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Rebuild finished, no output, no hum

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  • Rebuild finished, no output, no hum

    So I finally decided to recap and rewire my chassis of this home brew I bought a while back. It's all back together, nothing blew up. New pots, sockets, wire, caps. I have no output whatsoever. No hum, buzz, no pop or crackle when I was taking voltages (Except V2 seemed to be making a lot of racket when I touched some of the input points). The good news is it's REALLY quiet haha!

    As far as I can tell the output xfmr is hooked up right. It's one of those with only one wire going to the output jack. I have not found anything in the voltages that make me think I have something grounded or loose but here they are. Any help is appreciated before I take this poor thing to some one who knows what they are doing.


    2 406V DC 7VAC
    4 372 VAC
    6 372 VAC
    8 407V DC 8VAC

    V4 6V6GT

    2 3.36VAC
    3 388V DC
    4 352V DC
    7 3.36VAC
    8 22.46VDC

    V3 6V6GT
    2 3.36VAC
    3 392VDC
    4 352VDC
    7 3.36VAC
    8 22.46VDC

    V2 12AX7
    1 162vdc
    2 0
    3 1.45vdc
    4 3.36VAC
    5 3.36VAC
    6 205vdc
    7 19.5vdc
    8 51.61vdc
    9 3.36VAC

    V1 12AT7
    1 133.7VDC
    2 0.00 vdc
    3 2.3 vdc
    4 3.36VAC
    5 3.36VAC
    6 130.9 vdc
    7 0.00vdc
    8 2.3vdc
    9 3.36VAC

    I appreciate you guys answering all my previous questions too. I hope I'm not spamming the board. I'm starting with little knowledge and less money trying to learn about amps and this board has been a huge help.

  • #2
    The fact that "touching" V2 input plns made a racket tells me the output is working.
    Without a schematic, the voltage measurements say a few things.
    V1: pin 1 & pin 6 are conducting (that is good)
    V2: pin 1 is conducting (good)
    V2 pin 6,7 & 8 are odd, unless it is the phase inverter.
    Did the amplifier work before you replaced all those items?
    If it did, than you have to go back & nitpick your work.


    • #3
      When I say made a racket, it's making the tube rattle. The sound is coming from inside V2. Nothing comes out of the speaker. No hiss, hum, pop or noise of any kind as I was checking the pins. The amp worked ok before I took it apart. Lots of intermittent problems.


      • #4
        The voltages look like what is expected from a 5E3. The silence issue points to the output transformer as the first area to focus on.

        With the speaker connected, measure the resistance between the speaker terminals on the speaker. It should look like a short. Un-plug the speaker and you should see about 6 ohms. If you don't see a short, check that the output transformer frame is a low resistance to the chassis.

        If you still see the short, the cable is probably bad. If you see an infinite resistance, you have a bad speaker.

        Plug the speaker back in and unsolder the one wire from the output transformer. You should see the 6 ohm reading again. A short would indicate a problem at the speaker jack.

        Other than the above, look for a short to ground at V2 pin 2, possibly at the volume/tone controls.
        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 !


        • #5
          Ugh you are right I was just looking at that when you replied. I show an open circuit across the output of the OT. Stupid thing was working when I took it apart. At least I had output, maybe that was what has been going out all along...
          Last edited by Boxnix; 12-27-2010, 10:16 PM.


          • #6
            Ya know what though I just realized, I had an issue a while back where the bolts on the OT got loose, one fell totally off and the xfmr was dangling. The ground wire on the secondary is soldered to the tranny chassis so it would have been constantly loosing connection with vibration. You think that would burn up a transformer? I'm worried I have something screwed up that burned the transformer up as soon as I turned it on. I'm skeerd to put another $60 xfmr in there.


            • #7
              Originally posted by Boxnix View Post
              You think that would burn up a transformer?
              It's possible, but you would probably have to play the amp loudly for several hours. It's a slow process where an arc develops inside the transformer and gradually carbonizes a short.

              Originally posted by Boxnix View Post
              I'm worried I have something screwed up that burned the transformer up as soon as I turned it on. I'm skeerd to put another $60 xfmr in there.
              Not very likely. A bad transformer has very low output. If the amp is louder than a cell phone ringing, it's good. For a transformer to die instantly, it usually goes with a puff of smoke and a light show inside the 5Y3.
              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 !


              • #8
                I put a Hammond 1750E in to replace the really expensive MM tone clone I blew up. I am pretty sure now I did hook up the signal lead straight to ground and just let it burn out while I recorded the voltages on every pin. Now that I have the hammond in I realize this is the only place in the world where green is signal and black is ground.

                After all that the stupid amp is still farting out on all the low notes. I think I got ripped off on this speaker, need to borrow another amp with 8ohm load to check the spkr, I don't see any damage or holes.

                On a positive note, the upgrades I did sound AMAZING from the A string up. I swapped out the orange caps for Mallory's. The amp seems a lot less harsh but I'm also going through a different speaker and PT so who knows what I'm hearing.

                Thanks for all the help!


                • #9
                  Did you try changing the cathode bypass capacitor on V1?
                  Stock is 25uf.
                  Try a lower value.
                  I think what you are hearing (I call it Fender Blaat) is from too much bass passing through.
                  A lower value bypass cap may help here.
                  Turn down the bass & see if it goes away.
                  Then turn it up until it starts.
                  A much higher wattage speaker would also help.


                  • #10
                    I really thought that too but I went from a 20W speaker to a 50W tone tubby. I dunno the problem did start when I installed that speaker. I just got to put a 40W fender deluxe into that speaker and it handled it well. It was less bassy though... I dunno. I'll try swapping out those caps, I actually did not change those when I did the recap.

                    I just did the chopstick test. the signal wires into V1 and the wire off the volume pot to V2 both make a noticeable bassy pop when I chop stick em. I wiggled the tubes, no scratch or pop. Sockets are new. Solders are new, look good, I guess I can resolder them but I don't think it will help. I thought maybe I needed to stabilize the wires so I had a friend play and held them steady with the chopstick. No help.


                    • #11
                      Quick update. I finally broke down and totally disassembled the amp. I took the chassis out and took the speaker out of the cab and held it while I let the low E ring. The rattle is coming from the speaker. There is some issue with the voice coil rattling or something. I'm sending it in. Apparently there is a year where tone tubbies had some goofy coils but they said they will fix it no problem. Quick phone response to my email from A Brown Sound too! Anyway, I put the old speaker back in. While the amp sounds like total ass again the problem went away. I'm pretty sure this has been my only problem from the get go, which kind of makes me feel stupid for all the work I did. It was fun though, good learning experience for under $300.

                      Oh also a quick OT opinion... I replaced the MM tranny with a $30 hammond. To my ears it's not any worse. I seem to remember some one calling MM overpriced. Maybe I'll change my mind if this one smokes next week but right now I tend to agree.


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