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Troubleshooting amp problems


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  • Troubleshooting amp problems

    Hey everyone,
    I have a blackstar HT-40 that I have had for over a year. Just last weekend I was playing a show and had all noise cut from the amp. No buzz or humm at all. Tried different cables, visually inspected preamp tubes (haven't looked at power tubes yet), and checked speaker connection.

    I plan on doing the following:
    1. Checking power amps;
    2. Hooking up new speaker and testing the speaker through a different source

    Does anyone have any other recommendations of where the problem might be coming from? I did just replace my stock speaker a couple weeks ago but have been playing on it all this time, I know the speaker specs are the same as the stock speaker so I don't see how this could be a problem.

  • #2
    The first this is I'd be checking those pre amp tubes. Unlike power tubes which tend to die a slow death, pre amp tube can and do go belly up. A visual inspection will probably not work. Make sure you turn off your amp each time when replacing the tubes. I would start there. You should have a few extra 12AX7's always on hand. Test em one at a time if you want to find the bad boy.

    If your using that second channel as a higher gain lead tone, you're working the pres.
    I just looked up you amp.
    Might be something else too.
    Faith is taking the first step even when you don't see the whole staircase.

    Join Date: Aug 2001
    Location: N. Adams, MA USA
    Posts as of Jan 10th 2013: 82,617


    • #3
      The first thing I would do is suspect that I did something in replacing the speaker that is causing the problem and look there. Maybe a bad solder connection or a broken speaker wire that may not have been evident at the time.

      A friend of mine recently gave me a Blackstart HT-1 but I have yet to open it up and look at the construction. Is the HT-40 made in China too?
      the real voyage of discovery consists not in seeking new landscapes, but in having new eyes


      • #4
        I always check the simple stuff first. Speaker cable, speakers, guitar cable, fuses.

        If the cab and cable works fine with another head you can rule that out. This amp may have a pair of fuses. One for the primary and one for the secondary voltages. You could have the tube lit but no high voltage to the tubes. A bad standby switch could also cause that. Using an ohm meter to check the switches isn't very hard and switches can take a shot and get damaged transporting or eventually arc out inside, especially the plastic rocker type.

        From there you can see the power tubes change color when the B+ is applied. You can usually see them change from just the orange heater to a bit of blue when the HV is applied. If you have that its likely the amp is operational. If its bright cherry orange, power down immediately. You have bad power tubes and possibly some blown grid resistors.

        Hopefully you were running the head with the correct impedance load. If you weren't there's a possibility of a blown output transformer.
        If it has a switch to change impedance you want to check it with an ohm meter as well.

        Next would be to check for bad solder joints. Heads receive allot of vibrations and a weak solder joint can crack. The last head I bought wouldn't do any more then fizzle when turned on and there was a bad solder joint on a switch and bias connection and all it needed was resoldering and it powered right up.

        You can try swapping the tubes of course. Preamp tubes usually go microphonic before they go bad so there should have been some signs. Most last a good long time before having any issues, and power tubes usually fail before the preamp tubes, but there's always a rare possibility you have a bad one.

        If all that's looking good then you really need to signal trace the amp. You inject an audio signal into the front end then trace it stage by stage through the amp till you find where the sound stops. If you have sound going into a component, and nothing coming out either that component isn't receiving the proper voltage to operate (requiring you to troubleshoot the DV supply) or its bad.

        Oh, And I didn't look to see if this amp has a rectifier tube. If it does it would be one of the first things I'd check because its what provides all the DC voltage for the amp. If those go, you wont get any sound.


        • #5
          Thank you for the feedback. I haven't had time to really look into this yet but will definitely use this info. Thanks again.


          • #6
            Ok, so far I have checked all of the simple sollutions. Speaker and speaker cable are good, swapped out preamp tubes, changed cables and guitars. Today my plan is to:

            - Test and/or replace each fuse
            - Plug into the fx return to see if that produces any sound whatsoever
            - Check the stand by switch as recommended
            - Hopefully I find the problem there before getting into possible transformer problems (seems a little over my head)

            I have read that it is very rare to have both power tubes go out but I guess one of them could have a short and blow the fuse. So I assume I will replace the fuse, it will work for 5 minutes and then blow again... but I would be fine with knowing that it was just the power tubes.


            • #7
              if the fuse is bad, remove the power tubes before applying power. If the fuse doesn't blow again, then its likely you have a bad tube. If so buy a "matched pair" Don't just buy one tube or you'll likely have issues getting them to bias. If the fuse blows with the tubes out, then you're looking at power caps or possibly a transformer. Lets hope it was a bad tube or caps because its less expensive to fix. If a tube did go bad you must also check the grid resistors, and you must bias the new tubes (in most amps) You'll likely need to have this done if you aren't familure with the procedure. It will be cheaper if you buy the tubes yourself.


              • #8
                I'm at a loss for words here. Checked each fuse with a multimeter and they are all good. Removed old power tubes and did notice some brown rings on them but that wouldn't cause a total lack of sound (No humm, hiss, popping, nothing). I know the speaker is good I tested it with a 9-volt. I checked continuity on each input jack on the amp, guitar, and cables. I tried plugging into the fx loop. I switched out preamp tubes as well. Unless I have 4 bad preamp tubes... I don't see that being the issue. I am replacing each fuse and power tube anyways... I just don't know what other "easy" fix there could be. If this doesn't work I will just send it in. The amp is still under warranty was just trying to avoid sending it in and waiting a couple weeks. I guess I can have a loner for a few gigs though.


                • #9
                  With my HT-1, plugging into the Headphone or Ext Speaker jacks cuts off the speaker. Perhaps you are having a problem with one of those jacks or the logic circuit associated with them.

                  I have found with Fender amps that switching to printed circuit boards introduced a new set of problems related to poor solder connections. Maybe your amp "thinks" something is plugged into one of those jacks.
                  the real voyage of discovery consists not in seeking new landscapes, but in having new eyes


                  • #10
                    Does the power light come on?

                    Any output from the Headphone/Line Out?

                    Any output from the Ext Speaker jack?
                    the real voyage of discovery consists not in seeking new landscapes, but in having new eyes


                    • #11
                      Power light comes on, when i plug into the emulated output with headphones I only hear a slight noise when turning the amp on and off. Nothing when going from standby to on though. Just when the amp powers on and off.


                      • #12
                        I just noticed one thing, when I power on the amp and have my ear right on the speaker I can hear the speaker getting power... kind of like a very very slight hum...


                        • #13
                          Do you get anything out of the speaker if you plug your guitar (maybe through a pedal) into the Effects Return?

                          Do you get any thing if you plug your guitar into the regular Input and go from the Effects Send to another amp or PA?
                          the real voyage of discovery consists not in seeking new landscapes, but in having new eyes


                          • #14
                            I am giving up on this... just going to send it in and get it serviced proffessionally. If I keep messing around with it I am bound to destroy it.


                            • #15
                              That's probably the best thing to do.

                              It's a new line of amplifiers built with modern manufacturing techniques and a whole new set of potential problems - not your typical old point to point Fender style amp.
                              the real voyage of discovery consists not in seeking new landscapes, but in having new eyes