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Old 12th January 2015, 07:28 PM   #1
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Default Need help with microphone preamp.

I have an aphex 107, which is a starved plate tube hybrid microphone preamp. I'm very much a novice when it comes to diagnosing problems, but I'm trying to learn. The preamp is a two channel design and channel 2 still functions well. Channel one recently began a strange hissing and the gain pot did nothing to the little bit of signal that it was receiving. I switched out the pot thinking that it might be something to do with it, and the problem is still there. Any ideas would be greatly appreciated! I realize this project may be out of my hands, but I do have a strong desire to understand how things work!! 😎
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Old 12th January 2015, 08:21 PM   #2
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I'm not sure what you mean by "the gain pot did nothing to the little bit of signal that it was receiving". Do you mean that the gain was independent of the pot setting and was always quite low?

Was the strange hiss audible when you listened to the electrical output signal or was it an audible hiss coming out of the amplifier itself? In the first case, it might be related to a poor contact or a tube that isn't in perfect shape, in the second case, it must be some sort of spark occurring somewhere but not in vacuum.

Usually a good (but time consuming) approach to diagnose problems is:

-Try to find the service documentation or obtain a schematic by reverse engineering

-Measure the supply voltages and compare with the documentation or with the other channel that still works well

-Measure the bias voltages and compare with the documentation or with the other channel that still works well. Preferably put a 10 kohm resistor between the node under test and your meter to reduce the chance of causing instability.

-If this doesn't help you to find the problem, apply an input signal and follow the signal path using an oscilloscope, again comparing the bad to the good channel.
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Old 12th January 2015, 09:04 PM   #3
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Yeah the signal was pretty low and was independent of the gain pot. The hiss was only audible on channel one of the preamp. The second channel works and sounds great. There is only one tube which is used as the starved plate for both channels. I even swapped that tube with another 12ax7 and the result was the same.
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Old 17th January 2015, 08:31 AM   #4
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Have you had any luck trying to find service documentation or reverse engineering the part of the circuit that you suspect (presumably the part around the gain potmeter that stopped working)?
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Old 17th January 2015, 08:59 AM   #5
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Tube should be 12AT7 (not 12AX7) according to schematics.
https://www.gearslutz.com/board/atta...x-107-schm.pdf
maybe amp is oscillating because of high gain tube ?
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Old 17th January 2015, 10:22 AM   #6
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