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Ampeg V4B Plate Resistor Question

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  • Ampeg V4B Plate Resistor Question

    I'm trying to put a broken (and modded) V4B back to stock working condition.

    First question: There are four 5W ceramic resistors that I'd like to replace but I can't figure out the value. I'm pretty sure they're the plate resistors, and they're marked "SB-1 5W" on the schematic. I haven't tested them yet, but they've probably drifted anyway.

    Second question: is there any problem with over rating the voltage on the rectifier diodes?

    Third question: When I flip the standby switch, I get a really loud rising howl from the speaker. The volume control is off. The filter caps seem to be working. I've tested them with a multimeter at least, and they do charge up the way an electrolytic should. I've replaced the bias resistors, the two caps in the bias section, and all the diodes on the board. Any suggestions where I should be looking next?

    Thanks,
    Neddyboy

  • #2
    Neddyboy:
    I believe that the correct value for the 5 watt plate resistors is 10 ohms.

    If you mean using 1000 volt diodes in place of 400 volt diodes, no problem.

    Has the output transformer been replaced or rewired? That howling sound often occurs when the output transformer is phased incorrectly. If it has been messed with, try switching the two primary wires that go to the output tubes.

    Comment


    • #3
      Hi Bill,

      No, nothing's been changed on the output transformer since it worked.

      Here's the backstory: the amp was working, but if you turned it up very loud or hit a low note hard it would kind of fart out. Ugly distortion, no punch. This is my amp, but one of my son's friends had been playing through it for a while and just didn't know the difference.

      I finally decided to open it up and look and discovered a bunch of burned up resistors. The four 2 watt bias resistors were fried so badly the middles were gone -- someone had replaced them on the bottom of the board with 1 meg 5 watters and one of the connections had come loose. One of the resistors in the power tube bias section was gone as was the resistor in line with the standby pilot light. The diodes on the board tested okay but looked questionable.

      Anyway, replaced all this, plus the two 10mfd caps in the power tube bias section, and now I'm getting this awful howl. I've just flicked the standby switch on for a second at a time, just until I hear the noise. The first time I did it, the wire between the board and the standby pilot light got so hot it melted through and standby pilot light resistor looks like it's about to melt.

      I'm a little frustrated, since all I did was replace burnt components and it did work before I started. I haven't changed anything, except to go back to stock values.

      Comment


      • #4
        There is a real problem if the standby pilot lamp wires are running that hot. It is a neon lamp that should never draw enough current to heat up the wires. The lamp circuit connects to the hi voltage power supply, but you can just remove it from the circuit to test the rest of the amp.

        Double check all of the wiring on the circuits that you have worked on. Check for broken or shorted traces on the board. Are there any sections on the board that have been burned badly enough that they have become carbonized (electrically conductive)?

        I'd start by looking for a short in the Standby light wiring, because it's drawing way too much current.

        Comment


        • #5
          I think I'll pull the lamp out of the circuit and see what happens, good thought. I just realized that I can't remember if the lamp lit up when I flipped the switch!

          Thanks again,
          Neddyboy

          Comment


          • #6
            Neddyboy:
            I finally looked up the schematic for your amp, and after reviewing it and your earlier post, I have a few questions.

            Originally posted by neddyboy View Post
            The four 2 watt bias resistors were fried so badly the middles were gone -- someone had replaced them on the bottom of the board with 1 meg 5 watters and one of the connections had come loose. One of the resistors in the power tube bias section was gone as was the resistor in line with the standby pilot light. The diodes on the board tested okay but looked questionable.
            Are you refering to the 470 ohm grid resistors that go to pin 4 of the output tubes here? And did you mean that they have been replaced with 1K (thousand) 5 watters not 1 meg (million)?

            Check diode D5, as it is the ground connection for the entire power supply.

            Comment


            • #7
              Hi Bill,

              Yup, three of the four 470 ohm grid resistors had been replaced with 1k ohm 5w resistors. I said 1 meg in my earlier post -- sorry, slip of the keyboard. The fourth 470 was still in place, and one of the 1k resistors had come loose at one end. Yeesh.

              I checked the plate resistor values last night and they were all at around 3 ohms. I'm going to replace them and the four pigtail diodes today.

              I've already replaced D5, so that's not it. I've double checked my work, reflowed solder anywhere that looked iffy and everything looks okay. I've put in two different sets of power tubes too.

              I guess after today's round it's time to bring it to a real repair guy!

              Neddyboy

              Comment


              • #8
                I think Ampeg used 3.6 ohm/5W resistors in that position on the V4B. I've seen those in the past.

                Comment

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