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Old 26th January 2012, 08:12 PM   #1
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Default Cold solder joint?

First the background. I replaced the coupling caps between input and driver/invertor in a pair of tube amps. Now when I turn them on, both amps come up, but after about 10minutes one amps volume decreases. Does this sound like a cold solder joint? Thanks for any help. I ask this as the amps weight around 70lbs each are a real PITA to go back into.
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Old 27th January 2012, 12:07 AM   #2
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Doesn't sound like a solder joint problem to me. It's easy to test the joint though; see if the volume still drops with test leads on either side of the suspect joint.
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Old 27th January 2012, 01:01 AM   #3
tinitus is online now tinitus  Europe
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Originally Posted by john dozier View Post
Now when I turn them on, both amps come up, but after about 10minutes one amps volume decreases.
my thought was have this happened more than once

btw, remember to check your RCA connectors
they can do strange things if not making proper contact

dont know that much about tubes, but I guess its the same thing with tube sockets
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Old 27th January 2012, 01:06 AM   #4
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If you replaced electrolytics did you put them back in the right way around ?
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Old 27th January 2012, 01:21 AM   #5
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Changed out PIOs for Teflons. It happens each time I turn the amps on when they are cold. If I leave them on standby(heaters only) and turn on plates, volume is still low on one. Regards and thanks
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Old 27th January 2012, 01:48 AM   #6
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you might need help from tube amp forum
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Old 27th January 2012, 03:35 AM   #7
Loren42 is offline Loren42  United States
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Originally Posted by john dozier View Post
Changed out PIOs for Teflons. It happens each time I turn the amps on when they are cold. If I leave them on standby(heaters only) and turn on plates, volume is still low on one. Regards and thanks
Swap the caps. If the problem follows the caps then it is the cap. If it was a solder joint both sides will work (assuming you do a good job resoldering).

If the problem does not change, then something else is wrong with the amp.
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Old 27th January 2012, 05:11 AM   #8
ryuji is offline ryuji  United States
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measure all your components in the circuit comparing one side to the other, and check for visible decay of resistors, joints, capacitors, etc.

If your confident you can do it safely, probe the bias point of each tube as well as anode and (if applicable) screen voltages of each tube, anything with large differences one channel vs the other is suspect.


some thoughts: did you check the capacitors for proper capacitance/lack of shorts before you installed them? might want to check for excessive leakage with it turned on. the grid should measure near zero volts DC

Last edited by ryuji; 27th January 2012 at 05:15 AM.
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Old 27th January 2012, 01:35 PM   #9
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I think I have solved the problem. jiggling the input and one of the driver tubes seems to have cured it. I suspect either a bad tube socket or poor soldering when I changed caps (mea maxima culpa) Now to go in and solve the problem for good. My poor back! Thanks for all the help. As usual the board is a font of wisdom and knowledge. Kindest regards John Dozier I knew tubes were some trouble. These are my first tubes since rebuilding a pair of Fisher 50a's many years ago. But the sound is worth it!
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