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5f6a no sound

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  • pbiagi
    Update - the shorted coax cable to treble pot was the issue.
    When I installed them, I was careful to measure the shield to core wire to make sure they weren't shorting. It was OK then, but guess it was close, and something moved.
    The amp is working, quiet, but unfortunately it's too late here to really try out.

    Thanks loudthud for helping guide me.

    BTW, I'll post pictures if anyone's interested later.

    Leave a comment:

  • pbiagi
    Disconnected output transformer ground secondary. Speakers in parallel - like original layout - measure 2.4 ohms.
    The output transformer secondary measures 0.8 ohms. Primary in-circuit measures 86.5 ohms.
    These seem slightly high but still in line to me.

    I did hear clicking and buzzing when measuring voltages but not near as loud as expected.

    Re-measured resistance on input jacks, ~33K ohms.
    With plug in position 1, the resistance is about 1M ohms, in position 2, 67K ohms.
    All seems good there to me.

    To keep the amp quiet I had the bright idea of using coax on the 12AY7 and 12AX7 grids. They're grounded on one side..
    Measured them all for shorts and all seem good except possibly one.
    From V3 grid, pin 2, after the .02uF cap, the cable to the treble pot is measuring close to 0 ohms. Could this be normal?
    I'm thinking of disconnecting and just trying a jumper cable to find out.

    Leave a comment:

  • loudthud
    V3 looks ok. You meter won't get an accurate reading on pins 2 and 7 because it loads down the voltage too much and that changes the operating conditions on the tube. You probably have a short at the speaker jacks or possibly the input jacks. You should have heard a click or buzz when you measured voltages in the preamp. Unsolder one of the wires from the output transformer to the speaker jacks. Measure the ohms across the speakers and you should see about 1.5 ohms (assuming four 8 ohm speakers in parallel). The two wires to the output transformer should measure very low, less than 1/2 ohm.

    Leave a comment:

  • pbiagi
    started a topic 5f6a no sound

    5f6a no sound


    Tried turning on a 5f6a bassman for the first time but get no sound at all.

    B+ 458V, 451, 407

    Voltages in order of pins:
    V1 (12AY7) 156, 0, 2.5, 6.4 VAC, 6.4 VAC, 162, 0, 2.5, 6.4 VAC
    V2 183, 1.0mV, 1.3, 6.4 VAC, 6.4 VAC, 336, 183, 185, 6.4 VAC
    V3 269, 30, 49, 6.4 VAC, 6.4 VAC, 252, 31, 49, 6.4 VAC
    power tubes
    V5 39mV, 6.4 VAC, 452, 451, -53, 453, 6.4 VAC, 39mV
    V6 41mV, 6.4 VAC, 452, 451, -53, 454, 6.4 VAC, 41mV

    Everything to my eyes looks OK except V3 - especially the cathodes seem high. I checked the circuit resistance and seems inline.
    Here's a photo. I decided to use wire-wound on the cathodes here for some reason. Would that be a no-no?
    Click image for larger version

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    However it seems doubtful that it would cause no sound.?

    I measured the grids to ground resistance and all seem OK. The 12AY7 grids are about 33K ohm.
    I poked around with a chopstick and didn't really find anything.
    Tried different power and 12AY7 tubes - no change. (Didn't try changing out the 12AX7's, hmm?)

    Maybe there are two issues - high V3 voltages and no sound.(?)
    Thanks in advance for advice.