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Old 28th December 2005, 03:58 PM   #1
Trout is offline Trout  United States
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Question Tube Oscillation or Microphonic Question

Hi,
After playing on several sets of tubes in one of my guitar amps, I have found an issue that appears to be common of the type 12AX7's I select. Basically, I have a large number of NOS american made valves from various manufactures. And I am finding that SOME types oscillate? to varying degrees with anything Long plated being terrible at extreme settings.
RCA & GE Grey long plates being worst.

I have also seen mention of this at the eurotube website addressing its 12AX7/ ECC 803 S

Quote:
All preamp tubes are microphonic it's just a matter of where the threshold is and longer plate tubes will squeal before a short plate tube will
Is this squeal Oscillation? or Microphonic?

Has anyone else experienced this?
Gene
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Old 28th December 2005, 07:57 PM   #2
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Yes, I've experienced this. In most cases, extra decoupling of the power supply to the tube is enough. For the most difficult cases, unbypassing the cathode resistor, a grid snubber or a small (<100uH) choke inline with the plate was needed.
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Old 28th December 2005, 11:28 PM   #3
Trout is offline Trout  United States
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Quote:
Originally posted by Geek
Yes, I've experienced this. In most cases, extra decoupling of the power supply to the tube is enough. For the most difficult cases, unbypassing the cathode resistor, a grid snubber or a small (<100uH) choke inline with the plate was needed.
Adding an additional decoupling stage is fairly easy,I will give that a try
But, Do you concider this symptom an oscillation issue? or Is it a Microphonic Issue or are they basically the same thing.

I was under the impression that microphonic issues were basically tube componets mechanically vibrating.
Gene
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Old 29th December 2005, 01:24 AM   #4
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Gene,

Put high temperature elastomer "O" rings on the tubes aligned with the micas. Damp microphonics along with decoupling and grid stoppers. Take NO prisoners.
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Old 29th December 2005, 02:02 AM   #5
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A squeal could come from a microphonic tube which is picking up vibrations from the speaker. This is a particular problem in a guitar amp due to its high gain. A combo amp (speaker and amp in the same box) is real bad for this. Simple test: use an external speaker, and put it in the next room. If the squeal goes away, it was a microphonic issue. Long plate tubes are far more microphonic than short plates, because the grid rods are longer. This is why I tend to avoid 6SN7's. I have never met one that wasn't microphonic.

If that didn't solve the problem you may have an oscillation problem. No two tubes are exactly the same. If your circuit is on the edge of oscillating, it may go over the edge with certain tubes only. This may be because of slightly higher gain, or different capacitances.
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Old 29th December 2005, 03:45 AM   #6
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Speaking of elastomer "O"'s, get a silicone ring from an auto parts store for 50 cents, rather than an 'phool one for $15
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Old 29th December 2005, 04:18 AM   #7
Trout is offline Trout  United States
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Quote:
Originally posted by Geek
Speaking of elastomer "O"'s, get a silicone ring from an auto parts store for 50 cents, rather than an 'phool one for $15

haha, I think on my EL84 amp I will need Viton, They great purrrrteeeee hot lol



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