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I recently replaced the power tubes, sockets, pots, and jacks in a Mig50 that I'm fixing, and now I'm having some terrible troubles with it.
I've biased the tubes at around 30mV as best as I can tell. My cheap DMM won't read past 400v and overloaded when I tried to measure the plate voltage so I'm not exactly sure of the cathode resistor dissipation wattage to make sure it's within limits. I turned the bias pot down lower though just to be on the safe side. there is a loud high pitch squealing unless the Presence is turned all the way up. But then when I try to play anything, the amp just explodes into feedback-esque noise that shakes the earth and it won't go away unless I turn the presence down untill it starts squealing. I can fiddle between the squealing and noise but can't get them to go away. With the presence turned down, when I play it sounds horrible. The squealing is modulating, the amp sounds horrible, etc.... When the bias is up further, so the tubes are running hotter, the squealing is much more intense. Turning the treble pot adjust the frequency of the squealing. The volume pots go from 0 to LOUD from 0 to 1. Could I have wired the pot backwards so it gets the wrong end of the audio taper? But if I reversed them then the pot would work backwards right?
I have no idea what is wrong? The amp had 5881's in it before I burnt them up (cab accidently came unplugged and I turned the amp on and poof!) so i replaced them with Sovtek 6L6WXT's after repairing the sockets. These are supposed to be +20% more powerful than regular 5881's so is there just too much power from the tubes? I don't know much about how tube amps work so I really am without a clue...
Thanks for any ideas,
Michael
I've biased the tubes at around 30mV as best as I can tell. My cheap DMM won't read past 400v and overloaded when I tried to measure the plate voltage so I'm not exactly sure of the cathode resistor dissipation wattage to make sure it's within limits. I turned the bias pot down lower though just to be on the safe side. there is a loud high pitch squealing unless the Presence is turned all the way up. But then when I try to play anything, the amp just explodes into feedback-esque noise that shakes the earth and it won't go away unless I turn the presence down untill it starts squealing. I can fiddle between the squealing and noise but can't get them to go away. With the presence turned down, when I play it sounds horrible. The squealing is modulating, the amp sounds horrible, etc.... When the bias is up further, so the tubes are running hotter, the squealing is much more intense. Turning the treble pot adjust the frequency of the squealing. The volume pots go from 0 to LOUD from 0 to 1. Could I have wired the pot backwards so it gets the wrong end of the audio taper? But if I reversed them then the pot would work backwards right?
I have no idea what is wrong? The amp had 5881's in it before I burnt them up (cab accidently came unplugged and I turned the amp on and poof!) so i replaced them with Sovtek 6L6WXT's after repairing the sockets. These are supposed to be +20% more powerful than regular 5881's so is there just too much power from the tubes? I don't know much about how tube amps work so I really am without a clue...
Thanks for any ideas,
Michael
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