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Very low voltage on all B+'s.

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  • Very low voltage on all B+'s.

    Hi, Anyone here has a tip on what to look for when the voltage fre the filtercaps in a 5f6a Bassman 59 build is lower than 1 volt? I have checked most thinks, changed cathode resistors, but volt is around 0.900 ma all over, also in the V1 pin 1 and 6. I begin to wonder if the PT is blown, but the pilotlight is lit. At one point the amp gave a big hum in the speakers before the fuse blew. I have taken out all the tubes and put them back on one by one, seems like the GZ34 rectifier might lead to the fuse blowin, cause it doesn't when the tube isn't mounted?

  • #2
    Pull the rectifier and measure the resistance from that filter cap to ground. You should see a moderate resistance that steadily increase with time until it is very high. The Rectifier is probably damaged due to a short further down the B+ path. If Measure the resistance from the plate pin of one outputs to the other plate. Don't turn it on again until you find the cause or you will damage something expensive.

    You should see the rated DC resistance of the primary of the transformer when measuring between the two plate pins and half that value from both plates to the B+ node.
    For the speaker to be generating a loud buzz, a lot of current had to have been flowing through the output transformer. Find out why, a short on the plate side of the transformer could explain that symptom. A shorted rectifier could certainly cause high current and ruin the filters. There are a number of things that could cause it but every one of them is easy to find with an ohmmeter with no power applied.
    Some on here suggest using a series light bulb as a mains current limiter until you find the problem. I never bothered with one because they are not needed in on a repair bench set up for amps.
    One warning I make but is often ignored is "Don't replace ANYTHING" until the problem is discovered and its replacement is essential. By randomly changing things, just hoping the problem fixes itself, it adds confusion and makes the original problem harder to track down. The hardest repair a tech has to do are finding tech induced mistakes created when another tech tried swapping parts. Some shops and techs will not work on units that have been tampered with, for good cause. A "natural" failure is easier to track down because it is logical and fits the failure modes of electronics. A miswired or somehow damaged unit does not follow logic of failure modes since new paths and new stresses were introduced. So for your own peace of mind, knowing that if you get in over your head, a skilled tech can make swift work of it. But mistakenly miswiring or damaging something will often be refused for repair and of those who will take it in, they will charge a lot more.If that cathode resistor was faulty, it would have been 2 seconds to verify it at Zero cost.

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    • #3
      Thanks for that, good info. The problem is I don't get any voltege readings close to what I expect, anywher on the board. I don't see any obvious failures of any kind. Any sign of life is the pilotlight and there is a low hum in the speaker when it's on without the rectifiertube inserted.

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      • #4
        Don't read voltages. It is broken, don't put power to it until you discover what is the problem. Your own meter will tell you a lot, enough to identify the problem. Keep it unplugged until you found the problem and corrected it.
        Take the resistance measurements I suggested. When measuring the resistance to ground from the node of the first filter, plug in the GZ34 and measure again. Any change. If there is, the tube is a problem, without the heater hot, it should appear like an open circuit.
        Without the rectifier it shouldn't make sound out of the speaker because there is no B+ unless you are just hearing the hum of the power transformer laminations vibrating.
        Seriously, unplug it until later when you are confident you corrected the problem or else expect to buy new transformers and tubes.
        Did it blow fuses only with the rectifier in place?
        What test instruments do you have and do you know how to use their various functions and ranges? An ohmmeter is the bare minimum for testing now.

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        • #5
          Hi and thanks again. I have a Ohmmeter, and I'm fairly familiar with it. I shall try to do as you seuggest.

          Best.

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          • #6
            Hi again.

            There is no ohm readings on the cathode pin number 8 and ground?

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            • #7
              "No ohms" means what? Zero ohms or no reading, due to being open? But that reading does not cause very low voltage seen on the plate, if the cathode was open the voltage would be higher on the plate due to the load on the power supply being less.
              Take a reading from the plate to ground or from plate to the cathode of the GZ34. It should be less than 500 ohms. If it is high or acts like it is open, trace it back to the open, such as measuring between the cathode of the GZ34 to the primary center tap of the output transform. If that is a low resistance to the GZ34 but the plate or a power tube was very high resistance of open, the fault is probably an open primary in the transformer. Ohm meter reading should allow you to pin point where the break in the B+ supply to plate path.

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              • #8
                4 and 6 to ground is 44.1 and 46.2. No reading on 4 or 6 to 8. Between 8 and 2 is 0.2 ohm.

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                • #9
                  Did the amp ever work correctly?
                  "Take two placebos, works twice as well." Enzo

                  "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                  "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                  • #10
                    Best advice on this thread is by RG on ChuckH's sig.
                    Valvulados

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                    • #11
                      Has it ever worked? The values indicate it is miswired. The screen supply is shorted to ground

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                      • #12
                        Thinking about it now, that seems like a good possibility. I don't have your trouble shooting experience. I assumed it's a miswire, but just wanted to be thorough before insulting a mans program since the usual MO is to let us know how many times they re checked it and they're sure it's right. So I wanted to remove other possibilities before suggesting it.
                        "Take two placebos, works twice as well." Enzo

                        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                        "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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