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Old 28th March 2005, 06:52 PM   #1
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Join Date: Jan 2002
Location: Tyrone Ga. U.S.A.
Default Tube troubleshooting question

I have a Antique Sound Lab MGS 115 that is ill. This is a single
ended kt88 amp that is switchable between triode and pentode
operation. A few days ago I noticed one of the output tubes
seemed to flash or arc. Then the side with this tube started
playing about 6db lower than the other.

When i opened it up I found that the grid #2 resistor was burned out. When I replaced this I noticed that the bias was a lot lower
in the side with the trouble. It will now run in triode mode but
does not seem stable in pentode mode. Does this just sound
like a bad tube or what? By the way this is just a backup amp
my little 6c45pi sounds so much better but I am waiting on a replacement tube.....


Thanks,

Woody
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Old 28th March 2005, 08:19 PM   #2
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Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
Hi there......we need more info volts measurements and a circuit diag if youv'e got one.
A flash/arc in tube with screen grid resistor burnt out seems to indicate an o/c anode connection to the tranny.......i.e all the current in fault condition was going through screen grid.
You may have damaged the tube......depends on the value of g2 resistor before destruction. Find the problem before you fit another tube and replace with fresh resistor....otherwise you risk doing the same damage to a new tube....

richj
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