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Old 16th November 2010, 06:51 PM   #11
M Gregg is offline M Gregg  United Kingdom
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I think the H-K leakage as you say is the most probable!
You have got me thinking now I wonder what will happen if I reverse the DC on a "bad" tube?
Other guys use DC filaments I don't hear them having this problem.
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Old 16th November 2010, 06:54 PM   #12
20to20 is offline 20to20  United States
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Quote:
Originally Posted by M Gregg View Post
I have had it happen to 3 tubes now NOS RCA, Mullard, Now Groove Tube. The voltage is spot on 6.3V DC.
I had a 12ax7 make a kind of hissy, sizzly, muffled poping noise. Not constant but aggrivatingly too present. I swapped sockets and it was there. Tossed it.

I've also had a dirty socket, and tube pins sound like that too. But wiggling the tube gives that one away.
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Old 16th November 2010, 07:00 PM   #13
M Gregg is offline M Gregg  United Kingdom
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The tube seems to go microphonic and noisy! If you tap it the sound comes and goes! I have changed the socket " Still the same. I thought it was an Elec cap so I changed that too! Its a recent build. I would bin the circuit, however I want to know whats happening might I come across it again!
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Old 16th November 2010, 07:01 PM   #14
20to20 is offline 20to20  United States
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Quote:
Originally Posted by M Gregg View Post
I think the H-K leakage as you say is the most probable!
You have got me thinking now I wonder what will happen if I reverse the DC on a "bad" tube?
Other guys use DC filaments I don't hear them having this problem.
Since a good tube will usually work with a lower filament voltage, you might try dropping a volt to it if you can, for kicks, to see if it makes any difference at all.
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Old 16th November 2010, 07:07 PM   #15
M Gregg is offline M Gregg  United Kingdom
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If I'm honest its driving me nuts. LOL I just got to know why it's happening!
I will try a few things.
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Old 16th November 2010, 08:18 PM   #16
Sheldon is offline Sheldon  United States
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I find it "sounds" better with DC filaments. I must admit this has got me puzzled! Someone said to me in the distant past that the heater can age in a non uniform manner with DC. I don't want to post old wives tales, it's probably a load of hog wash.
That can possibly apply to DHT's because the filament carries the signal current as well as the heating current. The positive side of the filament will carry more of the load than the negative side. Probably only an issue when tubes are run near current limits.

With a IDHT, the heater is not part of the cathode current flow, so this would not be an issue.

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