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Ampeg svt vr channel 1 problem

Discussion in 'Amps and Cabs [BG]' started by markoSirec, May 28, 2011.

  1. markoSirec


    Mar 1, 2009
    Hello guys,

    i just bought a new SVT VR, made in Vietnam. After playing it for about 1 hour at our rehearsal space, i but it into standby and had a break. after turning standby OFF, there was no sound, just slight rattling that sounded like weird electricity problems. also one of the bias leds was flickering.

    the fan is working like it should, also the led on the front panel is green. If i put the cable into channel 2, it works normally. the place where we practice does have some weird electricity sometimes, but its weird that channel 2 is working, so i guess its not the relay problem some of the other guys have experienced. Any help would be greatly appreciated :S

    The head is still under warranty, but i would have to ship it back to England (i am from Slovenia - about 1300km away)... not something i wanna do if its only a small problem.
  2. markoSirec


    Mar 1, 2009
    edit: if i turn the volume knob of channel 1 to the maximum, you can hear the bass, but its very quiet.
  3. First thing is to switch the tubes in CH1 with those in CH2 and see if the problem follows a particular tube. If it does, replace that tube. If not then take the amp to a qualified tech. His bill may be well less that the return shipping.
  4. Paul's suggestion is a sensible first step. With the bias lights flashing, I would think you have a power tube malfunction, which is not uncommon. Paul is correct that you should take it to a local but qualified tech. Ampeg probably has a warranty repair person much closer to you than England. Contact them and find out.

    A failed power tube is a simple fix, if you have the right equipment to trouble shoot power tubes. Otherwise, you're dead in the water, so to speak. From my experience, Ampeg has been very reasonable about early power tube failures. It's a fact of life with power tubes these days, though I haven't had a single problem with Winged C brand tubes.

    While your tech has the amp, you might ask him/her to do JohnK's mod to bypass the brown-out relay. Not sure if your amp is 120V or 220V, so the relay may function differently; the mod may therefore be entirely moot.
  5. markoSirec


    Mar 1, 2009
    thanks for the tip guys. so it is not uncommon for new tubes to die with these amps?

    we have 220V here :)

  6. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Tube failure rates depend on manufacturer, but it's not uncommon for a new amp to come with an inherently bad tube. If it is a pre-amp tube, the ampeg warranty on tubes is 90 days but I'm not sure what hoops you'd have to jump through for that. The good news is you can get some decent quality new manufacture pre-amp tubes for cheap.
  7. Tim1


    Sep 9, 2005
    New Zealand
    I am on 240 volts and recently did the johnk standby mod. It is even easier if on 220/240 volts as you do not need to make up a splitter wire, you simply remove the wire from J26 and push it on to whichever of J27 or J28 is not used (in my case it was J28).
    And by the way, I have had my Vietnam SVT VR for two years now, it has been gigged regularly and behaved faultlessly throughout that time :) . Welcome to the club.
  8. Yes, but that's not specific to your amp. It's a general experience with current production tubes. Generally, power tubes can quiet easily fail within the first few hours of use. Your amp has six of them, and it only takes 1 failure to cause issues for you.

    My tech calls it "infant mortality" when a tube fails within the first few hours of use. He tells me that in his experience, chinese tubes in general, and EH power tubes have a higher incidence of this problem.

    Your amp probably came with Sovtek power tubes (made in Russia) and generic Chinese 12AX7s in the preamp/driver slots. Maybe ElectroHarmonix 12AU7s. If you just bought the amp, I'll bet that Ampeg won't give you any issues over replacing the power tubes. I know of someone who reported a power tube failure and was sent a new sextet of matched tubes from Ampeg. Follow up with them.
  9. markoSirec


    Mar 1, 2009
    guys,thanks so much for your help.i appreciate it...

    After spending the evening googling the sh*t out of this issue i'm almost certain it's a bad preamp tube so tomorrow i will try to swap them and see what happens. i'm no tech guy but i should be able to do it by myself.

    Will post results!

    The amp sounds sooo sweet,damn i feel like i had sex with a gorgeus lady for 5 minutes and then she put her clothes on and left before we...

    I want more:>
  10. Earth to Jimmy! SVT VR question. Light the "Jimmy signal light" asap!:)
  11. markoSirec


    Mar 1, 2009
    So i swaped the preamp tubes today.sadly with no luck,channel 1 is still very quiet.

    Visually all the tubes seem fine. I guess i have to ship it back:/

    Btw, the preamp tubes are all EH made in russia,the power amp tubes are sovteks made in russia.
  12. oinkbanana


    Oct 20, 2011

    does anyone know the tube layout of the preamp?

    is this accurate:
    i ran into the same problem as the OP just last night at practice.
    channel 2 is working no problems
    channel 1 is silent (volume knob of channel 1 to the maximum, you can hear the bass, but its very very quiet)
  13. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    I think V4 and V5 are reversed on there, assuming it's supposed to be like the vintage amps but really that's immaterial to the issue. If channel one is out but channel two is good it's probably one of the first three pre-amp tubes listed. Try swapping them for ones you know are good and see if that fixes the issue. Had this happen on my SVT once, made me glad to have a second channel ;)
  14. oinkbanana


    Oct 20, 2011
    thanks. i've got extra tubes, so hopefully it's all A-ok!

    and from what i've read you don't have to re-bias pre-amp tubes, so it's just a swap and test.
    hopefully taking it apart won't be too much of a hassle and i'll be able to swap and test, swap and test, swap and test without too much difficulty.

    looking at the schematic attached (taken from johnk_10's post), it looks like V5 is for channel 2.
    anyone have a better schematic, It doesn't tell me which jumpers should be connected... har har.
    J5>J7 for channel 2
    J6>J8 for channel 2

    just for reference, anyone have a full schematic of the VR? I justhave the preamp section I think (titled "SLM Bass Amp)

    Attached Files:

  15. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    i have all of the schematics for it (there's 4 of them). email me and i'll email them to you.
  16. oinkbanana


    Oct 20, 2011
    yup, preamp tube V1 the 12AX7 was the problem.
    I switched it with V5, and then it was channel 2 that wasn't working... har har.
    So i'll pick up a new 12AX7 tomorrow!
    maybe i'll look for an upgrade from the stock one. off to the search function.

    and extra thanks to John for hooking me up with the schematics!

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