tube amp debugging (jet city 20 watt combo)

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ubertar
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tube amp debugging (jet city 20 watt combo)

Post by ubertar » Tue May 21, 2019 6:31 pm

Just picked up a non-working Jet city combo amp, and I'm hoping to get it running...

Here's what I know so far:
1. When turned on, the tubes don't light up. The indicator doesn't light either, but the guy said it never did.
2. I checked all the tubes (5x 12ax7; 2x EL84) by putting them in other amps, and they all work. So it's not a bad tube.
3. I tested the power transformer with a volt meter, and all output voltages read as they should, including voltage to the indicator, so that must be a bad bulb. The transformer is handily labeled with what the three output voltages should be.

So... it seems it must be something in-between the power transformer and the tubes, not allowing the power to get to the tubes. Also the indicator bulb must be blown, but that's not important.

My best guess is it's diode 1 or 3, from looking at the schematic (see links below) but my knowledge is pretty limited, so I'm hoping someone can tell me if I'm pointed in the right direction. Since it's not a faulty power transformer, it seems like it must be something relatively minor, and a fairly easy fix if it can be diagnosed properly. There don't seem to be too many possibilities for what the problem could be, since the power transformer tests good and all the tubes test good, but the tubes don't light, and there aren't a whole lot of components in-between, so I'm hoping this will be a piece of cake to the right person. If that's you, can you help?

schematics:
https://music-electronics-forum.com/att ... 1452171994
https://music-electronics-forum.com/att ... 1452172017
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ubertar
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Re: tube amp debugging (jet city 20 watt combo)

Post by ubertar » Tue May 21, 2019 6:42 pm

Apparently those schematics aren't very good, and this amp is very similar, with a few component changes, and this is said to be a good schematic: https://music-electronics-forum.com/att ... 1452329663
FWIW
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Re: tube amp debugging (jet city 20 watt combo)

Post by I'm Painting Again » Tue May 21, 2019 7:27 pm

I'm not an expert on this stuff either but it's usually easy to fix once you find the spot

did you check the fuse?

the power supply filter cap/s might be bad ?

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Re: tube amp debugging (jet city 20 watt combo)

Post by ubertar » Wed May 22, 2019 5:06 am

The guy I got it from said he replaced the fuse and that didn't fix it. I haven't checked the filter caps yet... that's one of the first places I'd look if this was a vintage amp, but being relatively new with modern components I wouldn't expect them to go bad, but it's possible. Would that cause the tubes not to light and there to be no sound at all? I would think bad filter caps would mean loud hum. There's no sound whatsoever. If I wasn't reading correct voltages from the outputs of the power transformer I would think it's as dead as a stone.
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Re: tube amp debugging (jet city 20 watt combo)

Post by emrr » Wed May 22, 2019 5:27 am

Don't chase anything else until you get filament voltage to make tubes light up. That is possibly the only problem.
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Re: tube amp debugging (jet city 20 watt combo)

Post by ubertar » Wed May 22, 2019 8:19 am

emrr wrote:
Wed May 22, 2019 5:27 am
Don't chase anything else until you get filament voltage to make tubes light up. That is possibly the only problem.
How do I do that? I've checked the voltages on the power transformer's outputs, and they're all correct. I don't know what to do next. Unfortunately the way this amp is put together it's really difficult to access the underside of the board where all the connections are.
get a hammered sound from guitar or bass! http://www.stringhammer.com
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Re: tube amp debugging (jet city 20 watt combo)

Post by digitaldrummer » Wed May 22, 2019 8:58 am

If you can pull the board out to re-flow solder joints, that's going to be my guess. Seems to be the leading cause these days.

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Re: tube amp debugging (jet city 20 watt combo)

Post by I'm Painting Again » Wed May 22, 2019 10:10 am

ubertar wrote:
Wed May 22, 2019 5:06 am
The guy I got it from said he replaced the fuse and that didn't fix it.
perhaps they put the wrong or defective one in - try another one

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Re: tube amp debugging (jet city 20 watt combo)

Post by ubertar » Wed May 22, 2019 2:33 pm

digitaldrummer wrote:
Wed May 22, 2019 8:58 am
If you can pull the board out to re-flow solder joints, that's going to be my guess. Seems to be the leading cause these days.
You win! I did that and now it's working.

Oddly, I couldn't find a fuse on it, anywhere. Or the little holder where one would clip in.

Very psyched to get this working so quickly. I was worried that it was going to be a fried power transformer. It wasn't much of a gamble, at $25 including delivery to my door... the tubes or the speaker alone are worth more than that.

Thanks everyone who pitched in! Now if I could only have this much success replacing the door lock cylinder in my 03 Honda Civic. Can't get the clip out that holds in the old cylinder. At least something got fixed today. Yay Tapeop!
get a hammered sound from guitar or bass! http://www.stringhammer.com
hand-made version to raise money for manufacturing... kind of like kickstarter, but you get a fully functional item now

Album!
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