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Old 8th March 2008, 05:15 PM   #1
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Default tube amp being weird -- any ideas

just finished building a single ended kt88 stereo amp and it works fine for a while then one channel goes very load and the other very quiet. it is consistent i doing this i can turn it off leave it for a while then turn it back on and get perfect stereo sound for a while. i am confused.
any ideas
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Old 8th March 2008, 08:51 PM   #2
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Hey soundbadger, what are you driving the KT88's with? A schematic and/or layout would be helpful. It is possible that the problem lies in the driver stage. Also go ahead and double check all your voltages and currents. Make sure nothing is shorted. I'm no expert, but I know how frustrating it can be to troubleshoot an amp.....
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Old 8th March 2008, 09:27 PM   #3
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Yes, check the voltages and currents while it's working well and again when it's not, to see what is changing.
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Old 8th March 2008, 09:32 PM   #4
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if both channels use a single supply, especially if tube rectified,
it might be that one of the channels`s tubes and/or bias gets runaway.
what biasing method is used?
what value grid bias resistors are used?
resistor possibly going open when getting warm?
but just guessing now..

do you have and will post a schematic?
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Old 9th March 2008, 11:53 AM   #5
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I believe he's building Mikael's amplifier, is that right soundbadger?
I've built this amp myself, as well as many others, and it's a very simple design.
I'd do as Ray_moth suggested and take some voltage readings. I have voltage measurements on my page if this helps. It's got to be something simple.

Mikael's KT88 SE amp

Glenn's KT-88 build
"Imagination is more important than knowledge"
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Old 10th March 2008, 02:01 PM   #6
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sorry have been away,,
yes it is Mikael's amplifier i am building.
i shall take readings tonight to try and see what happens when the problem starts to occur.
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Old 10th March 2008, 03:40 PM   #7
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Beware using a 470k grid resistor on the KT88. I've had modern KT88's draw enough grid current that 470k is too high. The result is that the bias point goes weird, sometimes even oscillates.

Just for kicks solder a 100k in parallel (it may hurt LF response) and see if the problem goes away.

If that fixes it you might need bigger coupling caps to restor LF response.

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Old 10th March 2008, 05:15 PM   #8
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The great thing about stereo is you can always compare the voltages between the working/non-working channels. It's possible that one of the KT88's are defective.
"No man really becomes a fool until he stops asking questions" - Charles Proteus Steinmetz
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Old 10th March 2008, 08:43 PM   #9
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(sorry caps off now)
the bias resistors are also pretty hot, i measured them about 70 degrees centigrade , they are 12W resistors which i believe should be fine.
i am guessing that either one of the bias resistors or one of the bypass capacitors is not good.

the pre-amp stage seems fine even when the problem is occurring.
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Old 11th March 2008, 06:09 PM   #10
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i changed the bias resistors for ones of greater power and changed the bypass caps, still the same problem after about 10 mins.
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