Ampeg SVT-VR Balance Adjustment?

Discussion in 'Amps and Cabs [BG]' started by Slpbass81, Dec 10, 2015.


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  1. Slpbass81

    Slpbass81

    Dec 16, 2010
    Orlando, FL
    Endorsing: GHS Strings
    So a few months ago I was posting that I was having issues with my SVT-CL amp. Took it to the shop a couple times and no solution. Well 8 months later and a bunch of money spent I figured out one of my Mogami patch cables was bad. The reason it never clicked in my head before was I change my pedalboard setup for each show so sometimes it would be before my tuner that mutes the rig and sometimes after.

    Anyways, I decided to buy new power tubes for my backup SVT-VR. I took it to the shop and got everything set up but it was still making noise so I bought some new JJ's. I adjusted the bias but I don't know how to do the balance since I don't have a dummy load at home. What are the side effects if I don't adjust this for 6-12 months? Do I need to adjust it if the shop just did even though I changed my power tubes?
     
  2. B-string

    B-string Supporting Member

    Yes changing the power tubes can change the balance even with same numbered replacements. Will it do harm, NO. It just makes it so both the "push" and "pull" sides of output tubes are at the same level in the output transformer. Some people even like the sound/feel of imbalance there, but slightly less power is the price. Bias is a must for tube life and safe operation, balance is more "gravy for the biskits" (if I don't get shot for being untechnical here).
     
  3. Slpbass81

    Slpbass81

    Dec 16, 2010
    Orlando, FL
    Endorsing: GHS Strings
    When biasing should you leave the amp in standby when warming up or full on?

    Any recommendations for an easy/cheap way to build a dummy load?
     
  4. B-string

    B-string Supporting Member

    Bias use the lights with a cab hooked up, amp in operate with NO signal for a 20 minute warm up before adjusting. The balance you need a dummy load, signal generator and true RMS volt meter. If you want to build a load two of these would be a minimum for balance test "8 Ohm 200W Non-Inductive Dummy Load Resistor" from www.parts-express.com! wire in parallel, four and use the 2 ohm tap if you are going to power test. They will get HOT, that is normal, but burns or fire from laying them on paper or wood has been a concern. If you have the other test equipment then a load is a good investment, if not, have a tech do it (25Vrms of 40Hz is pretty darn loud with a speaker cab).
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The amp should warm up with standby off, in playing mode. It helps it to warm up if you play it.

    If the balance is not set properly, the THD will be higher.

    Will post an image a bit latter of a dummy load using the resistors that B posted.
     
  6. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    You had posted them here, if it helps save you some digging.

    Retubing my SVT II (non pro)

    I use a similar setup, a pair of 8 ohm/200 watt non inductive loads mounted in an aluminum chassis with a switch for 8 (single) or 4 (both in parallel) ohms. I picked them up from weber a while back as they were half the price as the parts express ones, but I think they've stopped selling them.
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Thanks Corey that's what I was thinking of. I found another post as well: Amp techs, what's on your bench?. You can see how it should be wired by looking at the images. On other dummy loads I also add a BNC connector in parallel, good for connecting test equipment. The BNC connector can be seen at the bottom in the last image.

    I use Arcol resistors (mouser part number 284-NHS300-8.0F ) now, they are accurate but unfortunately, very expensive. Those are the gold ones in the SVT II thread. Build in an electrical box bought at a hardware store.

    It isn't necessary to get this fancy though. You can slap together a couple of resistors on a board and wire it with a speakon cable to connect to the SVT.

    For the 40Hz signal, there are phone apps that generate tones. There are also tone files available on the internet. The signal can be fed into the input of the amp.



    dummyloadtop.jpg

    dummyloadinside1.jpg

    dummyloadinsideclose.jpg


    arcol dummy load outside.jpg


    arcol dummy load.jpg
     
    Last edited: Dec 10, 2015
  8. Slpbass81

    Slpbass81

    Dec 16, 2010
    Orlando, FL
    Endorsing: GHS Strings
    That is one heck of a contraption. I think I will let the amp roll the way it is (bias is done) since it is just my backup. Has anyone else compared the SVT-CL to VR and notice the VR has more noise/hum even when the volume is at 0 than the CL?
     
  9. B-string

    B-string Supporting Member

    Last edited: Dec 10, 2015
    beans-on-toast likes this.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Different tubes could be the reason for the hum. The small tubes that came with my VR were quite noisy. I changed them and it made a big difference. The amp also stopped picking up radio stations.
     
    B-string likes this.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Certainly a lot more than is needed for you to tweak your amp. This is a bench tool that gets used often enough to make all the fuss worth while. A big part is making it safe which is why it might be best to have a tech do this next time it is on the bench.

    Years ago, some amps had balance controls. Like a hum balance pot for the heater circuit, the power tube balance was done by ear. The pot was adjusted for lowest noise.
     
  12. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
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