Power resistor over heating

[Mr_Tickle]

 

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.

[jerrydyer]

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.

[sluckey]

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.

[HotBluePlates]

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.

[Mr_Tickle]

Cheers for the quick response. I'll have a good check of everything over the weekend.

[tubeswell]

Something is sucking the current through that resistor - could be a shorted filter cap maybe.