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APPJ EL84 SE mini Tube Amp, PA0901A

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Joined 2015
Paid Member
I've done the following modifications to my PA0901A with PCB dated 2011.11.11:
1) Replaced the 2A timed 5x20 fuse with a 1A timed 5x20 fuse. According to the switching controller datasheet, a fast-acting 3.15A fuse may be used.
2) Installation of mains-rated (CE-marked) wires from the power outlet to the PCB L and N contact points. I soldered the wires directly on the PCB traces (back side), because the section is bigger than the original holes. On the free L and N holes, I installed a S20 275V varistor for sourge protection. I also added a layer of kapton insulation tape on the live area of the PCB.
3) Removal of the wire connected between the mains outlet ground terminal and the PCB. Installed a 1 sqmm green-yellow wire from the mains outlet ground terminal to the top screw at the corner nearest to the RCA inputs. The wire is terminated with a flat electrical lug; the lug is securely trapped between the top plate and the bottom case and does have good electrical contact with both metal parts.
4) Replaced the two series-connected 1206 560K SMD resistors between live and neutral with 3 series connected 1/4w 330K trough-hole metal film resistors.
5) added kapton insulation on the bottom cover above the aluminium heatsink. This heatsink is connected to the primary side of the SMPS and it has a 2mm clearance to the metal bottom cover. The cover has a ventilation hole exactly over one of the fins. That's simply insane, I was speechless when I discovered this safety blunder.
6) Removed the big primary side capacitor (270uF 400V Nichicon GN, with a PCB adaptor) and replaced with a 100uF 400V Nichicon VZ, soldered on the main PCB. The Nichicon GN is a good and expensive capacitor, but I replaced it with a genuine Nichicon bought from RS Components to increase the clearance from the heatsink and because I was not 100% sure about the source of the original capacitor. This is the only critical electrolytic capacitor in this amplifier.
7) Replaced the two RIFA 300V rated coupling capacitors with standard 630V film capacitors. 300V is the bare minimum. 630V (or even 400V) gives a bit of headroom. Fancy Hi-Fi capacitors are not needed with the tiny output transformer (and the global feedback path is trough a SMD ceramic capacitor).
8) Modified the PCB to install a 1W 1K resistor on both EL84 g2 (pin 9) as protection measure. This is not strictly required, but the PCB modification is easy and clean.
9) Removed the 10K SMD resistors across each RCA input and ground. They lower the input impedence unnecessarily (the declared impedence is 50K, but without the resistors).
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