This little box of oranges came in with a report that it sounds off and makes a racket when it’s placed on top of the speaker cabinet. One of these amps with a selection of settings, switching out 2 of the 4 output valves and another switch reducing the power further. Whatever takes ya fancy I guess.
Power it up with a loudspeaker connected and see what happens. First thing, the amp was certainly sensitive to vibration. Tapping around, at least one of the output valves (EL84s) was very microphonic. Also after a few minutes of being powered up, things were getting a bit hot and sweaty around the EL84s with a couple starting to glow cherry red. The infrared digi thermometer was showing well over 200 dungarees radiating from these poor soldiers. If I remember, the amp has a nice straight forward good simple design with auto bias, so couldn’t blame a biasing circuit.
Out with the chassis. Now this is nice to work on as the handles on the front mean you can upend the chassis without damaging the controls and you can tip it upside down, with the chassis supports preventing the valves resting on the bench. Well done Orange men. Another nice thing to see is a total lack of silly Jfet transistors switching routing paths and silly OP amps doing silly things that OP amps like doing.
Out with the EL84s and into the AVO Valve tester. One valve showed and intermittent heater to cathode short, that one being the microphonic culprit. Another was very soft. Popped in some new one and another test. However, before, checked the resistors around the output stage and yes, a damaged screen feed resistor belonging to the naughty valve. The PCB is nice made and good tracks. Components are good and build quality also good. Peoples Republic of Chineesium done well with this one we think.
All back together and a soak test, all good. 30 Watts RMS output with the choke full out. For me, the tone circuit makes things sound quite toppy. But what do I know? I aint no guitar player.