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Post subject: Concert II (Hum, Blown Fuses)
Posted: Sun Dec 11, 2011 5:14 pm
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I'm having a problem with a Concert II amp ('80s vintage/tubes). On Friday, it started having a loud hum, even without any guitar plugged in. It was annoying. Today, I tried it. It hummed and then it blew both 500 ma fuses. It blows new fuses, too. My first thought is to replace the power supply filter caps. Any other suggestions as to what I should look for?

The schematic I have is hardly the best. Are those two fuses on the B+ line? The amp continues to get AC power after the fuses blow.

One final point: this amp was originally made for European power. An additional transformer was put in to adapt it for American power. Before Friday, this amp worked very well.

Thank you for any help you can give me.

Brian


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Sun Dec 11, 2011 11:05 pm
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" An additional transformer was put in to adapt it for American power. Before Friday, this amp worked very well. "

"Before Friday," meaning you were not using 120VAC/60HZ outlet before Friday? And how was that "additional" PT added to your amp? Really, should be a rewiring of the original PT or replacement with proper tranny for US use. I'm not a big fan of plug-in external step-up or -down transformers for tube amplifiers.


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Mon Dec 12, 2011 3:00 am
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A 500 milliamp fuse is always on high voltage . If it blow most of the chance it is power tubes are short or/and bias too hot.

Fotget for now replacing capacitor.

Remove all of your power tube , put a new fuse and power on. If fuse don't blow it is on your power tube circuit ; 6L6
Replace them and check bias , test output transformer before. IMO output transformer should'nt be bad.

I write that if your mod ; new power transformer is the good one and no more wrong mods on youre amp


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Tue Dec 13, 2011 2:34 pm
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Thanks, guys.

I tried it with the 6L6s removed and it still blew the two 500 ma fuses. What should I look for now?

The after-market conversion xformer was in the amp when we bought it a year ago. Until last Friday, this amp was a real workhorse and sounded tremendous.

On the fuses, should I be buying fast blow or slow blow?

As always, thanks for your help.


Brian


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Tue Dec 13, 2011 4:29 pm
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bconnor94 wrote:
Thanks, guys.

I tried it with the 6L6s removed and it still blew the two 500 ma fuses. What should I look for now?
On the fuses, should I be buying fast blow or slow blow?
Brian


This mean your 6L6's are not the problem for now. Same type of fuses you have if it the are the originals fuses. Don't try some more fuses until you find something wrong on the amp. Can you send us some picture inside the amp ?

What should you do for now; Find the schematic for us . I can't find on the web.


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Tue Dec 13, 2011 5:16 pm
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The only Concert II schematic I found have not your 500 ma fuses. Only one 2 1/2 amp fuse .

Rivera - area;

http://www.stratopastor.org.uk/strato/a ... nders.html


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Sat Dec 17, 2011 3:08 pm
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Thanks for your help, guys. Brian and I managed to fix the amp.

With a new batch of fuses, the amp stopped blowing fuses but the annoying hum remained.

We looked inside the amp and one of the filter caps (4 mfd/450V) was bulging. We replaced both filter caps. While we had the amp apart, we replaced the remaining six electrolyitc caps. Now it sounds even better than it did when we bought it. My hobby is restoring tube-type communications receivers so I had the caps on hand. I was amazed at the clean layout and quality of parts that Fender used. Much better than even most of the high-end radios I have worked on.

We couldn't find an accurate schematic for this amp. However, we did find one accurate parts layout diagram. That was helpful because it enabled us to make sure we had the right value caps before we took the amp apart.

Again, thanks for your help.

Joe Connor


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Sun Dec 18, 2011 2:35 am
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Good to know your amp is better, fresh capacitors bring a new life most of the time . And what about the hum ? You have it with new caps ?
As you know if you rebuilt radio ; same hum at any volume is filter caps. Hum change with volume is another components. Could be unmatched power tubes, or preamp tubes. Does your tubes heater have a center tap at power transformer ? I guess yes. No center tap = hum induction by heater.


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Sun Dec 18, 2011 6:23 am
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No, the hum is completely gone.


Joe Connor


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Post subject: Re: Concert II (Hum, Blown Fuses)
Posted: Sun Dec 18, 2011 6:52 am
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Good news.


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