That would work, especially with the original style board layout of a tweed Champ. If the Joyo values are true to a Fender it looks like R4 may be the cathode resistor for the input triode of V1 and like that empty capacitor pad might be for a bypass cap. Assuming you have the ability to verify the information and trace the bias resistor and the empty capacitor pad traces, something like the cap below might be easier to add. If you like the sound as it is, you could even add the bypass cap to a switch for a "boost". The axial Sprague cap would probably be easier to use if you wanted to add it to a switch on the chassis face. http://m.ebay.co.uk/itm/22uF-25V-SA...C22M-OSCON-QTY-10pcs-/141446547868?nav=SEARCH I believe there was another thread on here several months ago about these amps except they may have been branded something different from the Thomann website.