G3 first startup - R23 & R24 fried - help needed

    At initial startup I got light in the tubes but no sound. Turned amp off immediately, and the noticed smell of smoke. Turned over amp and saw that R23 & R24 were burned. Not sure where to start troubleshooting. I did not think to photograph the underside of the board before assembly, that would have been a good idea... But anyway, here is the top side with "french fries". :(

    Hi and welcome to the forum. If R23 and R24 are burned, R22 might be also wrecked. It seems that way to much current is going through these resistors, there could be various reasons. When you've replaced them and double checked the wiring of the power supply you should remove the wires to the tubes (points A, B, C) first and then measure the voltages to be sure that the power supply itself is OK.
    Regards,
    Martin

    Thanks for the quick reply. I have de-soldered R22/23/24, and connection poits A, B and C. Waiting for new resistors.

    I have checked my assembled board against the layout, and can't find any faults. I used my multimeter to test for continuity between the points that have jumper wires on the back, and got continuity where I expected it. Just to be sure that I did not read the layout drawing wrong, the connection wires B and C are supposed to go to the terminals 19 top and bottom, right? And negative legs of C12/C13 and C14/C15 go to terminals 23 and 20, respectively? If so, then my wiring is according to the layout, and I don't understand why the resistors failed. R24 fell out when I turned the chassis over, and it was less burned than R23. I had soldered R24 just resting on top of the terminal and not through the small hole. So maybe one solder joint of R24 was bad, could this have caused the meltdown?

    Should I worry about the caps or the rectifier, or is it Ok to try a new startup (just the power supply) with just 3 new resistors? I ordered 3W resistors, I assume it's not a problem to increase the wattage?

    If you consider what you gave us as input - a small cutout pic of the PS section - what do you expect?
    The error ist not in the schematic or the layout plan, which some of us may have, it's in your realisation of this amp which we don't know.
    So some good pics of the whole amp would help.
    BTW: I stopped guessing about errors on the basis of descriptions because in most cases the error is in that what the clueless doesn't see by himself...

    Regards
    Bernd

    Jaichweiß (Andy Pipkin)

    If you are not sure about reading the layout plan correctly please use BOTH plans - the schematic shows exactly the same circuit so I suggest to not only using the layout plan but also the schematic.

    When R24 is not soldered, the only effect should be no voltage at point C. Depending on what is incorrectly built, there might be capacitors etc. also destroyed but that's only guessing w/o knowing what's wrong. Before powering up again the amp you should disconnect A/B/C to first check the power supply and check if the C12-15 are correctly connected (polarity?) You can also measure the bridge recitifier - it's just 4 diodes ;)

    Ok, found the error. I had soldered the wires from point C and V1 pin 1 to R11 instead of R3/R5. Replaced R22, R23 and R24. Measured 324-325 V DC at points B and C. So I connected A, B and C wires and powered up again. And I got sound! Nice tube sound, but I thought it was rather low, had to crank both gain and volume to get up to conversation level. Played for 5 to 10 minutes like this. Tubes were glowing bright orange. Suddenly, the volume gets way louder (too loud for livingroom playing), but only briefly, then some crackling noises and intermittent sound. I am thinking it could be a bad connection somewhere and will check my connections tomorrow when I am rested. Or maybe some other component got destroyed on first power up, but thenI am not sure how to troubleshoot. But apart from the low volume it sounded normal/nice for the first 5-10 minutes.

    I have looked over my work, and can't find any clear faults. Sound is still sometimes good but low, other times louder and with a lot of crackling noise, like a record player where the arm has not lifted off at the end of an LP side. Probably a funny way of describing it, but that's close to what it sounds like. How to diagnose such a problem?

    Could be bad soldering of tube sockets or a bad tube socket itself, try to twist the tubes a bit (caution: hot!) when you hear the crackling noise. Could also be bad soldering or a thermal problem of any other part of your circuit. Try touching/pushing the parts with a wooden stick when you hear the crackling noise (caution: lethal voltages!)
    Regards,
    Martin

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