1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

1970 Ampeg SVT blue face 6550's power amp issue

Discussion in 'Amps and Cabs [BG]' started by Masterofsparks, Oct 14, 2018.

  1. Masterofsparks


    Oct 14, 2018
    I recapped the amp with a kit from Fliptops. I believe all the caps are installed correctly, but anything's possible. The problem WAS that the last 2 tubes were burning super hot. I replaced the diodes. Tested all tubes. When I try to turn the standby switch the R52 resistor burnt up. I've replaced it and checked the D11-14 diodes. I've gone thru the board and tested every resistor. Any help greatly appreciated.
  2. Masterofsparks


    Oct 14, 2018
    Also, before the R52 issue, R49 is/was running super hot as well.
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Rule number 1 in amp repair is to diagnose the actual problem AND correcting before doing any other "upgrades" or modifications.

    I see this sort of thing happen all the time. There is no way to know what the problem might be, you have a lot of work ahead of you to determine what and where your problems might be. I said problems because you likely now have more than one.

    WHY did you think you needed to "re-cap" your amp?
    bassinflorida likes this.
  4. Masterofsparks


    Oct 14, 2018
    It was humming and making the classic bad cap noises. You know, the thundering/white noise/hum sounds
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    White noise is unrelated to "bad caps" Hum MAY be related to a bad (filter) cap, but there are at least another dozen possible causes.

    Did you measure all the voltages and especially measure the ripple voltage at each node of the supply?
  6. Masterofsparks


    Oct 14, 2018
    No. Was getting set to do that when the internal lightening started. Can I turn it on, leave it in standby & test? Will the proper voltages be present in standby?
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The problem you have now is not knowing where to start. I suspect that you have 2 or more problems.

    The first thing I would do is pull the power tubes (label where they came from) and the two phase inverter/driver tubes, then start measuring voltages at the PS caps and then at the sockets. You will need to be systematic, I would suggest an 11x17 copy of the schematic and of the wiring layout and go wire by wire, connection by connection to be sure the layouts match up. Then make your voltage notes on the layout and on the schematic in another color than black. This way, hopefully, you can eliminate (or rule out) entire sections of the amp from consideration.

    Word of very real caution... since you don't know what's really wrong, and there are high voltages present, you better be darn familiar with safety principles AND I would also recommend wearing safety glasses so that in the event of a power supply cap failure, your eyes are protected. This kind of failure can be spectacularly violent.
    Old Garage-Bander likes this.
  8. Masterofsparks


    Oct 14, 2018
    Thanks. I will pull my tubes. (I labeled then when I put them in 6/16) - I'll power it & begin testing. I think I best wait til tomorrow morning at this point. I have schematic 591720 from the tube amp book. It's as close to what I have in front of me as I've been able to find. My amp never had all those diodes that ran parallel to the 22ohm to tube pin #4. Other than that it's quite similar
  9. Did you check VERY carefully over and over again to make sure you recapped it correctly? If it worked OK despite the hum before you recapped it, 99.99% you made a mistake, you might even might want to consider taking it to a good tech before you screw it up even more. I recapped a couple of SVT's years ago and took a LONG time to do it and then made 100 percent sure I had it wired correctly before I turned it on. Those are not the simplest amps in the world to work on and as agedhorse intimated, they can kill you with their 660-700 volts in places.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Man, you really need to have the correct schematic, otherwise you don't know what is your mistake and what is truly a version difference.

    Did you take pictures of every cap before you replaced them, being sure to mark "+" on each of the old ones prior to taking pictures? This is one helpful thing to do whenever working on something complicated.

    Often, I will do one section at a time and then test to be sure I have the correct results. Actually, whenever I design a new amp and build it up in prototype stage, I will build it up stage by stage, making lots of notes.
    MrCash likes this.
  11. Another lesson, hard learned, unfortunately.

    Those You Tube amp repair guys make it look so easy.
    Kind of like the cooking shows Mrs. GB likes to watch.
    She’s a great cook, who knows it isn’t often as simple as it looks.

    One word in either case... editing.

    You don’t get to see the mistakes and the problems that pop up when cooking, or repairing electronics.
    You just get to see the good stuff that happens. All packaged nice and neat for your viewing pleasure.

    What do I know about it?
    Been an electronics tech for about 45 years now. Full time regular day job.
    During that time I also did a lot of TV program editing.


    And if you’re taking amp repair advice from someone else by remote control, the person offering the advice is going on what you’ve told him. If you had to ask your guru in the first place, you may not be giving him/her the proper information for them to give you the right advice. Your guru DOES NOT have eyes on.

    Would you have a med student take out your appendix based on his diagnosis and advice from a doctor that he got in an email?

    Just like surgery, just like cooking, just like electronics repair, you start out with basic knowledge and work with an experienced person looking over your shoulder, seeing what you see and helping you learn how to learn.

    There is nothing wrong with learning some things via the internet and starting out in easy things, working your way up through more complex repairs. But if you haven’t bothered learning any basic electronics theory and service procedures, and jump right in on a complex and dangerous piece of equipment while only taking the advice of someone who street creds you haven’t verified, then it’s going get real real for you real fast. That is if you don’t kill yourself first.
    Last edited: Oct 16, 2018
    Bim1959, JimB52 and cerrem like this.
  12. Masterofsparks


    Oct 14, 2018
    I am not a complete novice. My father repaired tvs, stereos, vcrs as a hobby. Unfortunately he passed on. I have all his electronics parts & tubes which are quite vast. Have every HealthKit probably ever made. I have repaired this amp before. In 2002 abt 4 -5 of the 10 ohm 5watt resistors that connect to each plate 3 blew up. I've also repaired a Reverborocket and the monster altec 9440a 800watt power amp.
    Joe Piazza's 1970 schematic is like my head. However, nothing is labeled as far as R#. So I've had to refer to the aforementioned drawing. I've never worked on something 'live', so I will be extremely careful- I hope.
  13. cerrem


    Apr 4, 2006
    San Diego
    Keep in mind that B+ voltages may be way higher than normal when the amp is un-loaded with-out the power tubes....in some cases it may exceed the filter cap voltage ratting....
    Make sure that you properly discharge those caps after you shut-down the amp and before your ready to work on the circuit....
    One thing to verify is to check that the BIAS cap has it's + lead going to RETURN...
    Keep in mind that the very first filter cap comprises of two caps in series.... Typically the AXIAL C10 cap is in series with the large C12 can mounted on chassis....
    These cans are mounted on floating phenolic wafers and not chassis grounded until later on in the circuit.... Keep in mind this amp when I got it was cooked well done burnt traces..so excuse the mess....
    Here is a photo of how I have my SVT caps...
    IMG_5047.JPG IMG_5050.JPG
    Bugeyed Earl and Masterofsparks like this.
  14. Masterofsparks


    Oct 14, 2018
    I guess I should just give up according to you???
    Ppl ask questions and getting smart ass answers/remarks doesn't look well for the site, but such is life I guess.
  15. Masterofsparks


    Oct 14, 2018
    Cerrem- thank you!! That's what mine pretty much looks like. I find it kind of frightening how the caps are just 'floating there'. Thank you so much for the photos. I'll compare with mine in the morning. I appreciate it very much!
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Those parallel diodes are not needed. Ampeg has them on their schematic but there is a note DNP, no not place.
    Masterofsparks likes this.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There are so many possibilities. Start be checking all the 22 ohm resistors. With the power tubes removed, test from each power tube pin-4 to node E at the capacitor. You want to ensure that there is not an open circuit. The 22 ohm resistors can blow open when the tubes are badly stressed.

    Which two power tubes are acting up is importantin determining what is wrong.
  18. No you just did things in the wrong order.
    You should have come here first and asked questions.
    Rookie mistake. You’re not the first, you won’t be the last.

    I Don’t hold any ill will towards you. But hope you’ve learned something from all of this.

    This place is littered with people who’ve come here only after they’ve gotten into trouble.
    They could have just as easily asked some questions ahead of time.

    Even after 45 years, I know enough to contact tech support regarding a problem before I just jump in and start messing around. Knowing what one doesn’t know comes with experience. The more you know, the more you know you don’t know.

    Had you come here and asked about changing out all the caps, you would have gotten several people advising you to not do that.

    Hate me all you want. As long as you’ve leaned something from this, I don’t really care how you feel about me.
    I’m too old to be concerned with bruised egos. Mine or yours.
    ThisBass and Kelly robinson like this.
  19. Masterofsparks


    Oct 14, 2018
    The 2 power tubes that were burning super hot are the last 2 to the far left if looking from the front of amp, the far right from the rear.

    This guy made a video using a thermal gun. His problem, he said was the 10ohm 5 watt resistors. I've already tested every resistor on the board. So it's not those.
    svtb15 likes this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.