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Post subject: Help!!! SCXD bias way too high?
Posted: Sun Jul 15, 2012 6:51 am
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So I checked the bias on my scxd last night and it was running at about 55.2 mVs. So i tried lowering but the trim pot bottomed out at 52.1 mVs, idk what to do. Could there possibly be something else on the board going bad letting too much power through to the tubes. The tubes are the stock fenders and i have only had the amp for a couple of months. I love the amp so far and just want it to last, hence the reason for me checking the bias because of all the threads i've read that they are biased cold or hot from the factory. Any help would be greatly appreciated.

Thanks,
Shane


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Post subject: Re: Help!!! SCXD bias way too high?
Posted: Sun Jul 15, 2012 7:43 am
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Hi Scigy,

Take it easy, you're not drowning! From your numbers, assuming approx 375 volts on the Plate, that isn't really too high, it's really just about right. Remember that the reading at the test point is for both output tubes, and your numbers work out to about 70% idle dissipation per output tube. Depending on which tubes that you have, the bias might not be able to go any lower. The schematic indication at the test point of 40 mV is about 50% idle dissipation. The tube brand and type will largely determine the range of adjustment, and you can get a set that draws less current by talking to a tube vendor and tell them what you want. If you like the sound that you have now, leave the pot at the lowest setting and enjoy.

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Post subject: Re: Help!!! SCXD bias way too high?
Posted: Sun Jul 15, 2012 7:54 am
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shimmilou wrote:
Hi Scigy,

Take it easy, you're not drowning! From your numbers, assuming approx 375 volts on the Plate, that isn't really too high, it's really just about right. Remember that the reading at the test point is for both output tubes, and your numbers work out to about 70% idle dissipation per output tube. Depending on which tubes that you have, the bias might not be able to go any lower. The schematic indication at the test point of 40 mV is about 50% idle dissipation. The tube brand and type will largely determine the range of adjustment, and you can get a set that draws less current by talking to a tube vendor and tell them what you want. If you like the sound that you have now, leave the pot at the lowest setting and enjoy.



Thanks for the reply shimmilou. It still has the stock tubes in it, I was just worried i was going to smoke the damn thing and i just got it. I really like the amp minus the buzzy farty lowend but i will deal with the speaker eventually. Anyway i appreciate the reply.

Thanks,
Shane


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