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  • NAD not going so we’ll need advice.

    I picked up a used jet city 50 watt head. It’s just about mint. It sounds good as well. But there is one issue. The power light indicator dims after a few seconds of it being on. When I first turn on its very bright. But dims drastically to the point you can barely tell it’s on.
    the tubes seems to be glowing nicely and the all of the controls work. What could be some causes of the light to do that? Hopefully it is a simple cheap fix.

  • #2
    I'm not the right person to diagnose/respond, but here's a schematic showing what's in the circuit around it. Wild guesses - bad diode on either side of the pilot light? I don't know what those V components on the same transformer tap are. Maybe just the lamp itself. Try just replacing that to see?

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    • #3
      If it appears to be operating normally except for the power light, it probably is something simple.

      A lot of the power lights are LEDs these days. So that part of the circuit is usually a current limiting resistor and an LED coming off a post fuse supply voltage. Definitely inexpensive components, but I never like throwing parts at an amplifier.

      Any possibility of tracking down a schematic? Or share the model and I will search.

      Edit: ahhh, see it has a real lamp in it.
      Last edited by JamesPaul; 06-22-2021, 11:19 PM.
      I miss the 80's (girls) !!!

      Seymour Duncans currently in use - In Les Pauls: Custom(b)/Jazz(n), Distortion(b)/Jazz(n), '59(b)/'59(n) w/A4 mag, P-Rails(b)/P-Rails(n); In a Bullet S-3: P-Rails(b)/stock/Vintage Stack Tele(n); In a Dot: Seth Lover(b)/Seth Lover(n); In a Del Mar: Mag Mic; In a Lead II: Custom Shop Fender X-1(b)

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      • #4
        Originally posted by beaubrummels View Post
        I'm not the right person to diagnose/respond, but here's a schematic showing what's in the circuit around it. Wild guesses - bad diode on either side of the pilot light? I don't know what those V components on the same transformer tap are. Maybe just the lamp itself. Try just replacing that to see?

        ...
        The V components are the tube filaments/heaters.

        That's also the 100 watt power amp, but the 50 probably uses the same approach for the lamp.
        I miss the 80's (girls) !!!

        Seymour Duncans currently in use - In Les Pauls: Custom(b)/Jazz(n), Distortion(b)/Jazz(n), '59(b)/'59(n) w/A4 mag, P-Rails(b)/P-Rails(n); In a Bullet S-3: P-Rails(b)/stock/Vintage Stack Tele(n); In a Dot: Seth Lover(b)/Seth Lover(n); In a Del Mar: Mag Mic; In a Lead II: Custom Shop Fender X-1(b)

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        • #5
          Does the power light dim after a few seconds, even if you leave it on standby?

          If so, remove all the tubes and power it on and leave it on standby. If the power light dims it is the lamp or something after the lamp dragging down that winding. If the power light does not dim, one of the tube heaters might be dragging down that winding on the transformer. Power it off and start plugging tubes back in one at a time then power on with it on standby. If the power light dims after you plug in a tube, replace that tube.
          I miss the 80's (girls) !!!

          Seymour Duncans currently in use - In Les Pauls: Custom(b)/Jazz(n), Distortion(b)/Jazz(n), '59(b)/'59(n) w/A4 mag, P-Rails(b)/P-Rails(n); In a Bullet S-3: P-Rails(b)/stock/Vintage Stack Tele(n); In a Dot: Seth Lover(b)/Seth Lover(n); In a Del Mar: Mag Mic; In a Lead II: Custom Shop Fender X-1(b)

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          • #6
            Thanks for the input it dims when it’s on standby and stays that way. I will try what you suggested after I get some sleep. Some if it is something bigger than that lamp I guess I can’t complain. I bought it in a package deal with the 2x12 cab for $275. I just wanted the cab but the guy would not split them up.

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            • #7
              It's probably the lamp, try replacing it. As mentioned, the "V" things are the heaters in the tubes, so it is a 12V lamp.

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              • #8
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                • #9
                  I've had two lamps go out in my JCA20H. It's probably as simple as replacing it.

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                  • #10
                    It's just a bad lamp in the switch. While annoying, it isn't entirely necessary for operation. I myself am not a fan of integrated lamp switches. When the lamp dies, you have to buy and install an entire switch as opposed to just replacing a bulb or LED in the pilot light position. It shows where all the corners were cut in order to get the Jet City amps to the price point they are at. While the PCB and components may be status quo, the switches, jacks, and other non-essential parts are cheap as can be. Having built and designed a few amps in my day, I can say that they are not that expensive to build even at the homebrew market pricing. When you can buy in bulk and collect the deals that larger companies can acquire, you can build a pretty complex amp for next to nothing. I would suspect that an amp that retails for $500 likely costs about 1/3rd of that to actually produce. A more or less basic amplifier ( like a JCM-800 ) can be built from scratch with parts you can get from a couple of places for around $500 total.

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                    • #11
                      Thanks guys yeah the amp works fine besides the lamp. I will probably have to take it to a tech to get replaced. It has metal switches and the lamp is an led jewel type. It looks like a solid light not just a bulb. So it is beyond my skill set to replace.

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                      • #12
                        Unless you are planning to sell the amp, you could just wait until the lamp goes out completely before taking it to a tech to replace it. The lamp glows bright when you first power the amp on, so you know the amp is getting power. The day you turn the amp on and there is no light but you still have sound, it will be tech time. If you power the amp on and have no sound, it will be the usual check the fuses. Saves you some cash for now.
                        I miss the 80's (girls) !!!

                        Seymour Duncans currently in use - In Les Pauls: Custom(b)/Jazz(n), Distortion(b)/Jazz(n), '59(b)/'59(n) w/A4 mag, P-Rails(b)/P-Rails(n); In a Bullet S-3: P-Rails(b)/stock/Vintage Stack Tele(n); In a Dot: Seth Lover(b)/Seth Lover(n); In a Del Mar: Mag Mic; In a Lead II: Custom Shop Fender X-1(b)

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