SVT-CL output stage problems.

Discussion in 'Amps and Cabs [BG]' started by el_Kabong, Dec 15, 2005.

  1. el_Kabong


    Jul 11, 2005
    I've had a 6550 and screen resistors die twice in the last 12 months (new amp). It's happened with both the original sovtek 6550's and a replacement set of EH tubes. Am I just unlucky or is it likely there's an underlying problem here that's yet to be diagnosed? Is there a particularly rugged tube (other than mega $$$ nos) that's worth trying? Would changing to KT88s be a good move? What's your advice tube oracles?
  2. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    Changing to Kt-88's would probably be a good move. I am not sure if that would solve your problem though. Why do you think the tubes are the cause?
  3. el_Kabong


    Jul 11, 2005
    I don't know what the cause is, that's what I'm asking about. Just read that KT88's are supposedly a more robust tube. If they're a good idea, next time I need to feed it tubes I'll put them in. Do they sound much different to a 6550?
  4. N*Joy


    Nov 30, 2002
    Birmingham, UK
    Sounds like you need to dig out the receipt and flex the warranty mate!
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Depends on how old the unit is. If it's from the last 2 or 3 years, it should have a different screen resistor from older ones.

    There was a change made then, changing the screen resistor value, and removing a diode. Newer tubes seemed not to like the old setup.

    What's the serial number?

    If it has the resistor and diode in screen circuit, it's the older setup.
  6. el_Kabong


    Jul 11, 2005
    The serial number is BJNDPA0037. The tech who's working on it mentioned that the screen resistors were 'tiny' and that he was replacing them with '1 watters as they should be'. Does this sound right Jerrold?

    edit: ps Jerrold, are KT88s a good idea in these amps or is it best to stick with 6550s? Reliability and clean headroom are my priorities.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Your serial is right around the changeover, but the guy I need to ask isn't here this week. And I'm not, next week.

    If yours has a diode in parallel with the screen resistor, then its the older style.

    If there is no diode, it is newer style.

    It is perfectly possible for tube problems to cause trouble like that, although the point of the change was to minimize them.

    The "size" of the screen resistor can be deceiving..... the actual rating is more than it looks, AND the screen shouldn't be drawing enough to heat up the resistor excessively anyhow.

    If for some reason the plate is not getting the right current, the screen may draw excess current. That is something the tech ought to look for also. Probably not an issue if the right power out is obtained...but....
  8. Rickenbackerman


    Apr 17, 2001
    Laurel MD
    The screen resistors were always 1/2 watt in the older amps - I wouldn't exceed this since they can act as fuses in the event of a blown/shorted tube and help protect the OT.
  9. never had that problem with mine...

    Sorry to get in the Thread but I had a good experience with the Sovtecs and not with the EH... Do you guys recommend the KT88's?
  10. I just put a sextet of EH KT88's in my SVT-CL and so far I really like what I'm hearing. Definitely livelier sounding than the 6550's I had in it previously. Makes my TBird sound huge-r and my jazz stand out as well. I think the true test will be when I get in a room with band and play loud. I did a fair amount of research on them before I bought them and I read a lot of positive things about KT88's in SVT's. I did a lot of price comparison before I did buy them and a Tube Depot had good prices. 49.95 per pair and 153 bucks total including shipping from Tube Depot

    You may already know this but make sure to get the sextet matched.
  11. el_Kabong


    Jul 11, 2005
    Thanks Jerrold. So if I have the old setup do I just ask the tech to remove the diode? What should the screen resistors be in the newer style setup?

    Hey Rockit, do the user bias controls on the svt-cl still work correctly with KT88s? I've read some good things too, might have to get a set to try.
  12. It took me less than 2 seconds after the amp had been warming up on standby for 20 minutes to bias it.

    Worked perfectly!

    According to the guy @ Eurotubes, 6550's are sterile sounding in comparison to KT88's when used in an SVT. If my amp sounded that good and was considered "sterile" sounding I'll probably be pretty happy. Like I said, I do like what I hear but I can't wait to get into a room with a band and crank up my amp to reallyt hear the difference.
  13. el_Kabong


    Jul 11, 2005
    Let me know what you think after a gig Rockit. The only negative thing I've heard about KT88s is that you might lose a little growl. You still happy with your jazz on the bridge pickup? The sovtek kt88s are very cheap where I am, and PBG seems impressed with them.

    ps Jerrold, thanks for the info.
  14. No bands as of yet, stil trying to find the right situation. But when I go jam with someone I'll let you know my opinion of the KT88's.

    I always run both p/u's on my TBird and Jazz wide open and get tone from my amp, love the sound I get. Not a knob twiddler and the music I play is loud, fast, straight ahead rock and I need lot's of bottom and some mid.
  15. el_Kabong


    Jul 11, 2005
    Looks like at least part of the problem with my amp is that the bias led's are not accurate. My tech tells me that when he uses them the amp is considerably under biased, ie the tubes draw too much current and run very hot. I guess this is good news as now the problem can be corrected.