Product Catalogue

Fender Blues Junior MK1-4 Tube Board with bias test points

  • Model: FBJ_MK1-4
  • Shipping Weight: 0.2Kg
  • 7 Units in Stock


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Note there are different versions for Blues Junior MK 1,2, 3 & 4 early V1 Blues Juniors. The MK1, 2 & 3 have a cream main board. The MK4 has a green main board.  The early V1 type also has a green main board but has the power indicator wires going to the tube PCB so take a look at yours to see. Make sure you select the correct version.

This version is for the basic MK 1 to 4 and features bias current measurement test points.

We have other variants:

  • Premium MK 1 to 4 with bias test points and gold sockets and bias adjustment mod kit
  • MK 1 to 4 with bias test points and bias trimmer on board
  • MK 1 to 4 with intelligent auto bias and protection.
  • Basic Early MK1 "green board" with bias test points


The original manufacturer's PCB is single sided, i.e. has copper on the solder side only. The EL84 tubes run very hot in these and this weakens the glue that attaches the copper foil to the substrate material. The original substrate does not appear to be FR4 glass fibre but possibly an inferior composite such as SRPB. When the glue fails the heavy tube & socket is free to vibrate and soon the attached copper track fails due to metal fatigue. You hear crackles, popping and/or a loss of power or you can even get arcing and completely destroyed boards.  When that happens the PCB needs replacement.  In the pics above you can see examples of arcing failures due to insufficient conductors clearance.

The replacement PCBs are designed to drop in and are made from a reinforced glass fibre substrate FR4. They also use a double-sided copper foil with plated through holes. This overcomes the glue failure issue.

  •     Durable Glass fibre reinforced FR4 material
  •     Prevents glue failure issues by using double-sided PCB with plated through holes with generous pad sizes.
  •     Prevents arcing due to careful attention to creepage and clearance distances
  •     Drop in replacement is cheaper and much easier to wire in than hand-wired metal plate alternatives
  •     Bias current measurement test points make tube replacement easier 1mV/mA
  •     Just remove the old PCB, unsolder the wires and attach to the new one.The board is preassembled with valve sockets and resistors. (You will need to solder on the trimmer (supplied) for the bias-on-board version yourself. This to to avoid the risk of damage during shipping.)
  •     No need to remove the main circuit board.
  •     Heatproof ceramic tube sockets
  •     Much faster to upgrade than hand-wired solutions as the wiring has been done for you
  •     Better than uncertain hand-wiring as the location of 'wires' is fixed and optimised for you giving repeatable results. Greatly eliminates the chance of expensive miswiring errors
  •     Incorporates resistors for measurement of bias current on each EL84
  •     Oversized holes to make re-wiring easier
  •     Heater tracks optimised to reduce hum. No hand wiring of heater wires needed
  •     Instructions included. Anyone with decent soldering/desoldering skills can do this.
  •     Increase/preserve the value of your amp

There are lethal voltages inside these amplifiers. Always exercise extreme caution to avoid the risk of electric shock. If in any doubt refer to a qualified technician. Fitting and use of these parts is entirely at your own risk. No exceptions.


This product was added to our catalog on Saturday 16 May, 2020.

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