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 Post subject: Amp keeps blowing fuse
PostPosted: September 17th, 2019, 8:23 pm 
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Joined: July 17th, 2016, 6:24 am
Posts: 645
Haven’t used the amp for a while.
One of the channel was dead. Then entire amp is dead. Fuse blows after powering on.

This is a KT88 PP AMP.
What should I look at?

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PostPosted: September 17th, 2019, 9:06 pm 
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Joined: February 19th, 2017, 9:43 am
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I'd look at getting a good reliable, solid state amp. But that would not be a popular course of action around here.


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PostPosted: September 18th, 2019, 5:34 am 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
Need to check the tubes. KT88’s have the rep of shorting out


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PostPosted: September 18th, 2019, 7:36 am 
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Joined: March 2nd, 2013, 2:43 pm
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Location: Potomac, MD
An easy way to trouble shoot an amp with a short that does not have a switching power supply is to put an incandescent light bulb in series with one leg of the power mains feeding the amp. 100 watt bulb is appropriate for a power amp. If the bulb lights up brightly it indicates a short but the fuse doesn't blow. Then you can pull out your KT88s one by one until the bulb dims.

David


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PostPosted: September 18th, 2019, 7:51 am 
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Joined: February 28th, 2013, 1:19 pm
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Just read Dave Berning's suggestion, that is the first thing you should try. My comments below are further steps to isolate the problem if not a shorted KT88.

1. Do you have a variac? It is best to start up an amp that has been sitting a long while by starting at a reduced lines voltage and slowly bring up to voltage. This help reform old electrolytic caps.

2. Do you have access to a tube tester (either an emissions tester or transconductance tester). If so, check all tubes for shorts, emissions.

3. Does the amp have a tube rectifier. If so, this could be faulty, so check this (see above)

4. If the amp uses solid state rectification, the turn-on surge could be excessive if the caps are old and leaky after long storage or excessively large. It is possible the capacitors will need replacing.

5. Is there a way to turn on the heaters first to warm up the tubes prior to applying B+ voltage? This helps in getting the tubes up to emission temperature.

6. Is the fuse the correct current value and type for the amp? A fast blow fuse or too small of a fuse will blow on a startup surge. That being said, you do not want to indiscriminately increase the value of the fuse until it doesn't blow.

7. If the amp uses fixed bias (negative bias supply), make sure this is operating. However, if you have badly out of bias tubes (red-plating), this is not likely to cause the amp to immediately blow a fuse.


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PostPosted: September 18th, 2019, 10:00 am 
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Joined: March 12th, 2013, 11:12 am
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You also might want to first open it up and look for fried/burnt/cracked/swollen/leaky looking components.


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PostPosted: September 18th, 2019, 10:11 am 
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You could also post some pictures. Many eyes might see something.

Ray


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PostPosted: September 18th, 2019, 10:24 am 
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Joined: July 24th, 2015, 4:17 pm
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Location: Parkville, Maryland
I would add one more task to the excellent advice already rendered -- open it up and carefully inspect the amp. for any parts that appear stressed. Either it will be the stressed part a good place to start with repair or you can drop it from your check list if everything looks good.

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PostPosted: September 18th, 2019, 9:16 pm 
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Joined: July 17th, 2016, 6:24 am
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Thanks all for the replies.

I powered it on without the tubes and the fuse blew in about 10 seconds. Assuming the problem could in PS section? All the solders and the caps look okay. Attaching a few pics. Any suggestions?

Attachment:
19FCB713-3367-4C6B-BF85-48458F946104.jpeg
19FCB713-3367-4C6B-BF85-48458F946104.jpeg [ 1.18 MiB | Viewed 846 times ]

Attachment:
7727DDDD-0926-4295-B645-3132627103F9.jpeg
7727DDDD-0926-4295-B645-3132627103F9.jpeg [ 1.6 MiB | Viewed 846 times ]

Attachment:
2F39DD73-9F8C-46BC-BCA2-D689E520F410.jpeg
2F39DD73-9F8C-46BC-BCA2-D689E520F410.jpeg [ 1.96 MiB | Viewed 846 times ]

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PostPosted: September 18th, 2019, 9:56 pm 
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Joined: July 17th, 2016, 6:24 am
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David McGown wrote:
Just read Dave Berning's suggestion, that is the first thing you should try. My comments below are further steps to isolate the problem if not a shorted KT88.

1. Do you have a variac? It is best to start up an amp that has been sitting a long while by starting at a reduced lines voltage and slowly bring up to voltage. This help reform old electrolytic caps.

2. Do you have access to a tube tester (either an emissions tester or transconductance tester). If so, check all tubes for shorts, emissions.

3. Does the amp have a tube rectifier. If so, this could be faulty, so check this (see above)

4. If the amp uses solid state rectification, the turn-on surge could be excessive if the caps are old and leaky after long storage or excessively large. It is possible the capacitors will need replacing.

5. Is there a way to turn on the heaters first to warm up the tubes prior to applying B+ voltage? This helps in getting the tubes up to emission temperature.

6. Is the fuse the correct current value and type for the amp? A fast blow fuse or too small of a fuse will blow on a startup surge. That being said, you do not want to indiscriminately increase the value of the fuse until it doesn't blow.

7. If the amp uses fixed bias (negative bias supply), make sure this is operating. However, if you have badly out of bias tubes (red-plating), this is not likely to cause the amp to immediately blow a fuse.


See my reply above.

Fuses: fast-acting 2A 20mm.

I don’t have an variac. Do u think it is worthwhile based on the symptoms?

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