03-06-2021, 07:33 AM
(This post was last modified: 03-06-2021, 07:34 AM by Murphyv310.)
Good sound (pardon the pun) info from Andy.
We have ascertained that the hum is most likely to be from the first stage. We discussed the possibility of a hum loop inside the amp, possibly caused by one of the input Jacks earthed and the screen lead also earthed at the other end causing a loop. I've explained how to check this. Also how valve bases can leak or even heater cathode leaks can cause this.
As the hum increases with volume control without an input, I'd still think its down to a hum loop, possible inadequate screening, bad earth at the volume control or poor routing of cables. Single point earthing though usually cures many hum issues.
It's virtually impossible to trace hum loops at source on a scope due to the virtual earth effect.
We have ascertained that the hum is most likely to be from the first stage. We discussed the possibility of a hum loop inside the amp, possibly caused by one of the input Jacks earthed and the screen lead also earthed at the other end causing a loop. I've explained how to check this. Also how valve bases can leak or even heater cathode leaks can cause this.
As the hum increases with volume control without an input, I'd still think its down to a hum loop, possible inadequate screening, bad earth at the volume control or poor routing of cables. Single point earthing though usually cures many hum issues.
It's virtually impossible to trace hum loops at source on a scope due to the virtual earth effect.
