Perc no Work

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  • #776 Reply

    I’ve just completed my Percolator amp and I’m having trouble getting it to work. Here is some info:
    LED lights up.
    Tube lights up.
    No sound when speaker is connected. No hum at all.
    v1: 180 dc
    v2: 7.1 dc
    v3 1.8dc
    v4: 157 dc
    v5: 1.3 dc (not per chart)
    v6: 92.5 dc
    v7: 32 millivolt (not per chart)
    v8: 181 dc
    v9: 172 dc (not per chart)
    v10: 1.4 dc
    v11: 173 dc
    I just checked all the resistors and all but two agree per the installation chart.
    R7 & R14 both measure 750 ohms each. This should be 1.5 K per the chart. Don’t know if these two resistors when installed get divided somehow. They are both brown-green-red in color and I measured all resistors prior to assembly so I’m pretty sure these two are the right resistance. Maybe this isn’t even my problem! I’m stumped as far as what to do now. Any one have some advice? This would be much appreciated.

    #778 Reply

    Let’s get this fixed…
    According to your voltage chart (by-the-way, thanks for that…it’s very helpful) R8 only has 1 volt dropped across it, which tells me that there is very little current flowing into pin 4, which is why TP7 only has 32mV across it. The problem seems to be with either pin 4 or 7 on the tube. The fact that you are getting no sound at all leads me to think there is an open somewhere in your circuit (probably at pin 4 on the tube). Resolder pins 4 and 7 on the tube to make sure they are solid, as well as R8 and R7. Also check to make sure the tube socket itself is making good contact with the tube pins. If you pried the tube socket contacts open too far then try to pinch them closed a bit more. When you get TP7 and TP9 fixed then the voltage on TP5 will probably normalize. Make sure you drain the caps before soldering pin 4! Keep me posted on what you find.
    Good luck!

    #779 Reply

    …and yes, R7 and R14 would measure about 750R in the circuit because they are nearly in parallel. One side of both resistors are on the same node. R7 goes to ground and R14 goes to ground through the secondary of the output transformer which has a negligible dc resistance.

    #780 Reply

    Brach, thanks for the help.
    here is what I have done:
    1. retensioned socket pins
    2. reflowed solder on all tube socket tabs
    3. reflowed solder on resistors R7 R8 R13
    4. still no hum or sound
    5. test point T7 now has 1.4 Vdc instead of 32 mV
    What next?

    #781 Reply

    It looks like you fixed that issue. What does TP9 read now?…i’m assuming that it’s around 70V?
    Are all the other test points reading correctly?
    Just for good measure you may want to reflow the other joints around the signal path and tighten the other tube pins. To keep from having to pull the board out of the chassis just solder as many components as possible on the component side of the board and see if that fixes the signal path problem.
    Do you, by chance, happen to have access to an oscilloscope or other signal tracing tool? If not, no problem…just thought i’d ask.

    #782 Reply

    It’s alive! I retensioned the socket pins and resoldered the resistors R1-2-4. This seemed to do the trick. Lesson learned:
    1) Be careful with the socket pins when expanding them. The tube should be a little snug when inserting into socket.
    2) use a little more solder when putting in the resistors and capacitors.
    Brach, thanks for the help. The amp sounds good so far.

    #783 Reply

    Great! I’m glad to hear that it’s up an running. Yes, the socket should be tight enough to make good contact with the tube pins…I’ve also learned that lesson the hard way!
    Enjoy the amp!

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