|At first it was quite difficult. I powered it up on a current limited supply + variac. While the heaters
all came up okay I was getting very low HT and only on one half of the circuit.
The HT on the EL6420 is in two sections. The pre amp stages get the HT straight from the rectifiers + filter capacitors via a dropper
resistor. The output stage (EL34 valves) get the full HT.
My problem was that I was getting the 850v(!) HT on the output stage but nothing on the pre amp stages.
After some testing I found that the dropper resistor had gone open. After replacing this I got what seemed to be reasonable power.
Connected up a load and success! (almost). I still had a lot of hum but that was caused by a faulty limit switch which was fixed
just by pushing it back together!