[Penthode]

Still fussing over the power supply.

To accommodate the inherent forward resistance of the selenium rectifiers when replacing them with silicon diodes, I initially inserted a 10 ohm 15W resistor in series with the AC feed to both diodes. I had been powering the chassis and didn't initially pay much attention until I noticed the resistor leads appeared to be blueing! The ac voltage drop across the resistor measured 20 volts. At 2 amperes, the resistor was dissipating a whopping 40 watts!! And it survived!!

So rethinking this I initially determined the forward resistance of the selenium rectifier was 10 ohms at 600mA. That is 600mA per diode whereas my 2 amperes above was a culmulative measurement of both silicon diodes as the circuit is a full wave doubler where each diode conducts only 180 degrees.

I did find that the B voltages were low at an input of 118vac. The higher selenium resistance may be due to aging of the rectifiers? Just to note I left the selenium rectifier running under test at 1 ampere for half and hour and it was only moderately warm. My guess is the fins may help to better radiate the heat?

Nevertheless, I next tried attaching a chassis mount 50W 8 ohm resistor to the chassis upright with a little bit of silicon grease to reduce the thermal resistance. Boy did that heat the chassis! The surrounding metal after five minutes was too hot to touch and after 10 minutes or so the whole upright section was pretty warm.

As the B voltages were a tad low, it made sense to reduce the series resistance. with about 4 ohms and an AC input of between 115 and 120 vac at the set input, the B voltages were close. It is interesting to note that as this beast consumes 475 watts to load down the mains supply!

I had an old pack of 1.5 ohm Ohmite ceramic 11 watt resistors. With three in series, I measured a voltage drop of 8 vac which means that the resistors are dissipating each about 10 to 12 watts. I have mounted them suspended on a tag strip and left the set running half an hour. The surface temperature at the center of the resistors as measured with an infrared thermometer shows 205 degrees celcius! they are mounted directly under the old selenium rectifier (left in place for appearance) and the chassis remains relatively cool (45 degrees celcius and the adjacent electrolytics remain cool: one electrolytic has a cardboard shroud and the other measured 35 degrees celcius after half an hour, I am going to leave it here and it will be a good test to see how these Ohmite resistors last while running at 100% of rated capacity. (If they fail, they will simply go open and they are in seies with the 4.5 ampere fuse).

You will see below photos of the three Ohmite resistor location and the silicon diodes on the opposite side of the chassis upright. I have included a photo of my ballast resistor concoction with the surface mount resistors on the heatsink. The Infrared Thermometer reads about 80 degrees celcius maximum when pointing directly at the resistors from below as mounted. I am happy with this solution as it plugs in to the original ballast socket with no modification.