Troubleshooting

vetmed · 1480

0 Members and 1 Guest are viewing this topic.

Offline vetmed

  • Full Member
  • ***
    • Posts: 245
on: January 22, 2013, 05:35:38 PM
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

Robert Lees


Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19702
Reply #1 on: January 22, 2013, 06:28:19 PM
How are you getting the 7.5V to heat the tube?

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline grufti

  • Sr. Member
  • ****
    • Posts: 320
Reply #2 on: January 22, 2013, 07:03:00 PM
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?



Offline vetmed

  • Full Member
  • ***
    • Posts: 245
Reply #3 on: January 23, 2013, 04:10:57 AM
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

Robert Lees


Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19702
Reply #4 on: January 23, 2013, 04:31:36 AM
Hum that doesn't vary could be the filament supply, B+ ripple, or ground loop hum.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man