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Old 8th April 2003, 02:10 PM   #1
AuroraB is offline AuroraB  Norway
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Location: Norway, -north of the moral circle..
Default Microphonics

I had one of my brothers one the phone last night, in all agony about microphonics in his Cary SLP94 ( or RIAA modified SLP 90 ).
According to his description, the problem is worsening as the tubes ages, although the tubes in use has less than 200 hrs on the meter. Its Raytheon 12AX7 in the RIAA an JAN/Philips 12AU7 in the line stage.

Usually microphonics is associated with the mechanical contruction if the actual tube in question, but this seems to be a problem developing as the tubes burn in. It also seems that the bulk of the problem resides the line stage..

Any suggestions...???
Links to Cary schematics..??
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Old 8th April 2003, 02:38 PM   #2
Joel is offline Joel  United States
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Are you sure that the symptoms he describes are definitely microphonics? In other words, does he know what he's talking about?

If so, make sure he's using a later production 12AX7 - I don't know for sure, but very very early models may have had problems. Second, make sure he has the unit a suitable distance from the speakers! I have to ask just how loud is he playing the stereo to be getting microphonics in a line stage???
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Old 8th April 2003, 05:34 PM   #3
Gabevee is offline Gabevee
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Assuming your brother knows something about electronics, ask him to check two things:

1) Are there input capacitors on the line stage (after the pot to the grid)?

2) Are the capacitors ceramic, or large value?

Why? Capacitors can behave as microphones. Even a slight variation of the dielectric, or distance between the plates, or even flexing (piezoelectric effect) can make for microphonics in these very sensitive devices (tubes). The sensitivity increases with stiffness (ceramics) or size (large values of any kind).

If there is no capacitor, as some have the practice, what value is the potentiometer? The larger the pot, AKA grid resistor, the more sensitive the input is to variations of EM, vibration, changes in physical nature of components connected, etc.

Time breaking in makes the tube more sensitive also. Perhaps the tube has gone into its more amplificative stage. That is, the bias varied just enough to bring it into a higher mu.

On the OTHER hand... the 12AU7 has a mu of only about 20... What the....

I bet he reversed the tubes.... maybe? No? Sorry for the implication.

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