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Speaker output jack problem.

Discussion in 'Amps and Cabs [BG]' started by Hellbastard, Mar 31, 2009.

  1. Hello TB. I have an old fender bassman 135 tube amp. I've recently replaced a damaged resistor in one of the output tubes sockets. The problem is that now I don't get any sound out of the amp when I connect a cab to the speaker output jack, but when I connect a second cab to the extension speaker jack there's sound coming out of the 2 cabs. Any ideas on what might be causing this problem. Both cabs are 4ohm. Before I replaced the resistor this problem didn't occur and I'm worried that it might damage my amplifier.
    All of the tubes are new and matched.They have not been biased since I don't know which resitor controls the bias and there aren't tube amplifier techs in my city.
    Here's the schematic of the amp.
    The resistor I replaced is the 1.5k connected to pin 5 on V6. :help::help::help:
    Thanks in advance.
  2. Hi.

    I'd say the jack problem isn't related to the resistor changing.

    As You can see on the schematic, the extension jack serves two purposes. #1 connects a second cab. #2 switches the two cabs in series.

    Since the output signal is routed via that switch, any open on that circuit means the secondary side is open = no sound.

  3. MIJ-VI

    MIJ-VI Banned Supporting Member

    Jan 12, 2009
    I don't mean to seem like a smart a$$, but are you sure that you're plugging your first cab into the speaker out jack, and not the ext. speaker out?

    Have you tried plugging the other cab into the speaker out jack?

    Thank you for including a schematic with your inquiry. :)
  4. #3 Is that the extension jack changes the tap on the OPT to suit the series connected speakers.

  5. Are you talking about the external speaker output? Because that one doesn't have a problem. The problem is on the "normal" speaker output jack, I checked and it doesn't seem like the circuit's open.

    I'm pretty sure I'm connecting to the Speaker output Jack, if I do that there's no sound but if I plug a second cab to the external Speaker output then there's sound coming out of both cabs.
  6. well I rechecked and there's no sound coming out of the speaker output jack at all, it's only the external speaker jack that's working. Could this be an Output trabformer problem? Is it safe to run the amp only with a cab connected to the ext. speaker ouutput? I have a couple of gigs coming soon and I really like to play through this amp. please help
  7. The ext speaker jack has a switch that is operated when you insert a plug into it. Check that without a plug inserted that the contacts make. That supplies the feed to the 4Ω jack that you should be using.

    The 1K5Ω resistor that you changed stops parasitic oscillations in the output tube.

  8. I checked the jack and the contact's made. Now I found something very strange, while I was cleaning the Ext. Speaker output switch contacts I connected a cab to the "normal" speaker output and turned on the amp, the ext. speaker jack was not making contact with the chassis. But as soon as I put it on the chassis the Sound is gone. Is the ext speaker jack not supposed to make a ground connection through the chassis? is there something I'm missing here?
  9. Does anybody know if the ext. Speaker output jack shouldn't make a ground connection through the chasis?
    I'd appreciate the help.
  10. The ext speaker jack should not connect to ground/chassis. If it does it shorts out the normal speaker jack and thus no sound. Remember that when using two cabinets they are connected in SERIES. As the normal speaker jack is grounded if you ground the ext jack the normal jack is no longer in the circuit.

    Series connection for speaker jacks IS unusual. I've only ever seen a couple and this is one of them!

  11. Thank you very Paul, You have been of great help.
    One last question. If i connect two 4ohm cabs to the speaker outputs the amp will be working 8ohm right. so the output won't be the 135 watts the amp is supposed to deliver?
    Is it better to just use one 4 ohm cab volume/loudness wise?
    Thanks again.
  12. Hi.

    As long as the load (speakers) match the OT tap there's no difference in the power. It's ~135W in both cases (mine was ~100W back in the day).

    The same is true with every transformer coupled amp, the power is the same regardless of the impedance as long as the output devices "see" the correct load.

  13. Sam is, as usual, correct.

  14. Awesome, thank you guys for being so patient and helpful!

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