Hoffman Blues Junior Conversion

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Hoffman Pro Junior Conversion #1
A customer sent me his Fender Pro junior to do a total makeover with a hand wired turret board He had a feedback switch installed by a tech Your chassis would not have that switch shown in the images below Note: I have made some changes to the pro junior Turret Board shown below after doing this build The pictures below are not the build instructions
Note that Hoffman boards are not assembled by looking at my builds. I do test builds first and change the designs based on those builds. You assemble Hoffman boards using the documents listed below. The Layout Diagram, Schematic and BOM for this board are on this page
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Here's the stripped out guts of a pro Junior" Note that the EL84 sockets are soldered in and screwed to the chassis. The only way to remove them is to unsolder the wires on the back of the socket board Then pry off the sockets from the tube board
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Here's the bare chassis with the boards, post and jacks removed The main board ground is that metal standoff I had to grab it with channel locks and wiggle it to remove it It is too tall and in the wrong place for my board It's easier to work on the chassis by removing the Power and Output transformer Remove both of them before you continue
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The chassis holes are big enough to mount 9 pin sockets directly to the chassis. 9 pin Belton sockets fit into the chassis holes the best. I had to expand or create mounting holes for the screws. I used a Dremel tool with a cutoff disk to make the screw slots The screw holes were too close to the chassis holes to drill The drill bit would just slip into the big hole and not drill I used the existing power tube holes and expanded them I had to create the two pre amp tube mounting slots
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Here are the 9 pin Belton sockets mounted in the chassis
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I created the board layout in DIY Layout creator Then I printed the layout at 100% scale I laid the paper template in the chassis to make sure the board would fit Note that the power switch limits the board length to 8.5 inches
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The board was drilled on my CNC machine using the DIY layout creator layout
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I drilled four 3/16 inch holes in the board for mounting hardware You always have to look at the back of the chassis to make sure there is nothing in the way of the mounting hardware Here is a test fit of the drilled board Take a pencil and draw circles inside the four 3/16 inch holes This is where you will drill mounting holes in the chassis Use a center punch inside the pencil circles to make drilling easier
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Here is the board with the turret lugs installed
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Here is the board with the lugs laced together
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Here is the board with the rear jumper wires installed
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The stock plastic Output transformer grommets were too high Use two 3/8 inch grommets for the output transformer holes You can tape the wires down to the chassis in the direction they should go It's not a bad idea to use electrical tape on the wires Just in case you apply too much heat to the turret lug you do not want the wires to touch the under side of the board and melt the jackets
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Here is the board screwed into place using the mounting hardware
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Here is the board screwed into place using the mounting hardware I have also installed the pots and input jack
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I have installed the pot bus wire and some parts I used my Dremel tool cutoff disk to clean off a small area where the bus wire gets soldered to the pots. You will not be able to solder to the pots very easily unless you do this The bus wire goes over to the ground lug and switch lug on the input jack
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I soldered the output transformer wires to the power tube sockets I soldered the speaker jack wires also
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Hookup wires have been installed I also added a 3 lug terminal strip for the heater connections It is difficult to see the White/Green Heater wires They are tucked up against the top lip of the chassis They are not run as a twisted pair They are run just like they were on the PC board There is no problem running your heater wires this way Make sure you install tubes into the sockets before wiring up the sockets Tubes in the sockets align all the pins properly If you force the pins into non perpendicular positions, it may be difficult to insert the tubes later
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More wiring to the front panel area Note that the white wire going to the tone pot was replaced by a RG174 shielded cable I found that this made a difference in the amp noise level I changed the wire design after running the amp the first time You can see this wire replacement in the photo below
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After all the wires were run, I added the parts to the board Another design changed happened here The tone pot was too close to the bridge rectifier The hum level from the AC wires going to the bridge was very noticeable I ended up having to move the bridge rectifier You can see that I made a small board for the bridge I suspended the bridge board from a 1/4 inch standoff The threaded standoff is screwed onto one of the power transformer screws The center screw going through the bridge is screwed into the threaded standoff At this point I realized I no longer had a mains fuse I installed a standalone fuse holder on two of the power transformer lugs I used 1/2 inch threaded standoffs and screws to secure it in place A 2 amp slo-blo fuse was installed in the fuse holder The updated Pro JR. turret board will have these two changes on it The bridge and fuse holder are moved to the right side of the board Far away from the tone pot You can see the before and after DIYLC layouts down below
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Another close up shot of the finished amp
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Another close up shot of the finished amp
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The Pro Junior layout file, BOM file and build info is on the library page here Go here for Pro Junior Build documents
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