A couple ARC SP-6 questions, s'il vous plait.

Discussion in 'Tube Gear' started by GuyK, Nov 26, 2016.

  1. GuyK

    GuyK Junior Member

    My SP-6 has become quite noisy in the phono section. A suggestion in another thread was to clean all the tube pins and contacts. I've done this and there has been no improvement. The manual states that "if excessive noise should develop in the phono section only, it is most likely V1." This tube is easily found in the schematic supplied in the manual, but how do I identify this part in the case without the part numbers being labeled in the chassis? Then, once the correct tube IS identified, are tere any special biasing procedures required once the tube is replaced, or is it pretty much plug-n-play? The manual does not give any indication of this; it's only a user man, and not a service man.

    Last, I aquired this second hand a couple years ago and have listened through it extensively, so should I go ahead and re-tube the whole thing or just replace the likely culprit?
     
  2. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    As you face the front of the preamp, V1 is the tube in the right rear, closest to the phono inputs.

    You could certainly try just replacing that tube. V1 is a critical tube in that phono stage so if you do only replace V1, I'd invest in a good one.

    What brand(s) are your current tubes and do you have access to a tube tester? If you have some nice NOS Amperex, Telefunken or the like, try testing them to see how the others are aging.

    Personally, unless there are some nice NOS tubes in there, I'd probably replace all six.
     
  3. GuyK

    GuyK Junior Member

    It was supposedly re-tubed before I purchased the unit, and this indeed the case. All six are JJ ECC83S's. Apparently functional (for awhile), but about as cheap as one could get. Definitely nothing special. I don't have a tester or access to one. I 'spose I'll have to go get one nearby to be able to listen to lp's while I decide what to do for a better solution. Not entirely certain I want to spend big coin on a hextet of really nice (but possibly used) tubes, and I don't want to cheap out, either. This is too nice a pre for super cheap, so I've got some thinking to do.
     
  4. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    The JJs are not horrible tubes by any means. ;)

    Gold Lion makes a good 12ax7. I'm running Tung-Sol 5751 tubes in my Quicksilver Line Stage Preamps - you may want to try their 12ax7.

    Whoever you get your tubes from, make sure that they test and match your tubes on a curve tracer.

    To answer your other question: No biasing is required for your preamp tubes - just plug and play.

    I had an ARC SP-6B. Still have my SP-3 which has been updated to SP-6B status. Really good preamps. IMHO, some of the best that ARC ever built.
     
  5. GuyK

    GuyK Junior Member

    No, the JJ's aren't horrible. But neither are they anything special. Not by a long shot.

    How does the SP-6 compare to the 6b? Specs seem to show the 6b with lower distortion, a slightly revised power supply and a more sophisticated phono section.
     
  6. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    The 6B is, in my opinion, has the most balanced sound of the 6 series preamps. Incredibly musical and listenable. The 6 and 6A are also fine sounding preamps, I just prefer the 6B. The 6C was a little lean for my taste. The 6E is pretty much the same as the 8.

    You have a fine preamp, one that you could easily be happy with for many years to come. :)
     
  7. JohnVF

    JohnVF Administrator Staff Member

    I can 2nd the recommendation for the newer Gold Lion 12ax7s. I had been rolling a bunch of healthy NOS tubes in my Juicy Music phono stage and the 12ax7 slot was best filled by the Gold Lion I tried, which surprised me.

    I'm nearing the nerve to tackle the noisy phono section in my VAC preamp so I can use the Juicy Music one elsewhere, and will probably just go about cleaning the sockets and replacing the aged tubes in there with GLs.
     
  8. GuyK

    GuyK Junior Member

    Just got a Tung Sol for replacement while I ponder what to do for the long run. It has made absolutely no difference. Sounds like a filthy lead-out groove when phono (and only phono; all other inputs are dead quiet) is selected, even without a tt being on. Irritating to say the least.
     
  9. GuyK

    GuyK Junior Member

    Just a follow-up here. I cleaned the pins and sockets for no improvement and replaced V-1, also for no improvement. So a week ago I took the unit to a shop and they cleaned things again for a BIG improvement. But it's still not as quiet as it was. All the other inputs are DEAD silent, phono is noticeably noisy, even on the low-gain setting. Does anyone know how low the noise floor is in one of these? If this is normal? Should I try to source some extra-low noise tubes?
     
  10. GuyK

    GuyK Junior Member

    And the noise just came back like someone hit a switch.
     
  11. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    I had an ARC SP-6B and still have an SP-3 that was updated to an SP6B.

    In both cases the noise floor of the phono stage is more than acceptable. I have used the updated SP-3 with 104db efficient Lowthers and not had an issue with excessive noise in the phono stage.

    You can certainly try some ultra low noise tubes (I never had to in either of mine) but there may well be another issue impacting the phono stage noise floor in your SP-6B.
     
  12. GuyK

    GuyK Junior Member

    And tonight phono is absolutely silent. I think I'll have to just monitor things for a bit and see what happens.

    edit:
    An hour in and the noise returned again. I'm going to take back to the shop and insist they re-examine it.
     
  13. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    Is there an appliance or fan that kicks on periodically and runs while you are listening?

    Are there any ham radio operators or CB radio base stations with linear amplifiers in the neighborhood?
     
  14. GuyK

    GuyK Junior Member

    So, I took the thing back to the shop and explained the situation. They took it back for a week and then called me to say their tech had run the thing for three hours, as in hooked it up in a system at the bench and played records through it while he worked on other things, and was unable to duplicate the issues, so could not do anything further with it. Therefore, I could come pick it up again. Got it home and installed and phono is dead silent. For an hour. Beyond that is unlistenable. I'm ready to unload at these folks, they're starting to appear very incompetent.
     
  15. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    Do you have another preamp you can insert to make sure the noise isn't coming from another source?
     
  16. GuyK

    GuyK Junior Member

    Yes, but it's going to mean dismantling another system to do this. I'm certain it is the ARC unit, but will do this regardless just to prove it, if only to myself. Then I think I'll take it to another repair facility.
     
  17. GuyK

    GuyK Junior Member

    So, here we are just over a year later. I took it to another tech who had it for an entire month and some. Got it back and it was quieter but ran HOT, and just didn't sound quite right. And just after his warranty expired started popping fuses. So I took the system apart AGAIN, and took it down to the only fellow in the area I can really trust to work on stuff like this to see if he can suggest somebody to do some work on a tube unit, since I know he only works on solid state. He asks me "What tubes?" I reply "12ax7's." And then he remarks that these aren't power tubes, so he's willing to take a look. So after another month, a handful of zener diodes, a couple resistors, a large transistor, and a new set of tubes I have it back, it's dead silent, not overly warm, and sounds pretty good again.

    Sure wish I had known that John would do small-signal tube equipment in the first place, it would have saved myself a bunch of stress and a bit of money.
     
    TubeHiFiNut likes this.
  18. TubeHiFiNut

    TubeHiFiNut Administrator Staff Member

    Glad to hear that you got it sorted out.

    The SP6B is one of my favorite ARC preamps of all time. :)
     

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