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SVT VR balance. Ampeg guys come in.

Discussion in 'Amps [BG]' started by thedudebrah, Jan 8, 2014.

  1. thedudebrah

    thedudebrah Supporting Member

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    I've been trying to set the balance on my VR for a while just by ear. What is the proper "tool" to find the correct balance? The light seems to illuminate at different points around the dial so I'm unsure which is correct. Also, should the LED be lot at all times and stay illuminated under load, or should it only illuminate when the load is present? The bias on this thing seems to be finicky with the recent cold temps we've had and I'm trying to get it sounding good again.
  2. Selta

    Selta

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    Do you mean bias?
  3. thedudebrah

    thedudebrah Supporting Member

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    No, I mean balance. There are 2 bias lights and a balance light.
  4. Selta

    Selta

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    Ah. Yellow light on means it's balanced. IIRC, unless you're getting a ton of buzzing I wouldn't sweat it a whole lot. To set it perfectly, you'll want to run a 40 Hz sine signal through from a signal generator (though any low frequency should be OK, too, between 40 and 100hz).
  5. B-string

    B-string Supporting Member

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  6. B-string

    B-string Supporting Member

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    It's not hum balance it is push/pull balance on the output tube triplets. Can only be done with a steady signal applied.
  7. thedudebrah

    thedudebrah Supporting Member

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    I've been roughly setting it with a low E, but I want to know the proper way to run the signal through it
  8. B-string

    B-string Supporting Member

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    There are smartphone apps that generate tones as well as free PC/Mac programs. All you need from there is the correct cable to the amp's input jack and start from a low output on the phone/computer so you don't distort the preamp. OR buy a signal generator.
  9. thedudebrah

    thedudebrah Supporting Member

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    Nice, I'll check for an iPhone app now. How do I measure the 25vrms output?
  10. thedudebrah

    thedudebrah Supporting Member

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    And what is a good start for the balance? I can set it so with no input signal, both bias lights are green and the balance light is yellow. I can also set it so both bias lights are green and the balance light is not illuminated. The manual doesn't mention where the starting point should be.
  11. B-string

    B-string Supporting Member

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    I would start with green and yellow. You will need a meter (DVM, DVOM) to measure AC volts across the SPEAKER cab's cable, just unscrew the barrel on the 1/4" plug (cab MUST be connected) and use clips so you don't short the output or have to hold onto the meter leads.
  12. beans-on-toast

    beans-on-toast

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    It is important to let the amp warm up in playing mode (not in standby) for AT LEAST 20 MINUTES. You want to ensure that the amp is warmed up and nothing is drifting.

    Set each bias control so that the associated green LED is lit. Then with a 40Hz signal applied, the output level is adjusted to 25V[SUB]RMS[/SUB] with a meter. Then adjust the balance unill the yellow comes on. Then recheck everything again.

    With 25V[SUB]RMS[/SUB] on the output, it is going to be LOUD. The amp puts out around 35V at 300W out. At 25V you are around 156W. Normally a dummy load is connected to the amp instead of the speakers to avoid the loud volume level.

    The balance is adjusted to minimize THD. Each trio of power tubes is balanced so that one side isn't working harder than the other. The amp will be a little cleaner when it is set properly.
  13. P Town

    P Town Guest

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    If you plan to measure rms voltage, be aware that many meters are designed to accurately read rms voltages at 60 Hz, and the farther away from that frequently, the less accurate, the reading will be. 40 Hz is not too far off, so you may be close enough, I would use a meter that lists "true rms" in its specifications. I use mostly Fluke brand meters, and I pay to have the newest, and fanciest one calibrated once a year. Then I check the other meters against the calibrated one. I would be leery of a very low cost meter, especially if it has not been at least compared to an accurate meter once in a while.
  14. beans-on-toast

    beans-on-toast

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    That's true about the volt meters. Their standard is to use 25V at 40Hz but the procedure can be done at a close voltage. They want you to drive the amp hard enough to get a good clean signal, not so hard that it is distorting. Taking that distortion out of the picture and you can address the distortion due to a power tube trio imbalance.

    Basically, use clean signal to adjust the balance for minimal THD. A tech might use an oscilloscope and a spectrum analyzer to monitor the signal if you want to see what is going on. The vintage SVT procedure specified that the best way was to use a distortion analyzer. But the amp was designed with the LED's to do this without expensive equipment. You will obtain the best result with this stuff though.
  15. thedudebrah

    thedudebrah Supporting Member

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  16. beans-on-toast

    beans-on-toast

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    When it comes to test equipment, get whatever you can afford. It can get expensive fast. Starting with an under $100 meter is a good way to go. The radio shack one's specs look not to bad for the price. You will never know how well it will perform till you try it and see.

    It's nice to dream big. If you have lots and lots of money, check out the Agilent 34461A volt meter or U8903A audio analyzer or a product from Audio Precision (ap.com). :p
  17. B-string

    B-string Supporting Member

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  18. beans-on-toast

    beans-on-toast

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    Hey Jon, check this out. There are two followup videos on this dummy load. I've heard of using heater elements in the past and that those what have tried it say that it works well. In this case you would need four of them to get four ohms. 3500W at 240V each works out to around 8.1 ohms for two in parallel. 7000W@ 8Ω and 14000W@4Ω!

    A can't seem to have enough dummy loads on hand. I have a set of the cement resistors in a dummy load that you pointed to. They work quite well. I recently built one with two big 8 ohm Arcol non inductive resistors that I got from mouser (284-NHS300-8.0F). Even better. I'm going to try out the heater elements some time.
  19. B-string

    B-string Supporting Member

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    That is an interesting idea! I've seen people use toaster grids and that is a bad idea, too much inductance. IIRC the Calorimeter we used for transmitters had similar elements to water heaters (load was 50 ohms, mineral oil bath). Would be interesting to compare crossover notch with these and "non-inductive" resistors.
  20. beans-on-toast

    beans-on-toast

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    It reminds me of the Heathkit HN-31 Cantenna. A 50 ohm dummy load in a paint can filled with mineral oil that hams used. I still see them for sale at hamfests.

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