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PostPosted: Tue Feb 05, 2019 11:13 pm 
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Hello, I'm at the last stages of my champlifier build and hit a snag. The short version is that I blew a fuse, but the whole story is below for context.

I had pretty much finished the build and tested it about a week ago but couldn't get it to power up. I later realized that I had not connected the blue OT wire to the 6V6 socket (Though I don't see why that would keep the power lamp from coming on). Connected it, and also did my best to get that fuse to screw in...because it was not quite flush. Powered it up and got light. Great.

So next I unplugged the amp, plugged in the rectifier tube, turned the switch to the "on" position as recommended in the manual, I saw a slight spark at the outlet as I plugged in the power cable and did not see the power lamp come on. Unplugged the amp again and checked and it looks like the fuse is blown--no continuity and there is a tiny black dot on inside of the glass.

I'm sure I'll need to buy replacement fuses, but first I wanted to check in and see if there is anything I should be concerned about, given that the fuse blew. Any thoughts?

Thanks in advance!


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PostPosted: Tue Feb 05, 2019 11:18 pm 
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You should post pictures of your wiring so that we can help validate it for you.

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PostPosted: Tue Feb 05, 2019 11:27 pm 
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OK I can do that!


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PostPosted: Tue Feb 05, 2019 11:38 pm 
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Here are some pics of the wiring


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PostPosted: Wed Feb 06, 2019 11:34 am 
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I can't see anything glaringly wrong. Here are some notes:

- I cannot see the keyway / center pin mark for the rectifier tube socket, so I cannot tell which pin is which. Ensure that the keyway is facing the same direction as the keyway on the 6V6 tube socket
- From the photos, I cannot tell if there is any space between the volume pot power switch lug with the black wire attached to it and the component leads behind that switch lug. Ensure that there is no way that excess solder or untrimmed wire leads on the switch lug can contact the leads from the 470r resistor or associated 25u cap
- The green heater circuit wires can often cause issues where the transformer wires and the lead wires connect at the pilot light holder. If you look closely at the connections, you'll see that the two solder lugs on the pilot light assembly are insulated from each other and from the lamp base (which is grounded). Have a close look and make sure that no stray solder blobs or untrimmed lead wires are shorting to the opposite lug or to the lamp base. These connections need to be very neat and solid.
- BYOC has you install an uninsulated wire underneath the board from the 22k/8uf high voltage tap to the 100k/100k plate resistors. I'm not a fan of hiding an uninsulated high voltage wire like that underneath the board. Measure resistance or continuity from the 100k/100k junction to ground to ensure that wire is not shorting out to a circuit board standoff or component lead or something.
- Those unused yellow and brown wires from the output transformer that have wire nuts on them should be tucked out of the way into the corner of the chassis where they don't interfere with the rest of the lead dress.
- Measure resistance/continuity to ground from the positive ends of each of the electrolytic caps. Each one should have very high resistance (>1meg)

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PostPosted: Wed Feb 06, 2019 1:34 pm 
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A fuse blows because something is drawing more current than it should be. This almost always suggests that something is shorted out to ground.

If Morgan's suggestions don't solve the problem, disconnect the red wire from pin8 of the rectifier tube socket (but not the yellow). If the fuse doesn't blow and you are getting proper current at pin 8 (it will be a little higher than suggested since it's not under load), then you can assume the problem is somewhere on the turret board or further down the power rail.

If it still blows, remove the green wires from the lamp (coming from the transformer). If the fuse doesn't blow, then you can assume the problem is with the heater wiring. Reconnect the red wire. If the fuse still doesn't blow, then you know for certain that the problem is with the heater wiring.

If the fuse is still blowing after disconnecting the B+ and the heater wiring, then you've likely made a mistake connecting the mains to the primary, and we'll need some pics that show that wiring a little more clearly.

Also, you may want to swing by the hardware store and pick up a few extra fuses.

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PostPosted: Wed Feb 06, 2019 11:45 pm 
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Thanks to both of you, these are really helpful posts. So:

Quote:
I cannot see the keyway / center pin mark for the rectifier tube socket, so I cannot tell which pin is which. Ensure that the keyway is facing the same direction as the keyway on the 6V6 tube socket
- From the photos, I cannot tell if there is any space between the volume pot power switch lug with the black wire attached to it and the component leads behind that switch lug. Ensure that there is no way that excess solder or untrimmed wire leads on the switch lug can contact the leads from the 470r resistor or associated 25u cap

These are ok.

Quote:
- The green heater circuit wires can often cause issues where the transformer wires and the lead wires connect at the pilot light holder. If you look closely at the connections, you'll see that the two solder lugs on the pilot light assembly are insulated from each other and from the lamp base (which is grounded). Have a close look and make sure that no stray solder blobs or untrimmed lead wires are shorting to the opposite lug or to the lamp base. These connections need to be very neat and solid.

Interesting. So, at the lamp, right now I get continuity between the outside solder lugs, inside solder lug, and the chassis ground. Those connections weren't particularly neat :( But I'm not sure I see how or why they are all showing up as connected.

How should these connections look? Is it the case that the inside and outside lug should be discontinuous with each other and discontinuous with the chassis ground?


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PostPosted: Thu Feb 07, 2019 11:29 am 
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ch ra wrote:
So, at the lamp, right now I get continuity between the outside solder lugs, inside solder lug, and the chassis ground. Those connections weren't particularly neat :( But I'm not sure I see how or why they are all showing up as connected.

How should these connections look? Is it the case that the inside and outside lug should be discontinuous with each other and discontinuous with the chassis ground?

I forgot to mention that you cannot trace this circuit using continuity because the power transformer heater circuit will show as a closed circuit. You just need to visually confirm that there are no potential shorts on the lamp base. If you suspect this area, do as Keith suggested and desolder the green transformer wires from the lamp base and try the rectifier again.

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PostPosted: Thu Feb 07, 2019 1:41 pm 
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I did find a potential short and cut off the excess solder. I was thinking of resoldering the joint anyway, so should I just replace the fuse, remove the green wire that had the excess solder and try powering it up again?


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PostPosted: Thu Feb 07, 2019 2:45 pm 
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Yes. Then follow byoc’s guidance.

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PostPosted: Sun Feb 10, 2019 11:25 am 
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I got a couple of fuses from the hardware store yesterday--they are labeled MDL-2A Time Delay 2 Amp fuses. The guys at the store told me these would work as replacements for the ones listed in the Champlifier BOM. I removed the green wire going from the power transformer to the lamp outer lug. Plugged it in and no fuse blowing, but I also get no light. I assume this means no power, right? Is that how it's supposed to work? I checked continuity in the components going into the lamp and it looks good. Ground for plug is continuous w/chassis ground, and the two plug components show continuity to the red wire lug on the volume switched pot. I also get continuity b/w the two lugs on the fuse assembly. Is the reason that I'm not getting power in the lamp because green wire to the power transformer is disconnected?


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PostPosted: Sun Feb 10, 2019 12:56 pm 
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You’re getting power. The lamp is not lighting because you disconnected it’s power source - the green wires coming from the power transformer.

Are you able to plug in the rectifier tube now without blowing fuses?

NOTE: now that you’ve disconnected the green wire, DO NOT install the 6v6 or the 12ax7.

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PostPosted: Sun Feb 10, 2019 1:47 pm 
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I popped in the rectifier tube (tube closest to power transformer) and no blown fuse. Is the next step to pop that green wire back into the outer lug?


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PostPosted: Sun Feb 10, 2019 2:23 pm 
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The heater wiring on the 12ax7 looks off in the photo. I don't see pin 4 and 5 tied together, and it looks like 4 might be touching 8 from the photo.

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PostPosted: Sun Feb 10, 2019 2:31 pm 
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Spin, thanks for your careful eye. Actually the wiring was ok--4 and 5 are connected by a black insulated wire because I did not quite have enough wire on the 4 lug to reach to #5. There was a lot of excess wire on 4 that I forgot to trim, so I did that. Should be clean now. I tried to get pics but it is really hard to get a decent pic in such a small space.


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PostPosted: Sun Feb 10, 2019 3:39 pm 
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ch ra wrote:
Spin, thanks for your careful eye. Actually the wiring was ok--4 and 5 are connected by a black insulated wire because I did not quite have enough wire on the 4 lug to reach to #5. There was a lot of excess wire on 4 that I forgot to trim, so I did that. Should be clean now. I tried to get pics but it is really hard to get a decent pic in such a small space.

Ah, I see now, I wasn't looking at the black wire. Carry on

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PostPosted: Sun Feb 10, 2019 7:21 pm 
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You’ve removed the 6.3 volt heater circuit (the green wire) and now your fuse is not blowing. So the heater circuit is probably the issue.

I recommend removing both green transformer wires from the lamp base (it’s not clear if you removed one or both of them), remove the lamp from the socket, make sure no tubes are installed, and then check continuity to ground on both green wires connected to the lamp base. You should not have continuity.


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PostPosted: Sun Feb 10, 2019 11:18 pm 
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Morgan, I was thinking that I had a solder bridge between the two lugs before and that was what caused the fuse to blow. As it stands, the green wire at the outer lug is not showing continuity to ground, only the green wire at the inner lug (which is also connected to a wire from the transformer).

In any case, I'll follow your advice and disconnect the other wire to confirm that there should be no continuity w/ground for the inner lug green wire.

If that's ok should I hook them back up and fire up the amp again?


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PostPosted: Sun Feb 10, 2019 11:29 pm 
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Confirmed. Removing the green power transformer wires eliminated continuity to ground. Does this mean I can reattach and fire it up? Or is there a problem w/the wiring to the tube sockets?


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PostPosted: Sun Feb 10, 2019 11:52 pm 
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Yep, go ahead and reattach the heater wires and try it again.


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PostPosted: Mon Feb 11, 2019 12:45 am 
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Damn, I reattached the wires, powered it up, but no light and no glow in the rectifier tube. I measured voltage at the volume switch and it was ~120V AC. I checked the light bulb and it looks ok from what I can see. What could be going wrong here?


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PostPosted: Mon Feb 11, 2019 10:50 am 
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Take a step back. Remove all tubes from the amp. Visually check the green wire circuit for shorts. Check the lamp carefully to see if it's been burnt out. Check the fuse again. Try the amp with no tubes to see if the lamp comes on.

You can check the voltage of heater circuit. Set your meter for AC voltage, not DC. Low range. Place a probe onto each terminal of the lamp base - we're reading AC voltage across the two terminals, not voltage to ground. You should have between 6.5 and 7 volts or so with no tubes installed. If you are not getting the right voltage across the lamp base, remove the green transformer wires from the lamp base, make sure the ends are separated (not touching themselves or anything else), turn on the power and carefully measure AC voltage across the two disconnected green transformer wires. See if the transformer is putting out the correct voltage.

Also, check you center tap grounds. Make sure the green/yellow and red/yellow wires are actually grounded.

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PostPosted: Sat Feb 16, 2019 10:49 am 
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Morgan, thanks for sticking with me. I took some time off and went back to the amp. As far as I can see, the lamp is not burned out. I powered it up and checked the voltage of the heater circuit w/no tubes and it was spot on at 7 volts, so that is good, right?

Finally, I did notice that the red wire from the output transformer had broken off from the turret board, so I soldered it back on to the board. The amp still does not light up though.

Next step?


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PostPosted: Sat Feb 16, 2019 3:34 pm 
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OK I tested the bulb and it is fine. Fuse, likewise, is fine. The bulb does not light up when amp is plugged in and turned on, but I get 7VAC at the heater wires. What else should I check at this point?


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PostPosted: Sat Feb 16, 2019 5:00 pm 
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Curious how you tested the bulb.

Go ahead and plug in the rectifier and 12ax7. If the tubes are flowing and the fuse is not popping, plug in the speaker and the 6v6.

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