Although most of our work is repairs, we also do mods on amps. Here are a couple of recent examples of our mod work.
The Fender Blues Junior is a very popular model in the Fender valve amp range. However, like many amps that use EL84, they do run the output valves v-v-v-very hot. In this case the very knackered looking output valves were dissipating nearly 20W each at idle, which is slightly more than the data sheet maximum of 12W!
Although the Blue Junior is fixed bias, there is no means of adjusting the grid bias voltage. Here I installed a trim pot on the PCB that allows adjustment of the idle current to a less incendiary level.
The second mod was in a Musicman RP-65 amp. These amps date from the late 70’s/ early 80’s when Leo Fender was involved with the Music Man company. Like most amps Fender was involved with, these are well built and reliable amps. Although Musicmans have a solid state pre-amp and run the power valves almost in class B, they do sound good, are well built, and can be picked up for not a lot of cash.
This series of Musicman amps has gain and volume controls, employing a circuit similar to that used in a TS9 Tubescreamer to generate distortion. The owner of the amp like the distorted sound and the clean sound, but wanted to be able to switch between the two sounds.
I added an extra pair of controls for gain and volume, conveniently where the (unused) line out jack sockets were located, and only needed to drill one hole in the back of the amp for a footswitch. As the pre-amp is solid state running off +/- 15V rails, it made sense to use JFET switching. Below is a picture of the switching board. Don’t worry it wasn’t left flapping in the wind, but was bolted to a conveniently placed piece of cardboard underneath it. (The card is there to stop anything being poked through the line out jacks touching the fuses).