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I've replaced my el34's in my jcm 2000 and there is a problem. The right set of tubes biased just fine to 85mv, then when I switched the multimeter to the left bias needle, the meter starts to read VOLTS not mv, between 20 and 30! Obviously there is too much voltage going to the left set of tubes. Is this a bad resistor or something else?
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If this amp has fuses on the power tube cathodes, and I think it probably does, check them. Sounds like one of them is open. Usually there is an LED that lights up to tell you the fuse is blown, unfortunately the resistor that feeds it usually burns up because it can't handle the dissapation.
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Cool, thanks....it looks like the fuses are fine, so I just don't know....I think I'll just have to take it in to a shop.Comment
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There are two 1 ohm current sense resistors from the cathode to ground. One for each side and one of them is most likely open. These resistors are where the voltages your reading comes from. If one is open the tube will go into thermal runnaway and will glow cherry red and hopefully blow the fuse Rick was referring to. If you take your meter and read from the Cathode pin 8 to ground chassis and it's open, that's your problem.KBComment
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Hello, i realize this is a very old thread but i was just wondering if this guy ever figured out what was going on im having the exact same problem only its the right side, same thing between 20 and 30 volts, is this a serious problem or an easy fix? i bought this amp used and used it a couple months before i changed the tubes and had no problems other than the clean channel being pretty quiet and the gain knob on the clean channel crackels a little bit when i turn it.Comment
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Well certainly the whole problem is not so much how poorly it's designed more than the fact that it doesn't respond very well to bad tubes out of box or failing tubes. Fuses that are supposed to protect don't blow before other parts do. I've seen plate load resistors fry and blow the whole trace off of the board to open screen resistors and current sense resistors open but the source is usually a bad tube. If you are red plating monitor the bias voltage on pin 5 and see if it's drops when it red plates. If it does than I'd suspect a leaking coupling cap after the PI. Also check the 1 ohm resistors and the plate load resistors and verify they are ok.KBComment
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hey thanks man i really appreciate the info, that sounds a little out my league, i think ill probly just take it to a professional, i would hate to spend the money if its just a crappy tube though, how complecated would that be, and what kind of tools would i need?Comment
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Switch the right two tubes with the left ones and see if the bias problem changes sides as well. If it does, it would indicate bad tubes. If it stays the same it would indicate a circuit problem.Comment
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that is genious buddy thank you i will let you know what happensComment
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oh man, i guess its a circuit problem bc i tried that and got the same result. dont think i can handle much more i think im just gonna take it to a tech, thanks for your help guys i really appreciate it at least now i know its not just something simple i can do on my own.Comment
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