Joyo JTA-05 Mods



Joyo JTA-05 Mods

 

This popular 5W single ended all tube combo made in PRC has been around for a few years under various badges on various continents. In Australia it's the Joyo JTA-05.

The circuit is based EXACTLY on the published schematic for the 1956 Fender tweed Champ 5f1. This is a good thing, as the Champ is a legendary practice/studio amp, but to make it harp-friendly, some changes are required.

Other sites will advise you to do the following:

1. Replace the supplied 5AR4/GZ34 rectifier with a 5Y3.

2. Add a 25uF/25v bypass capacitor across the 1K5 cathode resistor on V1 (first triode). This cap adds grit and gain. It is not shown on the original 5f1 schematic but is apparently present on most vintage champs.

These are easy changes and worth doing, but most harp players will also want a treble cut Tone control to tame the teeth-grinding high frequencies. This post shows you how I did that. It's not super difficult, but requires a deft touch with the soldering iron, so you don't ruin the circuit board. Proceed at your own risk.

My basic aim is to turn the 5f1 circuit into a 5f2a Princeton circuit. Here it is, with the proposed changes outlined.

 

There's not much space on the control panel to add an extra knob. Here's how I did it. You may choose a different location, like using one of the inputs.

 

 

Next step is to assemble the Tone pot. I used a small 250K log pot because it's what I had to hand. A 500K or 1M linear pot (16mm) would probably be better, and the value of the capacitor could be varied. Your choice.


 


OK. Put your Tone pot assembly in the hole and check for fit. Cut two lengths of hookup wire and solder them - one to the RH lug on the pot, and one to the leg of the cap. For soldering them to the circuit, I bent a little hook at the free end of each, attached it to the leg of the appropriate component, then quickly soldered it with a tiny dob of solder. Do NOT try to solder to the circular pads on the PCB. If you use too much heat, disaster awaits!

 

 

Just another word of warning. While you are poking your soldering iron down into the depths of the circuit, be careful not to accidentally fry those ribbon connectors nearby.

That's about it. Check your work, tidy up and try it.

One other possible mod to try is to change the 12ax7 tube to a lower gain 12ay7 or even 12au7. Then of course there are speaker swaps, or try to wedge a 10" speaker into that little box. Endless fun.

Any questions? Email me at bluemark@hotmail.com

 

 UPDATE !!

I had a call from Ken, a guitar player and Joyo owner from Redland Bay. He wanted more treble, not less. I consulted www.chasingtone.com/guide-to-single-knob-tone-controls/ and found the Gibson GA-30 tone control. This uses just two capacitors and a 1M pot, and importantly avoids any connection to the Volume control, which is difficult to access on the circuit board of the Joyo JTA-05.

It promises treble cut at one end of the Tone knob, and treble boost at the other end. Sounds good. I haven't tried it, but am offering here the diagrams to help you add it to your Joyo if you wish. Please let me know how you go. One point - This tone circuit connects to the 1K5 cathode resisitor (R6) and will not work properly if you have added a bypass capacitor to R6.

 

 


 

Happy soldering!