Hum adjustment

Comity · 3309

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Offline Comity

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on: February 16, 2021, 09:12:59 AM
Hi All,
I have a question that may be obvious to the more experienced.  Should one do a hum adjustment each time the power tubes are changed out?  I recently noticed a hum in one channel of my Stereomour (previously masked by the crackling noises from having my WIFI router too close to the amp), and when I switched the power tubes the hum changed channels, so it is definitely tube related.  I have been using JJ's 2A3 tubes since about 2015, and the tube that has more hum appears to have a much darker glass support at the bottom of the tube compared to the non-humming tube. The glass support is almost black compared to the other tube in which the glass support is still clear.  This got me to wondering if I should have done a hum adjustment when I first installed the JJ's.  I did the hum adjustment when I built the Stereomour with the original Sovteks.  Also, would there be any reason to go back and do any resistance or voltage checks?  Those were all within normal range after I built the amp. Thanks for any input on this topic.

Greg Kelso


Offline Doc B.

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Reply #1 on: February 16, 2021, 10:02:24 AM
Yes, you need to check the hum trim any time you change the tube.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline Comity

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Reply #2 on: February 16, 2021, 10:34:08 AM
Thanks Doc....I should have realized that.   Do you think that not adjusting the hum trim could have played a role in causing the tube to hum prematurely and also darken the glass support?

Greg Kelso


Offline Doc B.

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Reply #3 on: February 16, 2021, 11:17:48 AM
Nope. You say the tube is 6 years old. That's a pretty healthy lifetime for a 2A3. Have you tried trimming out the hum on the tube in question? It certainly wouldn't hurt to do a voltage check to see if the tubes are drawing extra current from age.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline Comity

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Reply #4 on: February 16, 2021, 12:33:56 PM
Thanks Doc.  I appreciate your input and will go ahead and trim out the hum on the questionable tube and check voltages.  It's probably a good idea to go ahead and confirm everything is still in spec.

Greg Kelso