Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: Comity on February 16, 2021, 09:12:59 AM

Title: Hum adjustment
Post by: Comity 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.

Title: Re: Hum adjustment
Post by: Doc B. on February 16, 2021, 10:02:24 AM

Yes, you need to check the hum trim any time you change the tube.

Title: Re: Hum adjustment
Post by: Comity 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?

Title: Re: Hum adjustment
Post by: Doc B. 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.

Title: Re: Hum adjustment
Post by: Comity 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.

Title: Re: Hum adjustment
Post by: Pkay on March 25, 2025, 08:12:16 PM

Is it okay to trim the hum trim while the amplifier is connected to speakers.  ie, check the hum trim by ear rather than instruments?

Title: Re: Hum adjustment
Post by: Paul Joppa on March 26, 2025, 05:15:07 AM

Yes, it works quite well unless the hum is inaudible - in which case, you don't need to get it better  :^)