Fender Blues Junior Ultimate Repair Kit


Available on back-order


After the venerable Hot Rod, Fender Blues Junior repairs are the second most common jobs on my bench. I probably see about 1 a month. The most common issue by far is the bias issue and the knock on effects it causes. The second most common issue is filter capacitor failure.

PLEASE NOTE: these items will often appear as ‘available for back order’. I hold a small stock level for this site and a large stock level for my working repair shop, where I see many FBJs each month. I usually do have stock in the repair workshop that can be transferred to fulfil your order.  Back ordered items will usually ship within a week.


1. Bias issue:

This is the most important issue to fix, also causing issues 3 and 4. Sometimes causes fires. Yay! You can buy this board individually here: https://www.rsdsound.co.uk/product/fender-blues-junior-bias-mod-kit/

I’ve seen stock units with 130% bias, which is frankly insane. You can find out more about the bias mod on the kit’s own product page.

2. Filter capacitors (caps).


Odd sounding harmonics, ring mod type sounds, Crackles, Sound cutting out, Humming, Loss of Power, Nasty Distortion.


Fender use unreliable IC TTAA filter caps which have a tendency to fail. This fault is overstated by some forums and social media users, but the problem is real. I always offer the option of replacement as it’s worth doing whilst the amp is open to make the amp as reliable as possible, but in some cases you may prefer to decline to replace unless they’re the root cause of the problem.

3. Valve board failure

The bias issue causes the EL84s to overheat, delaminating the pads. Sometimes it causes a fire if the grid goes open circuit, just for fun!

This board was designed by my friend Brad Webb of Brad’s Guitar Garage in Sydney, Australia. It’s a great product with through hole plating, sensibly laid out heater traces, and good separation of high voltage and low voltages traces (*cough cough*, are you listening Fender? *cough*). You can usually re-use the old valve bases. If not, I’d recommend Belton PT9 bases, which I don’t currently sell. Brad also sells this product individually here: www.bradsguitargarage.com.au

4. Screen Grid failure

It’s a great idea to increase the 100 ohm resistors (R35, R36) to 1K resistors to reduce the stress on those poor EL84s with no detriment to tone.

The untimate Blues Junior repair kit includes:

Instructions and Safety

  • No instructions are provided for this repair kit and no installation support will be provided. However:
    • View the bias kit installation video below.
    • I’d recommend looking  at the Hot Rod Instructions for tips on how to install the cap adapters. Builders of the  Blues Junior package should be able to understand what to do for their amps by looking at the Deluxe instructions and the appropriate schematics.
    • The screen grid resistors and valve replacement board should be self explanatory
  • This repair involves working on a unit that can contain hazardous voltages up to 350VDC. These hazardous voltages can still be present when the amplifier is turned off.
  • DO NOT attempt this repair unless you’re absolutely certain you can do so safely and produce a safe working product at the end.
  • At Keld Ampworks I can fix your amplifier reliably and safely. I have repaired countless Fender Blues Junior amps and I’ve never seen one I couldn’t fix (even the ones that had fires!). I would be happy to take this job on for you. Unmodified amps are simpler to repair.


Additional information

Dimensions 350 × 250 × 20 mm

Hot Rod Deluxe or Blues Deluxe RI, Hot Rod Deville or Blues Deville


LT Supply / Footswitch only, Filter caps only, Fix everything!


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