis These items are all on the Turret Board catalog page See the PDF BOM file for notes on all the AB763 models to see the parts differences between the models. The Hoffman AB763 Bill of Materials BOM file is located here. On the Hoffman AB763 board, I have separated the pre amp tube cathodes so you can voice each tube half differently :) Nornally, two tube halves share a common 820 ohm resistor on the original Fender schematic I have separated the cathodes and use two 1.5k resistors and each 1.5k has it's own bypass capacitor. Bright Switches are used on some models. Some people don't like or need the bright switches. Leave them off if you don't want them 120p bright caps - Use these only if you are using bright switches. 10ka Pots are the Mid pots used on some models, they are not used on all models. They can be added as an option or deleted. 6.8k - 1/2 watt Resistor - Use these resistors if you are not using Mid pots Fender Knobs - The quantity depends on amp model and if you are using Mid pots. Slo-blo fuse size - The size depends the amp model. Power transformer - Depends on amp model. Output transformer - Depends on amp model. Choke - Depends on amp model Rectifier tube - Type can vary with the amp model and is also a personal choice. Twin reverbs and Dual showman reverbs have diode rectifiers Power tubes - Type and Quantity vary with each amp model. 6V6 or 6L6. Quantities can be 2 or 4. 4 power tube amps - Twin Reverb and Dual Showman Reverb notes Twin Reverb and a Dual Showman reverb amps have two more power tubes and so you need two more of everything that connects to these tubes Two more 1.5k - 1/2 watt power tube input grid Resistors. Two more 470ohm - 3 watt screen grid resistors. Two more one ohm cathode bias checking resistors Two more power tubes A Twin reverb has 4 power tubes and a Super Reverb has two power tubes but they are the same AB763 circuit #12A input Jacks - You don't have to have two input jacks for each channel. You can just use one input jack. See the common hookups PDF file for more info. #12A or 11A Speaker Jacks - You don't have to have two speaker output jacks. You can only have one if that's all you need. Reverb cables - You can make your own or buy pre made RCA cables. If you make your own you can use RG174 cable and RCA plugs. Hookup Wire - The parts list below includes PVC jacket wire and Bus wire to lace up the turret lugs. Use whatever type of wire you like and however much you think you may need. Resistors - Use carbon comp or metal film, it's your choice. The parts list all include 1% metal film reistsors SPDT carling ground switch - You don't need this switch if you are using a 3 prong grounded power cord. You don't need the .047 death cap either (shown on Fender layout diagrams) 22/500v, 20/500v, 16/475v and 100/350V Filter Caps - See this page for different ways to configure filter caps You can use 5 x 22/500v or 20/500v caps in parallel, instead of configuring the 2 x 100/350v caps in series If you decide to configure the 2 x 100uf/350v caps is series, you will also need 2 x 220k 3 watt resistors. The AB763 parts list does not include a filter cap circuit board. I prefer to install the filter caps inside the chassis along the front edge, under the pots and not use an outboard dog house cap can. I sell filter cap boards on my eyelet board page if you want to use a filter cap board. You will have to also add a cap can to cover that board. I stock several sizes of cap cans in my parts catalog. Another option is to mount the filter caps on terminal strips inside the chassis. Filter cap boards are on my eyelet board page here Amp channel notes - You can eliminate channel one, the non reverb channel and just have the reverb channel. If you do this, you can eliminate all the parts that go to channel #1. I have a AB763 One Channel Parts list. All the parts for V1 on the two channel layout diagram have been removed. Below is what has been removed from the two channel parts list to make a one channel parts list. All the caps and resistors on the board for V1 1 -12AX7 4 - Pots for V1 2 - 12A input jacks and the 3 resistors on the jacks 2 - 220k mixing resistors where the channels were combined and went to V6 Make sure you look at the Hoffman Turret Board installation instructions here. My library page is here with all the links to my Hoffman Turret boards |
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