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Ampeg V-6B Transistor Substitutions

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  • #16
    I do have a scope, but I still don't know how to use it. I have a signal generator that I downloaded on my phone, if that is ok to use. My question about the scope is, how do I make connections from the scope to the amp? I can possibly figure out the rest if you can help me with that. I know its a dumb question, but scopes have eluded me so far and I can't find anyone to teach me. here's what I have.
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    • #17
      That's a very nice scope. A lot better than the old Hitachi I have to make do with
      Anyway, in the bottom pic, on the right, there's a probe. On the left, there's what looks like a ground lead.

      Plug the probe's BNC connector into the CH1 input of the scope. Plug the ground lead into the hole between the CH2 and CH3 inputs. Clip the other side of the ground lead to the chassis of the piece of gear you're working on.
      Power up the scope.
      Set the main MODE selector (between CH2 and the SEC/DIV knob) to A.
      You should see a horizontal trace appear across the screen. Also, the scope will show its settings on screen.
      Set Vertical Mode to CH1
      Adjust A Inten(sity) such that the trace is nicely visible but not too bright.
      Adjust FOCUS such that the trace is as neat as possible
      Hook the probe tip onto the CAL eyelet.
      Fiddle CH1 VOLTS/DIV and POSITION, and SEC/DIV until you get a nice square wave on the screen.
      Observe what each control does.

      The probe has a little switch that says X1 - X10. In the X10 position, the probe attenuates the signal by a factor of 10. At the same time, the input impedance goes up from 1M ohm to 10M ohm. This is useful if you are measuring in high impedance circuits and you want to avoid loading down the circuit you're measuring on. It is also useful if you're measuring the output of a large power amp, which wouldn't fit on the screen otherwise. In the x1 position, the maximum amplitude you can visualize is something like 100V peak-to-peak.

      The inputs have an AC/DC setting. In the AC position, the channel will ignore DC voltages so a small AC waveform riding a large DC offset won't make the beam disappear from the screen.

      These are the basics, more or less. Experiment and have fun. See if you can find an owners manual for your scope, this model has lots of neat stuff, way beyond what a simple scope like mine has.

      Learning to interpret what you see on the scope is another matter still, it takes quite some experience.

      Oh, and whatever you do, STAY WELL CLEAR OF ANYTHING CONNECTED TO THE MAINS VOLTAGE. Unless you're using an isolation transformer, anything between the mains lead and the power transformer is strictly off limits for the probe.


      • #18
        Thank you so much! How about the signal generator? It's an app on my phone. It has 20Hz-20kHz adjustable frequency, volume from -60db to 0db, and either white noise, pink noise, or sine wave. I would go from 1/8" jack from my phone to 1/4" input to amp. Will that work, or do I need a fancier generator?


        • #19
          It should work without any problem at all, and it's more than fancy enough for most audio work.
          For testing instrument amps, a good setting is 500Hz, -20dB.

          Myself, I use a very simple home-brew sinewave generator which goes from 10Hz to 31.5 kHz in calibrated 1/3 octave steps. It has a 0dBV output level, which goes into a home-brew three-stage calibrated step attenuator that goes to -80dB in 0.5 dB steps.
          I also have a little pink noise generator. I've built this stuff more than 15 yrs ago, and I've been getting by quite OK thank you.

          I do have a proper function generator, also home-brew and now more than 25 yrs old, I use it only in the very rare occasions I need a square or triangle waveform, but this is mostly when I'm doing stuff that's not strictly audio related.


          • #20
            What about the black lead that is clipped to the side of the probe? What do I do with that? and where should I start probing in the amp?


            • #21
              That's the probe ground lead.
              If you can use the chassis ground lead, the probe ground lead is superfluous and can be removed. Or maybe I should say, should be removed. Otherwise, it may get in the way and since it's connected to ground, if it accidentally contacts something in the equipment you're working on (like supply lines or whatnot) a nasty short could result.

              If you have to work with high frequency or very feeble signals, it's better to use the probe ground rather than the chassis ground wire.

              If both the scope and the equipment under test are powered by three-pin grounded mains sockets you might be able to dispense with the ground connection altogether especially if the sockets are next to each other.
              Note however that some equipment may not have a solid connection between signal ground and chassis ground. In that case, you must connect either the probe ground or the scope chassis ground wire to a true signal ground, such as the sleeve of the input jack.
              Be careful with speaker output jacks, the sleeve might not be signal ground.