Thursday we powered up the Mars Mullard 3-3 ST for the first time.
We started off with the negative feedback loop disconnected.
First we checked the power supply, and voltages were on target. Next we monitored the voltage across the EL84 cathode resistor as we turned up the variac. One resistor went over the 28V target. We checked the circuit. Next we moved the two tubes from the functioning channel to the over heating side. Problem disappeared. One EF86 turned out to be broken. After replacing the EF86, all was well.
Next we used crocodile clips to check the NFB loop. Once we confirmed that the NFB phasing was correct, we soldered the NFB loop in place.
The sound: What a great sounding circuit! Absolutely no hum. This seldom happens from a DIY build, but there it was. I can recommend this circuit, in spite of its apparent complexity. It seems to be immune from power supply ripple, much less so than regular SE circuits.
The sound is very clear and the bass was impressive. One happy amplifier owner went home.
There is another build of the kit about to be tested. I will report the outcome later.