Acceptable level of hum at max volume?

Dear Roggae: Pbnaaudio posted:

++++++++++ A Phono Preamp has a LARGE amount of gain - some hum will almost always be present - if you cannot hear it at your listening position between tracks at normal to loud listening level - don't worry about it. ++++++

and Stringreen posted about some hiss level.

IMHO both are unaceptable by high-end standards. If we have it in our systems then we choosed the wrong electronics designs for the " job " or the wrong set up, period.

Regards and enjoy the music,
R.

Hum is not inherent in tubes even if there is no shielding, any more than in transistor circuits.

However, there are compromises that can occur in a design wherein a slight amount of hum at full volume might show up. I would contact the manufacturer and see if they think it might be normal.

Actually, I may have overlooked one factor that does differentiate tubes from transistors, and that is with regard to the filament supply. I suppose it is possible, even in an indirectly heated triode, that noise in the filament supply could leak into the cathode, and the AC component might then be amplified as hum. In a well designed tube preamplifier or amplifier input stage, the filament voltage should be fully rectified to DC, with very low AC component, and regulated. If the filament supply circuit is malfunctioning or if there is a "leak" between filament and cathode within a given tube, the result might be a faint hum. In either case, repair is possible. (If the source is a filament to cathode voltage leak, the tube should be discarded and replaced. Perhaps that is what Jburidan experienced.) A competent tech should be able to figure that out.

If you have regulated filament supplies, even with a cathode/filament short in a tube there is no hum.

One thing have have found though is a lot of tube equipment manufacturers don't know how to ground their gear. I can name a few famous ones off the top of my head. *That* can certainly contribute to hum but also has nothing to do with tubes.