Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mr_Tickle on May 17, 2012, 12:03:53 pm
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:BangHead:
I've built a JCM 800 2203 however the 1st power resistor(the one supplied by the dual can cap) keeps over heating.
The schematic states a 10k 2watt. it died. so I tried a 3watt which also died.
Could it be a problem with the cap or should I go up to a 5watt resistor?
The 2203 model is a late 80's with 2 dual caps on the power side and 1 dual cap on the preamp side all are 50+50uf 500v.
Heres the schematic.
Cheers for any suggestions.
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check farther down the line like preamp tube plate connections. Your pullin more juice through. like maybe something just barely touching ground. something miswired further down.
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I've built a JCM 800 2203 however the 1st power resistor(the one supplied by the dual can cap) keeps over heating. The schematic states a 10k 2watt. it died. so I tried a 3watt which also died.
3 watts should be plenty. I would expect a problem somewhere downstream.
Try this... Measure the voltage across the 10K resistor. Now use this formula to calculate the power that is actually being dissipated by that 10K...
P = E2 / R, where P is power, E is measured voltage, R is 10,000
Now double that number for a safety factor and use the next larger wattage resistor that's available to you.
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There are 6 triodes in the preamp/phase inverter. I'd bet none of them pull as much as 1mA each, under normal conditions.
6mA * 10k = 60v dropped across this resistor (measure your's to see what it's dropping)
60v * 0.006A = 0.36w
Note the schematic calls for a 1w resistor, which should be plenty in this case. If you had a 3w resistor burn up, you have a problem in the preamp resulting in too-high current draw.
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Cheers for the quick response. I'll have a good check of everything over the weekend.
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Something is sucking the current through that resistor - could be a shorted filter cap maybe.