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Can hum pots drift?

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Posted on February 10, 2016 at 04:49:00
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Have noticed that the right channel 300B tends to "drift" back to having some filament hum after a couple of hours even though I had it set with no hum at all originally. In each case the amp had been warmed up before I set it and again when it was drifting off. Is it more likely the tube that is doing this? I hope not. Guess I will have to switch positions of the 300B's to see. Anyone run into this before?

 

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RE: Can hum pots drift?, posted on February 10, 2016 at 10:23:53
drsx
Audiophile

Posts: 369
Location: around
Joined: September 11, 2005
 I once made a CCS board that used a trimmer to set the current. Over time, the current would cause the trimmer to turn and to ultimately turn itself off.

 

RE: Can hum pots drift?, posted on February 10, 2016 at 11:44:00
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Was that in days or hours? The thing about this is it happens in just a few hours.

 

RE: Can hum pots drift?, posted on February 10, 2016 at 12:17:41
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 If you re-adjust the pot to eliminate the recurrent hum, does the "cure" last longer or indefinitely the second time? That would suggest that maybe the pot itself is changing value slightly due to heating up over time. If so, you could choose to tolerate a little hum early after turn-on that then should go away after temp stabilizes.

 

RE: Can hum pots drift?, posted on February 10, 2016 at 12:55:01
drsx
Audiophile

Posts: 369
Location: around
Joined: September 11, 2005
 About an hour in my case. No idea if this is your issue.

 

RE: Can hum pots drift?, posted on February 10, 2016 at 15:55:43
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 It certainly could be the tube. Filament wires do change shape when heated, and the very tiny filament-grid distance controls transconductance. Some tubes - I seem to remember the otherwise excellent sounding Svetlanas - would sometimes warp until the filament actually contacted the grid. The experience, while entertaining, is not recommended :^)

 

update, posted on February 10, 2016 at 17:38:42
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Switched channels with the 300B's and now I am really puzzled. I figured if it was the tube it would follow the tube and if it was the pot it would stay with the pot. To my surprise it did neither, it just disappeared. After two hours the hum still had not returned. What the ????? Does this make sense to anyone?
Going away for a few days, but when I get back I am going to give it a really good listen and see if it shows up again.

 

Absolutely, posted on February 11, 2016 at 05:00:24
Chip647
Audiophile

Posts: 2652
Location: The South
Joined: December 24, 2012
 Your tube pins/socket were not making good connection. There could have been dirt or film on either surface that was scraped off when you swapped the tubes. Always make sure your tubes have good connections before troubleshooting other causes. Check you solder connections to the socket as well.

 

RE: Absolutely, posted on February 11, 2016 at 16:16:20
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Sounds like good advice. Now to see if if goes awhile without doing it again.

 

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