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*Solved Minerva Tropic Master model w117 "early"-crackle pop & no vol ****SOLVED
#82

(06-29-2018, 08:21 PM)jcassity Wrote:  I know what you mean Tom but the resistor is starting to smoke if I leave it on too long now

Guess I missed the resistor wattage.

The string consists of five 6.3v tubes totaling 31.5v, along with 75v for the two 50L6 in parallel and the 25Z5 equal 106.5v. To make up difference, one needs to drop a additional 15v. So ohm's law says set needs additional 50 ohms @ 4.5 watt. Since there is virtually no air movement, it's std practice to increase wattage approx 3x. Resistor will still run hot, has to dissipate that 4.5w. If all the set's  460ma is routed through resistor, it's going to smoke. B+ should be derived from AC line, fed from switch directly to 25Z6 plate(s) and B+ from the cathodes. This current should not be routed through above resistor.

Basically I'm saying we can't cheat ohm's law. A 40 ohm resistor @ 300ma is going to drop 12v, that was OK in the 40s/50s but with today's line voltage, ought to be increased.  BTW 460 ma through 40 ohms is 8.5w. At 3X it should be at least 25 watt.

Once you have the heater string sorted out we'll figure out why it's drawing a extra 80ma. Correction of string may fix that.

Tom


Messages In This Thread
RE: Minerva Tropic Master model w117 "EARLY" - by 35Z5 - 06-29-2018, 11:34 PM



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