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Deluxe Reverb strangeness

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  • Enzo
    replied
    OK, how about apaply a signal, and when this happens, leave it that way. Use a signal tracer (listening amp) and see where the signal collapses. Hey, if you turn up the reverb, do you get reasonable reverb signal? Or is the reverb the same garbled stuff?

    You can probe grids without it resets? Fine, scope or listen to the grids of the two stages. SOund OK? Or do one or the other or both go bad?

    Did we do the whack test? Whack the chassis with a mallet, any reaction?

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  • Randall
    replied
    "WHich plate? And by channel 2 we mean vibrato channel?"

    Either one. And yes vibrato channel.

    " Just for science, instead of measuring at the plate pin, measure at the end of the plate resistor or the coupling cap lead."

    This is how I am measuring.

    "Or instead of looking at plate voltage, try checking cathode voltage instead. "

    Cathode voltage looks fine in normal operation. The next time I can get it to do it, I will put a probe on and see.

    "The heater is the only thing I see in common between those circuits, and you said it was a heater to cathode fault so who knows. "

    That's what I was thinking also, but remember the bad tube is removed from the picture when it failed this last time. And I measured good filament voltages on a couple of tube while in fault condition, so I don't think this is the issue.

    edit:

    OK, so I just turned it back on after a 40 minute cool down, and it went into failure mode, signal low and scratchy and fading out to almost nothing. Cathodes measure fine, and do not change anything when probed. Measuring the grids at the socket looked good, but made no change, but when I probed the resistors at the input jacks, it corrected. I do not know if this was from the inevitable pop this made. I hit them with freeze spray with no change. This is all at V2.
    Last edited by Randall; 04-01-2020, 04:14 AM.

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  • g1
    replied
    My only guess would be that it pulled down the heater voltage a bit, and one of the reverb tubes was more sensitive to low heater voltage than the other tubes.
    The heater is the only thing I see in common between those circuits, and you said it was a heater to cathode fault so who knows.

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  • Enzo
    replied
    Touching a probe to the circuit completes a circuit with the impedance of your meter. It suggests to me perhaps and open grid return resistor. WHich plate? And by channel 2 we mean vibrato channel?

    Also, putting probe to plate pin pushed on the tube socket pin, which may be involved. Just for science, instead of measuring at the plate pin, measure at the end of the plate resistor or the coupling cap lead. Electrically the same but gets you away from the socket. Any difference?

    Or instead of looking at plate voltage, try checking cathode voltage instead. Does that also bring it back, or does the cathode voltage look right?

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  • Randall
    replied
    Well dang, I thought I had it beat, but alas, it did it again just now.

    At least now I can be more susinct about conditions, and hopefully less confusing. The bad tube in question has been sidelined before the most recent episode, so that rules it out. New tube in trem position, and two new plate resistors for V2. I left it on for about an hour and when I randomly checked it, it was doing the low fading out gravelly sound, but only in Channel 2, channel 1 was clear.

    I measured good filament voltages on both channel 2 and the PI, and as before, the second I touched my probe to measure plate voltage, it snapped back to normal. The entire area around V2 has been resoldered on both ends of the wires, prior to this. Also, A strong signal from the guitar will put it back to normal as well.

    This is a sticky and tricky one.

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  • Randall
    replied
    Through the fog of war, I am not certain of the timeline, but I think it was both, yes and sometimes no.

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  • g1
    replied
    When it was working without trem and reverb, it was normal sounding? Or always the 'low and gravelly'?

    Leave a comment:


  • Randall
    started a topic Deluxe Reverb strangeness

    Deluxe Reverb strangeness

    Hello shut ins. I have a DR that came in for a loud buzz, turned out to be reversed reverb switch connector with Tremolo. That problem solved, I checked out the other functions and noticed the reverb and tremolo didn't work. That led me to the tremolo tube with the first triode shorted cathode to filament. Now here is where the oddness come in. When swapping out the bad tube, the reverb and trem both worked. I don't yet understand why, so I put the bad tube back in just to confirm my findings. It was then that the sound of the amp got quite low and gravelly, and a couple of times faded to nothing, until I put a probe on the reverb channel plate, which made a pop, and then the sound returned to normal. I probed and resoldered around that area, but found no change.

    This happened several times. I'm still collecting data, but here is what I think. The times it has done this is when the bad tube is in place when it is powered up after a long cool down period. Every time I touch a probe to a plate, it returns to normal. Once while it was in this condition I popped out the bad tube, and it stayed in this condition until I tried to measure with a probe. I even connected the probe to a few different plates before powering up, and in each of these cases it did not do it. As far as I know it has only done this condition when the bad tube was in place. Most of the time it powers up and sounds fine. Indeed, if it does power up in this condition, once I either probe it, or hit a solid guitar chord it will come back and as far as I can tell stay good.

    Now I know someone is going to tell me not to put the bad tube back in, but when it is not in the odd condition, everything looks and sounds pretty normal. I don't know what is the deal here, and can live with tossing the tube and just accepting that as the problem, but I would like to understand what is happening here. How does a bad trem tube make the reverb not work?

    http://ampwares.com/schematics/deluxe_reverb_ab763.pdf
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