Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on December 27, 2025, 09:19:04 am

Title: Tube Bias with a hum after effect
Post by: pbman1953 on December 27, 2025, 09:19:04 am

Happy Holidays!
I need some advice. I’m done with a re-cap and now I’m down to biasing. I’m in good shape with the bias and both sides ( A-C and D-F) seem to match well. I sent a picture of the results. I was told years ago to take 60-70% of the tube dissipation and set the bias to that range.
For this tube , the7581a has a dissipation of 35 watts, so the range is 21- 24.5 watts. You can see from the picture that I’m in good shape across the tubes. There is one issue, I’m getting a low-level hum. That hum can be taken down by the output balance pot but one side is hotter than the other.
The questions are-
Since its matched , leave it?
Lower the bias? which will reduce the hum
Or, something else?
 

Title: Re: Tube Bias with a hum after effect
Post by: tubeswell on December 27, 2025, 11:50:28 am

If the hum is coming from the output tubes, then swapping them around may help to hit a magic spot. Or not.


If the hum is coming from elsewhere, then no amount of fiddling with the output tubes will make it go away.