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Ampeg SVT-CL Strange problem

Discussion in 'Amps and Cabs [BG]' started by RissKippery, Mar 3, 2016.


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  1. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    Hello TalkBass! I am new here. I have lurked the forums for years for information whenever I am in a jam.

    However, I've been digging for a while now and am finding no solution to this new jam I'm in. I recently put new tubes in my amp. New Sensor Tung Sol reissue 6550's to be exact. I power on the amp and let it warm up for 20 minutes, give or take. After the warm up, I bias it, play it a bit, not cranked but fairly loud (maybe 2 on the master volume, 3 at the loudest). After about 5 minutes of play, the sound cuts out and the fault light begins to flash red and green. Every time it has happened (which is about 3 times now) I power it down and power it back on only minutes later and it works great for the entirety of my band's set, which is about 30 minutes.

    Like I said, it started once I replaced the tubes. It never did this before. Has anyone ever had this issue? Any and all information/help would be greatly appreciated!

    Lastly, please forgive me if this problem has been solved over and over in past threads. I was unable to find someone with the exact problem.

    Thanks!
    -Rick
     
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Sounds to me like one of the new tubes is kaput or at least not matching.
     
    Mike_1978 likes this.
  3. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    The tech I bought them from told me he'd pay the extra to have them matched, all the numbers on the boxes of each tube say "56". They were purchased from tubedepot.com. I will see if the amp tech has the receipt.
     
  4. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Well I could be wrong bu that's generally the case when you get the red/green.
     
  5. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    It's odd because they're brand new. They it also works fine after powering down and then on again. If only there was a cheap way to get new tubes again to test them against the "new" ones I have.
     
  6. Why did you replace the old tubes?
     
  7. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    How the run in the amp is the best way to tell if they're actually tested/matched. I've had plenty of problems with tubes that tubedepot calls "matched" which is why I don't buy tubes from them anymore. Your tech should have caught this when they were installed if they were giving the amp a good play test and bias check. The red/green fault light indicates an issue in the power amp, so it's either the tubes or an associated component that should have been checked when the amp was serviced.
     
  8. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    The tech I went to diagnosed a fault light a few months ago as a bad tube. He replaced just the bad one and said it was imperative that I get new ones. I ran it for a few shows with just the stock tubes and the 1 replacements.
     
  9. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    If it were a bad tube, do you think it would only fault in the first 5 minutes of use and then function fine after being powered off and on again?
     
  10. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Do the bias lights still light up red and green?
     
  11. ultra60

    ultra60 Supporting Member

    Sep 16, 2010
    Herndon/Chantilly Va.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Even though the tubes are new, there could be a problem with them. If there is an intermittent connection inside the tube, tapping the glass envelope with a wooden dowel or pencil while the tubes are cool may reveal it. When the metal heats it expands, it could be making a better contact and this might fixing the issue.

    Another possibility, tube pins come in different sizes. If the new tubes have smaller diameter pins and the socket terminals are stretched out, this could result in an intermittent contact. More often, a loose connection results in a crackling noise but this could still be you cause of the problem. When a tube is inserted into a socket of is pulled out, you should feel it to be solid. If the tube slips or out of the socket too easily, there is not enough tension on the tube pins. The solution is to retension the socket. This involves removing the tube, and carefully pinching the socket contact inward with a small jewelers screwdriver or needle. Ask your tech to check the socket terminal tension.
     
    Garret Graves and amusicalperson like this.
  13. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    They light up only green when I'm not playing. I biased it as instructed in the manual and several instructional videos I've seen. They both light up when I'm playing.
     
  14. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    Interesting. Will check when I'm with my amp. I've pulled them before and they seemed to be pretty snug into the sockets.

    I actually had a crackle recently and it was a loose preamp tube.
     
  15. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Try the tap test then. If you can tap at different locations around the bottle. Not too hard so as to break the tube.

    If it doesn't lead to anything, I would return the tubes outlining your problem and stating that they need to be checked. They are still under warranty.
     
  16. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    So when I'm tapping the tubes when they're cold, if there is a loose connection, it may go into the fault situation by disconnection with a few taps?
     
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It's possible that the shock could have an effect and reveal an issue. Pinging a tube is one way to test them. Normally the internal structure is pretty rigid but faults can occur.

    With an amp, there are always a usually number of possibilities and they all have to be examined. Since your amp was performing well before the tube change, likely places to look are the tubes and the socket contacts.
     
  18. cerrem

    cerrem

    Apr 4, 2006
    San Diego
    A few thoughts come to mind....
    Matched tubes from where you mentioned is a bit hookey....
    The other problem is those 6550A tubes need to be checked at high voltage for intermittent arcing....

    Matching tubes is simple...it becomes convoluted when rationale is used justify inferior test equipment.....
    Then there are no longer standard results....just lots of mystic numbers on the box....
    The industry standard for testing 6550A tubes back in the day was 250V PLATE / 250V SCREEN / -14V GRID .....
    The "ideal" 6550A would produce a gm = 11,000 @ 140mA
    This way everyone is on the same page and same standard...
    Brand new NOS 6550A tubes would usually be within 10% of this ..... I have Very strong sets in my SVT with gm =12,800 ....
    Some tube distributors would have TC # on the tube base, which would be the gm

    Purchasing "matched" tubes is only half the battle... You need to know at what "STRENGTH" these tubes were matched to ....
    For example someone can sell you perfectly matched tubes, however they can be very weak and useless...
    If your paying for "NEW" tubes, then you should be getting STRONG tubes...
    I have had customer show up with "new" 6550A matched tubes that measured up at 8,000....
    These were useless...installed into his SVT the amp barely made 290 WATTS clean output power...
    The same amp with a strong set of matched tubes produced 360 Watts clean..
    I had to put his "OLD" tubes back in the amp ...because the old tubes were stronger then the new tubes he just purchased..
    Tube vendors love customers who do not have the ability to test tubes....they send all their junk and rejects out to them...

    My first question is where are your old tubes ??? Try putting those back in the amp and see what is happening ...
    Please don't tell me your tech still has them or your tech threw them out ..... that is classic BS...

    Second question: Are all 6 tubes matched??? or did your tech buy 3 x Matched Pairs....
    The reason I ask, is if the 3 matched pairs are way far off from each other in strength, they can goof up the current sense circuit, since some tubes are pulling too much current and some too little ...
     
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Tubes should be matched at the operating voltage of the amp. You tell them what amp you have, they set the plate voltage to what the amp uses, then match the tubes. It is also important that they burn in the tubes before matching. This can help eliminate faulty tubes. They should be tested for microphonics. tctubes.com vintage vacuum tubes is one good seller that does this, some others do this as well. Antique Electronic Supply has a new tube testing and matching system in place that looks interesting.
     
    99Z3 likes this.
  20. RissKippery

    RissKippery Gold Supporting Member

    Mar 3, 2016
    Per the amp tech, all 6 tubes were mathched. I still have the old ones. I will test them today.

    In the event that these tubes weren't sold to me as what they actually are, where do you suggest getting tubes? Is there a reliable source or am I rolling the dice every time I have to get new ones?
     

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