Announcement

Collapse
No announcement yet.

Hiwatt custom 20 tube 60 hz hum ideas?

Collapse
X
Collapse
 
  • Page of 1
  • Filter
  • Time
  • Show
Clear All
new posts
Previous template Next
  • #1

    Hiwatt custom 20 tube 60 hz hum ideas?

    I've got this Hiwatt korean custom 20 tube which
    has 60 hz hum and some high static.
    My repair person thinks it's the rectifier diodes and the power supply.
    Any suggestions for a fix?
    The schematic is nowhere to be found and they're telling me it's going to cost an arm and a leg without it to figure out what's going on.

    When you turn the master up even with the gain low, the hum and noise go up like you wouldn't believe.
    The transformers aren't at 90 degree angles either.
    The laminates are facing out. Could that be part of the problem as well?
    Tags: None
  • #2
    I would shap the preamp tubes first. I've seen soom really noisy Chinese tubes. Next I would suspect filter caps. Rectifier diodes fail but usually short. Do you have access to a scope to check the ripple voltage on the B+ line?

    Comment

    • #3
      If you Google the amp, you'll see quite a few posts regarding the noise. It looks like what LoudThud is saying about the cheap Chinese tubes being noisy & hummy is ringing out.

      It's the easiest place to start, always. After that you might be looking at most likely cheap design which can be remedied, but yes gets a bit pricey. You also have to look at how many of them all have the same issue after tube replacement to determine if it's just yours or a global thing.

      glen

      Comment

      • #4
        Originally someone modded the board and so the pcb had to be restored .
        2 traces were cut and low pass filters were put in.
        They cut the gain and screwed up the grounding, but the noise and hum were still there.

        The tubes were replaced and the diodes are good.
        Problem is still there.If it's a bad design is it still fixable?

        How much would a repair like this generally cost?

        From reading online some amps didn't have the problem but quite a few did.

        Comment

        • #5
          Are you sure it's 60Hz? 60Hz and 120Hz hum have different sources. Does the hum stop immediately when the amp is put in standby mode or does the hum fade out over a period of several seconds?
          WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
          REMEMBER: Everybody knows that smokin' ain't allowed in school !

          Comment

          • #6
            They told me 60 hz.
            I'm trying to remember what happens on standby,
            maybe it faded out along with the sound.
            The hum gets very loud when you put the master up and the gain low.
            If you put the gain high and the master low, the minute you move the master up here comes the hum.
            With the master and gain at 9 am that's pretty much your clean.
            Sounds like they're saying it's going to be very expensive to repair.

            Comment

            • #7
              60Hz hum that fades out usually comes from a bad tube or a problem with the heater supply wiring like a broken center tap, bad hum balance pot (if so equipped) or a bad virtual center tap (if so equipped). Bad tubes can usually be isolated by swapping them around or substituting know good ones.
              WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
              REMEMBER: Everybody knows that smokin' ain't allowed in school !

              Comment

              • #8
                I really appreciate all your help!
                What if it doesn't fade out
                What can cause it then?
                This amp is cathode biased
                and has 2 12ax7 ,1 12at7 and another 12at7. The power tubes are 2 jj el 84.
                I have to assume the tubes are good based on what they told me.

                There are also appears to be high static noise.


                What worries me is that it may be unfixable.
                Is that possible?

                Comment

                • #9
                  The important clue is that the hum is coming through the Master Volume. Most likely a ground loop problem or possibly a bad filter cap. It originates in the B+ supply and is getting into the preamp somehow. This type hum will cut off immediately when the amp is put in standby mode.

                  Note: Comments about the standby switch are made under the assumption that it is the conventional type that interupts current between the power transformer and the filter caps.
                  WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
                  REMEMBER: Everybody knows that smokin' ain't allowed in school !

                  Comment

                  • #10
                    I'm going to pass all this info on and see what they want to do.
                    I get the feeling I may have to find someone else to repair it,
                    because without a schematic they seemed insistent that it was going to be
                    expensive.

                    Comment

                    • #11
                      I would be tempted to gut the amp, except for the transformers, and make my own replacement turrent board. If you have someone else doing the work that probably isn't acceptable. If you're doing it yourself it would be a nice project and you would have a decent amp with a small investment of parts and time.

                      Comment

                      • #12
                        I'm certainly considering that option.
                        But I'm not sure how it would fit in here and whether I can use the 2 transformers.

                        Comment

                        Previous template Next
                        Working...
                        X