Philips EL6420 Amplifier

"Philips EL6420 Repair and Restoration"

This was a project I had sitting around for the better part of a year. I finally decided to make a start on it. It's a Philips PA amplifier from around 1952. It uses two EL34s as the output along with some very pretty looking AX50 mercury vapour rectifier valves.

Looks like something that might burn the house down! Did you manage to get it working?

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!

Fantastic! How does it sound?

Incredible actually!
And at 75w its more than capable of driving my 100w 8ohm Technics SB-CS7 speakers!