Fender Blues Jr. III Repair / Mods
October 2020
Here is another small Fender amp that we worked on. This is a Fender Blues Jr. III that came into the shop sounding horrible.
Fender Blues Jr. III
Turned out to be a simple issue - one of the power tubes was actually physically broken. Substituting in a used pair of EL83 seemed to bring the amp back to life. A decision was made to replace all of the tubes with a set of gold pin JJs.
Power Tubes - glass tube on one cracked at base
While the amp was in the shop, the customer decided to have us install a pair of mods from Fromel (fromelelectronics.com), which, basically, swaps out some parts for some better quality versions (more of an upgrade than a mod). This includes all of the power supply filter capacitors, the preamp cathode bias capacitors, several of the capacitors involved in the tone stack, as well as the input jack (the stock input jack is plastic, and is prone to be broken if the input cable is ever yanked on - the replacement is a standard metal Switchcraft 1/4" jack).
The second mod (also from Fromel) was a replacement power switch (this is a mod). The stock switch is a simple 'On / Off' switch, which means that the tubes get the full plate voltage immediately, before they warm up, which can shorten the life of the tubes. The mod incorporates a different kind of power switch switch - a 2 Pole, three position, 'progressive' toggle switch. Sort of an 'Off / Partial On / On' switch. Down is Off, middle is one pole On, and Up is both poles on. Thus, Down is 'Off', Middle is 'Standby' (power to transformer, but plate voltage still disconnected), and Up is 'On' (power and plate voltage both on). Interestingly, the circuit board on this amp has all of the terminals already present to handle this type of switch, so it is a simple swap
The following basically describes the mod process, which is pretty straight forward on this amp. The first step for these mods it to remove the circuit board from the chassis in order to get at the solder side of the board
Circuit board removed from chassis, showing parts to be replaced.
Filter Caps removed, ready for replacements.
Cathode Caps removed, ready for replacements.
Once all of the parts to be replaced have been removed, the replacements are soldered in place. The new input jack mounts to the chassis and is wired to the circuit board via wire leads, as opposed to being soldered directly to the circuit board as was the original. This also means that any stress on the input jack is isolated from the circuit board.
New input jack installed.
New filter capacitors.
New cathode capacitors.
New tone capacitors (red and blue).
One other interesting thing about this upgrade/mod is that Fromel suggests dressing up the power supply wiring. This actually looks messier, but the original bundling of all of the power wires together actually can produce hum and noise into the amp. The green wires that power the filiments should be twisted together to cancel out any AC hum, and it is suggested that the other wires not be bundled, but cross each other at as close to 90 degrees as possible, and as far apart as possible. This ends up looking worse, but potentially can reduce the background hum.
Original wire bundling.
New power switch, with wires no longer bundled together.
After reassembling the chassis back into the cab, we installed a new set of JJ gold pin tubes, and tested the amp. It sounds great now, and is really quiet at idle.