Microphonics - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th April 2003, 02:10 PM   #1
AuroraB is online now AuroraB  Norway
diyAudio Member
 
AuroraB's Avatar
 
Join Date: May 2001
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..??
  Reply With Quote
Old 8th April 2003, 02:38 PM   #2
Joel is offline Joel  United States
diyAudio Member
 
Joel's Avatar
 
Join Date: Oct 2002
Location: Brooklyn, NY
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???
  Reply With Quote
Old 8th April 2003, 05:34 PM   #3
diyAudio Member
 
Gabevee's Avatar
 
Join Date: Feb 2002
Location: Richmond, VA.
AuroraB,

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.

Gabe
__________________
Gabe CGV Electronics
Home of the CGV-300B amplifier on a budget
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Microphonics? Carlp Tubes / Valves 5 28th August 2008 12:19 AM
Microphonics that go away by themselves jlsem Tubes / Valves 2 15th November 2007 07:52 AM
Crossover microphonics? DSP_Geek Multi-Way 1 26th October 2007 10:04 PM
Tubes and microphonics nhuwar Tubes / Valves 13 3rd February 2007 05:22 AM
Microphonics........... Alastair E Tubes / Valves 4 3rd June 2005 04:02 PM


New To Site? Need Help?

All times are GMT. The time now is 08:58 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2