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RE: Could this even be possible to have a 12Ax7 hum?

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colebearanimals

In Reply to: RE: Could this even be possible to have a 12Ax7 hum? posted by airtime on November 18, 2015 at 16:14:45

Hey Charles. Looking at my S5000 I see that V5 is the pre amp tube that's right next to the power tranny. Although I've never had this issue, I wonder if there still isn't some kind of shielding problem.
PaulB

Follow Ups:

Probably heater to cathode leakage. - Alpha Al 08:52:43 11/19/15 (1)
This seems to be a common problem with certain new production tubes.

Shielding will not fix it. Running heaters on DC or floating a DC voltage on the heater circuit usually fixes it, although there may not be room in some units for the additional circuitry.

Or you can try various tubes until you find some really quiet ones that don't have that problem. I have had good luck with the 12AX7LPS.
- RE: Probably heater to cathode leakage. - Jim McShane 09:19:11 11/19/15 (0)
  In some cases I agree - but I routinely check these tubes on my AT-1000 which shows H-K leakage. IIRC I have never seen over 1 ua from a Genalex, I can't recall the Tung-Sols' readings though, sorry.
  
  Edits: 11/27/15
Oh this is definitely some freak shielding type of problem - airtime 06:28:00 11/19/15 (2)
I have NO doubt about that!

However, I've tried the Belton shields, doubling up on tube sleeve shields, attaching a second ground from the shield to chassis.

And all this with the full steel case shield screwed down in place covering the entire preamp section.

The only variable, obviously besides the actual tube, is the tube plate orientation.

Check out your tubes and try that experiment. Remove ALL the shields and see if it hums as described with what ever position the plates are in.
- RE: Oh this is definitely some freak shielding type of problem - colebearanimals 14:03:50 11/19/15 (1)
  I removed all my shields. No hum. Moved all the pre tubes around checking plate orientation. No hum. I do have the s5000 with the slanted pre tubes and the stock shields but I don't think that should matter. Tis a puzzelment.
  PaulB
  - RE: Oh this is definitely some freak shielding type of problem - airtime 14:30:46 11/19/15 (0)
    post the plate orientation
    
    You know I have to check about that two channel hum thing????
    charles

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