Symptons of a dead tube in preamp?

Discussion in 'Amps and Cabs [BG]' started by Dave Metts, Oct 21, 2001.

  1. Here's the story. I recently purchased a used SWR Grand Prix preamp from a business on Ebay. It was listed in working condition. When I hooked it up to my rig (Carvin DCM1000 and Eden 410xlt) and powered everything on, I got nothing but static. However, when I plugged my bass straight into the power amp, things went alright. Furthermore, when I hooked up my friend's amp head to the power amp through the preamp out, everything worked. Is it possible the tube in my preamp is bad? Any other ideas? Also, how can I tell when my tube does go bad? Will the unit just fail to power on?

  2. When in Doubt, Pull it Out!
    Just replace the tube. No big deal.
    Once that's done, go from there.
  3. Maybe you aren't giving it enough time to warm up. I just know that in my ADA, you have to wait a couple of seconds (maybe 30) for the tubes to warm up. Just turn on your preamp, then turn on your power amp, then wait about 40 seconds, then try playing.
  4. Well, right now the amp and everything is back in my home town while I'm back at college for the weekend. Will the preamp really make a static/popping noise when it's warming up? The noise sounded to me like it's something that would be bad on the speaker cab were the volume turned up.
  5. My Alembic F-1X arrived with a bad tube from the seller. It was heavily microphonic, and responded with echos, etc, to light tapping on the case. I replaced it and no problems since.

    Spend a few extra dollars and get the best tube available. It does make a difference. Also make sure the replacment tube will fit inside the metal tube shield. Some are fatter than others, and will not fit.
  6. I can say for certain at this point that it's definitely not lack of warmup time. I let it warm up for about 5 minutes today before playing anything through it with the same results. Let me try to make my problem as clear as possible. When I have this all hooked up, I get ONLY the static noise (i.e. the signal from my bass doesn't even appear to make it through). As a matter of fact, the signal light on my power amp doesn't even light. However, the headphone jack on the preamp appears to work (just tested it) although it is overly hissy. I also did notice some major microphonic effects (I could hear my straplocks shifting through the headphones).

    Since I purchased this preamp from Ebay, I'm a little worried that I may not be able to get my money back, even though it was purchased from a business through Ebay. Additionally, I'm sure leaving negative feedback will in no way affect this business' reputation (1400+ positive feedback).

    Any other ideas? If it's not an easily repairable situation for this preamp, I'll probably start looking into an Eden WT-100, even though that will run about $700. All told I'll end up spending $1000 on a preamp where I should have had a working one for $300. I think I may have learned my lesson with ebay...
  7. It's the tube!
    The SWR Gran Prix is constructed with the finest components available. Noble Pots, Wima's, etc...

    Contact SWR if you put in a good tube and there are still problems. Tell them the problem and see how much they'd charge for a repair.

    Or you can sell it to me cheap and I'll take care of it.
  8. I'd say talk to the person that sold it to you before simply leaving negative feedback. With 1400+ positive responses, he's probably a legitimate seller and would be anxious to resolve any issues/problems with you.
  9. Open up the inside, count how many pre-amp tubes are on the inside. go to GC (return policy). Go buy a cheep Groove Tube's 12ax7. its $10. Put them in the amp, and set it up. If it starts to work congrats, you've got a good amp. Then, if you dont like the tone you've got, buy some more expensive fancy tubes (gc will let you return anything if its within 30 days.). Try it out.

    If the amp doesnt work, take back the tubes, complain to the seller. problem solved.

    have you talked to the seller at all after this? Possibly a tube got damaged while shipping. Maybe you can convince the seller to split the cost of a re-tube.
  10. The Grand Prix only has 1 tube. :D

    Be a big spender, and get a nice tube for under $20. This is normal maintenance, just like replacing your strings.
  11. I haven't had a chance to talk to the seller because the business I got it from wasn't open all weekend. Also this preamp was not shipped. I picked it up directly from the business that sold it on Ebay where it was listed in working condition.

    One question I have. If it is the tube, why would the headphone out work but the preamp outs not work?

    I think I'm going to have a tech I know look at it so I can tell whether or not it is the tube. I don't want to spend money on a tube only to have that not be the problem. Muttluk, while I love your idea about just returning the tube, there isn't a GC within at least a hundred miles of here. :)
  12. Take it back and have them make good on it.
  13. LowRanger


    Dec 24, 2000
    Maybe a dumb question, but have you tried a different cord for connecting the preamp to the poweramp? I would think that if you've got bass at the headphone out, you should have something at the preamp out.

    If your pre has a direct-out, can you patch that into your power amp to see what you get.

    I feel for you, hope you can get it resolved.
  14. I tried using the 1/4" and balanced XLR outs on the preamp to the poweramp, same problem. However, when I hooked the line out (balanced XLR) of my friend's amp head (some Genz Benz) up to the power amp, it worked as it should have (no EQ, but the signal was getting through).

    So just to get straight on the situation, is it safe to rule a bad tube out? I'm going to have a qualified tech look at the preamp before I go back to the place asking for my money back. However, if it's likely just a bad tube it would probably be cheaper to replace that then to have a tech look at it.

    You guys have been super-helpful (as you always are)!
  15. RebelX

    RebelX Guest

    Oct 27, 2001
    Merrimack, NH
    Sounds like a problem I had once. After playing a wet outdoor gig, the next day I couldn't get any signal other than snap, crackle and pop from the pre-outs on my Grand Prix. Thinking it was the tube, I swapped it out with no luck. I did find that if I plugged the line out (pre master) into my power amp, it was working. I ended up sending it to SWR for repair. They had to replace the "Master" pot and one of the caps. I don't remember how much it ran me, but I do recall that it was short money. I had my pre back in a week or so. Sounds like you may have a similar situation.

    On a note of dependibility, I bought the thing used 4 or 5 years ago and this was the only issue I had. I'm generally not that nice to my amps, and this one has taken all of my road abuse including being left out in the blazer overnight during the winter.

    Good luck,

  16. I think I'm just going to try to get my money back from the business I bought it from. The item was listed in working order, so I basically should not have to pay a dime in order to get it working. I bought it for $300, and it will cost at least $30 to even have it looked at. That's not a lot of money, but it's money that I don't feel I should have to pay. Sadly, this experience has soured me on buying used from ebay (and there are no stores in my area with decent used equipment) so I'll be paying full new price for everything from now on. I'm looking into getting an Eden Navigator pre if I get my money back on this Grand Prix. If for some reason I don't get my money back, I'll be in a bad situation as I don't have the money to cover the $300 spent on the SWR plus the cost of another good preamp.

    Thanks to everyone that has helped!