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Thread: How do I figure this cathode resistor wattage?

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    How do I figure this cathode resistor wattage?

    I have a Morgan AC40 Deluxe in that blew up. I think a tube shorted and cooked the cathode resistor, which melted the wrap off the F&T 250uF bypass cap. The resistor measures 62 ohms, which believe is correct from an email from the builder, but the lettering is completely burnt off, so I don't know what wattage it is. I want to replace the resistor and cap since they obviously took a beating. I asked Morgan, but he didn't answer the question, only saying he would send me out a few of the failed parts (the power scale board also lost a resistor and MOSFET in an unrelated failure.) But then he hasn't sent them, and does not answer any further emails.

    I have waited over two weeks, and the customer is getting eager to get his amp back. So assuming Morgan has dropped the ball for good, how do I figure what wattage Cathode resistor to get? It is a four EL84 design with a 250uF bypass cap. Looking on Mouser I see an Ohmite 62 ohm 10 watt wirewound at approx. the same dimensions. I am going to make an educated guess this will work, but am I wrong?

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    Lifetime Member Enzo's Avatar
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    What voltage drops across it at idle? If you don't know, tack your 10-watter in there for test purposes and find out. That will let you calculate the dissipation. Then at least triple the wattage from what you calculate there.

    SOunded familiar, and sure enough 60 ohm 10 watt was used in a Crate VC series with four EL84. When I converted a PV Classic 30 to cathode bias, I raised it to 100 ohms so the tubes ran a bit cooler.


    And I would not be averse to using one of those aluminum finned deals from Dale or similar, but then just for size I'd use 25w, they are very compact. Mount to the chassis and run two wires.

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    Supporting Member Randall's Avatar
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    Enzo, that makes sense, but I wonder if the fact that the MOSFET driven power scale circuit isn't working at the moment will give me accurate measurements. It is stuck to the lowest setting, approx 1/2 watts, so wouldn't that make idle current that much less as well?

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    Lifetime Member Enzo's Avatar
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    Of course it would. But does the power scaler normally go "all the way up"? I mean is the max setting just the basic B+? If so couldn't we just bypass the scaler for now and apply full voltage to the system, at least long enough to measure cathode voltage?

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    I think that is how it works, yes, by lowering B+. No schematic of course, but it does appear that it adjusts the plate voltages. I'm not solid on it, but with the mosfet failed it is sitting at the lowest setting. I will investigate to see how to bypass it and do as you suggest.

    Thank you.

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    OK, so shorting past the power scaler board I have a functioning amp, and 9.5 volts across the 62 ohm cathode resistor. This is the math I am seeing:

    9.5v = 0.153A * 62 ohms

    1.45W = 9.5v * 0.153A

    Is this correct? If so, a 10 watt cathode resistor will be plenty, yes?

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    Lifetime Member Enzo's Avatar
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    Looks like it from here.

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    The voltage across the cathode bias resistor will increase at high signal levels, perhaps double at full overdrive. But that would still be under 6 watts, so your resistor should stay well within its rating and so last a ling time.

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    Thanks fellas!

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