Troubleshooting

[vetmed]

I've built a headphone amp with vt-25 tubes using a S.E.X. amp as the donor chassis. I get a loud hum in both channels which is drowned out by moderately loud and up levels of music. I've read all the posts here about tracking down hum, and I am perfectly willing to spend the time necessary to reflow all ground connections when I find the time. Is there any significance to the fact that the hum is constant and does not vary with volume setting?
             Thanks
                 Robert Lees

[Paul Birkeland]

How are you getting the 7.5V to heat the tube?

[grufti]

You may just have to post a schematic. It sounds like you might have used nothing except the SEX chassis. In which case nobody can really help with your question. What exactly did you build?

[vetmed]

Heat is provided by a pair of Pete Millett's filament supplies, each has its own transformer for AC supply. As a "proof of concept" I use these in a DHT preamp using '01A tubes; its dead quiet so I am fairly confident that this should work with the vt-25. Perhaps something to do with the separate AC transformers? At any rate the original question concerned whether or not the unvarying level of hum is a clue to the origin of the problem remains unanswered

Regards
       Robert Lees

[Paul Birkeland]

Hum that doesn't vary could be the filament supply, B+ ripple, or ground loop hum.