Quote:
Originally Posted by Kamakiri
Just to clarify, I did a soft start on a variac, but not too slowly....I took about 20 seconds to bring it up to voltage. Initially, the reading was 305 volts, and dropped to 235 on the first try, with the one resistor in place. The voltage reading was tested at the B+ point that was on the schematic on the previous page in this thread.
I then shut it down, put in a second resistor in place, then again with a quick variac power up, the reading briefly stabilized at around 253 volts. I noticed that I had a full raster, snow, and static in the audio....exactly the way it should be. Observed it only for a few seconds, then went back to the meter and noticed that it started jumping all over the board, ranging from 240-260 volts. When I caught a light odor of something, I shut the set down immediately.
This is a picture of the resistor, along with brand new ones from comparison. Obviously I caught it in the very beginning stage. The picture doesn't show it, but there is a very light browning on one spot.
I clipped the lead of the resistor to check it afterwards. It's reading at 39.5 ohms.
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Are those wirewounds? The look like film to me. What's the series or part number? The wattage?
With one resistor, your B+ is low. This is indicative of too much series resistance. 20 Volts low by the Sams, but the original B+ with the seleniums was what?
You've changed the seleniums to silicon, check. Added series resistance, albeit too much, check. With 39 ohms, you are too low by the Sams, so you need to
decrease the resistance, and thus the voltage drop across the resistor, resulting in more B+. Simple - just replace the existing resistor with one of a smaller value, or parallel another beside it. With lower voltage drop, the wattage dissipated will also decrease. If you are still overheating, recheck everything with the wiring you've added.
As to the wattage, and the other comments - BS. I've got several working sets with simple 10 -75 ohm
5 watt resistors working just fine. My Penncrest in the den has a 270V B+, and has had all 4 of it's seleniums replaced, and a 47 ohm 5W resistor added to bring it down.
Take a look at R90 on the original Sams - it's dropping some 40 volts across it, and it's rated at
5 watts. You rate the resistor based on Ohms law and Kirchoff's law, not something you pull out of a hat. Take a look at
w3hwj.com/index_files/RBSelenium2.pdf - I'm not the author, but it backs up what I'm putting out, and look! -> page 3 of 6 shows a dropping resistor....of 5 watts.
The voltage drop across the original seleniums is all that has to be accounted for. It's not a loading resistor of any sort, just a series dropping resistor. R92, the filament dropping resistor performs a similar function, dropping 30V (120 line minus the 90V at pin 2 of the 12DQ6) in the 600mA series string of the filaments. P=IxE, so .6A x 30V = 18W, and it's sized at 20W, a standard value. Solid theory, put into practice...
You can obviously listen to whom you choose...