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Old 2nd July 2004, 04:15 PM   #1
squid6 is offline squid6  United States
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Default amp troubles

this is my first tube amp build. i found the schematics at http://www.ax84.com/moonlight.html. when it worked i was happy how it sounded (althougth it didn't have much tone control). but now whenever i turn the amp on it makes this pulsating popping sound (like a machine gun). when i first power up and strum the guitar i hear audio for a second then the pulsating noise starts up again. i don't see any burnt components. could it be a bad tube or power supply. please someone help me.
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Old 2nd July 2004, 06:03 PM   #2
SY is offline SY  United States
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Sounds like classic motorboating, a low frequency oscillation caused by coupling through the power supply. I'll guess that you didn't include the "optional" power supply decoupling RC network. Add it and I'll bet your troubles go away.
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Old 2nd July 2004, 06:31 PM   #3
squid6 is offline squid6  United States
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i didn't see anything about the optional decoupling rc network. how do i make it, and where do i install it.
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Old 2nd July 2004, 07:44 PM   #4
SY is offline SY  United States
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Quote:
Originally posted by squid6
i didn't see anything about the optional decoupling rc network. how do i make it, and where do i install it.
Look at the schematic. The optional decoupling network is the group of parts labelled "optional."
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Old 8th July 2004, 03:31 AM   #5
squid6 is offline squid6  United States
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i checked my build, and i did install the additional resistor and capacitor. is there anything else i could add that will fix this problem. do you think that another capacitor before the optional 10k resistor would help?
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Old 8th July 2004, 06:50 AM   #6
dhaen is offline dhaen  Europe
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I agree with SY's idea; The trouble is most likely instability due to a supply decoupling inadequacy.
Next, check that all decoupling parts (3 electrolytics) are connected properly. if so, then maybe one is faulty. Try adding another in parallel across each in turn. Observe polarity and watch out for stored charge.
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