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  • #1

    HiWatt DR103 problem

    Hey all!

    I have this HiWatt DR103 from a friend in that keeps on having the same problems........ I need some help with this one.

    Schematic 1; poweramp

    Schematic 2; PSU


    It is a nice HiWatt made by Joyce; his sig is in the unit. The former owner never used the amp really loud..... the new owner really pushes it. When this happens (amp is loud, adds a Hartman Fuzz in front of it) the amp suddenly stops working.

    When I open her up I notice that the 100ohm resistor between HT1 and HT2 is loose. I fixed her so much by resoldering her in various ways to keep her down but every time I open her up the resistor is loose. Solder free.

    I replaced the resistor with a screw down resistor I can screw to the chassis. It is of a greater wattage then the original.

    It worked for a while, now the unit blows her main fuse.


    Any thoughts on this issue? I use 6CA7's in the amp biased correct (modified the bias section slight to get the right bias voltages). I recapped her but she also had the same issues when she still had her original caps.
    Tags: None
  • #2
    That resistor is between the Plate and screen supplies. Are these the original tubes? Have you tried a different set of tubes? Are any of those 100 ohm screen resistors at each power tube OK? ANy look like they got hot?

    Pull the power tubes and power up the amp. Does it still blow fuses? If so, check the filter caps in that cap stack under HT3.
    Education is what you're left with after you have forgotten what you have learned.

    Comment

    • #3
      I've heard of that 100 ohm power supply resistor being under-rated for the job and blowing. With screens at such a high voltage perhaps higher screen resistors per output tube would make it more reliable.

      jamie

      Comment

      • #4
        Originally posted by Enzo View Post
        That resistor is between the Plate and screen supplies. Are these the original tubes? Have you tried a different set of tubes? Are any of those 100 ohm screen resistors at each power tube OK? ANy look like they got hot?

        Pull the power tubes and power up the amp. Does it still blow fuses? If so, check the filter caps in that cap stack under HT3.
        Yes; I checked with diff tubes and still the problem stays. I'll try the screen resistors today with some higher values! They don't look they got hot.

        The fuses only blow at max use; turn the amp up high and add a fuzz. I guess I can't simulate that with the powertubes pulled

        Comment

        • #5
          AH yes, I meant to mention it earlier. How much voltage is dropping across that resistor? And by extension the current through it. And calculate its dissipation compared to its power rating.
          Education is what you're left with after you have forgotten what you have learned.

          Comment

          • #6
            if the 100R, 5W is getting hot enough to unsolder itself, the 470R, 10W must be getting even hotter! Unless, that is, it's not 470R. Check its resistance.

            I'd also check the owner is using the right impedance of cabinet with it, and look at modifying it to drop the screen voltage. Either increase the screen resistors themselves, or the 470 ohm 10w resistor, which is more or less a shared screen resistor. You have to tread carefully here, though, in case you spoil the Hiwatt mojo.
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

            Comment

            • #7
              Originally posted by Steve Conner View Post
              if the 100R, 5W is getting hot enough to unsolder itself, the 470R, 10W must be getting even hotter! Unless, that is, it's not 470R. Check its resistance.

              I'd also check the owner is using the right impedance of cabinet with it, and look at modifying it to drop the screen voltage. Either increase the screen resistors themselves, or the 470 ohm 10w resistor, which is more or less a shared screen resistor. You have to tread carefully here, though, in case you spoil the Hiwatt mojo.
              The 470R is not getting hot....... No problems with that one! I've replaced it as a try to get the unit to work but no changes.

              Comment

              • #8
                Well in that case, the schematics you posted must be wrong. According to them, all of the screen current that flows through the 100R also flows through the 470R, so the 470R should dissipate 4.7 times the power.

                Maybe some clever repair guy bypassed the 470R to sell more replacement tubes.
                "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                Comment

                • #9
                  Yes; I had the same idea but I triple checked the unit and it is still there; the 1k gets desoldered, the 470 ohm is unchanged. No bypass or something else

                  Comment

                  • #10
                    Earlier you said that resistor was 100 ohms, which is also what the schematic says. Now you're saying it's 1k?

                    At least that explains why it gets hotter than the 470R one.
                    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

                    Comment

                    • #11
                      Woops; miscommunication. It is 100 ohm

                      Comment

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