Introduction
I've developed this mod kit in response to customers asking me to help make their Pro Junior better suited for "blues harp" use. I've noticed over the years quite a few players using, or attempting to use Pro Juniors for blues harp. Reasons are simple: it's a good looking, small, portable, compact amp, 10" speaker, all-valve, low wattage yet loud enough to gig with, has a simple control layout, and it has the Fender name! However, most, if not all stock amps, suffer from self-generated hum, hiss, and buzz. They also run the output valves very hot, leading to premature valve wear and a harsh tone. Anode dissipations were calculated at 15 watts per valve at idle, this is way too high as EL84 valves are only rated at 12 watts absolute maximum anode dissipation. The modified amplifier anode dissipations were calculated at just over 10 watts per valve at idle. (All calculations based on new JJ Electronic EL84 output valves running with a measured 245V UK mains input voltage). The amp also has a huge amount of gain, weak bass, and a "honky" pronounced upper mid tone making controlling feedback a big problem, which is the last thing you need for blues harp.
So, I've added another filter stage to the HT power supply to lower the constant background hum, and converted the EL84 bias circuit from fixed bias to cathode bias (also known as "self bias" or "auto bias" - this ensures correct valve operation whatever valves you fit. These first two mods also have the added benefit of reducing the crazy high HT voltage to something more suitable for EL84s. I've converted the AC heater supply to DC to remove the "buzz saw" heater buzz. I've also altered the tone circuit for maximum bass, changed the tone control to a simple "treble cut" type, removed the negative feedback loop for a looser, less brittle, more organic tone, and have tuned the amplifier to run low gain 12AU7 or ECC82 valves. Please note: the kit does not include the valve, you will have to source one yourself, but they are usually reasonably cheap and readily available. I've also changed the rubbish plastic input jack to a proper genuine Switchcraft USA metal type. Lastly, all relevant capacitors in the signal path are upgraded to Mallory 150M Series for the smoothest possible tone.
The kit also includes a Carling 3-position off/standby/on switch. It replaces the stock power switch, requires no extra drilling, and preserves the stock appearance. Using it will put less stress on the HT filter capacitors and valves.
Instructions (18 pages), pictures (121), schematics, valve data, are kept in a folder. The folder is stored on a download site (dropbox). You will receive a download link via email which will be sent to you at
the same time as you receive confirmation from eBay that the kit has been shipped.
I will use your PayPal registered email address unless you inform me of an alternative preferred email address.
Important! Dropbox has recently changed and now requires you to open an account with them before you can access the files.
You will need a pdf reader to access the instructions. You will need a picture viewer to view the pictures.Check out this noise comparison video: https://youtu.be/jYmCmnLk3iw
Check out Mat Walklate giving an amp a quick soundcheck: https://youtu.be/DXIGir-_zGY