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Old 5th November 2008, 03:09 AM   #1
hwong is offline hwong  Canada
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Default Tube amp repair

I need to find someone or place that I can take (or ship if it is close) my AudioSpace AS-8i ( KT88 PP) integrated amp for repair. I am located in the Edmonton. I can take this to one of the few high end hi-fi shop here but I am afraid that the repair bill will be more than what I paid for the Amp. Anyone know of someone or place that will do it for a decent rate?

I picked up the amp in HonKong couple of years ago and it went south on me yesterday. No music is coming out of the left channle, only thumping from the speaker driver. I disconnected the left speaker from the amp and there are still music coming out from the right speaker.

I am totally clueless when it comes to electronics or circuitry.

Any info/help is appreciated.
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Old 5th November 2008, 12:20 PM   #2
erikb is offline erikb  Netherlands
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Well, i don't think it will be that hard, you probably have to replace a tube (or two). You could even find out which tube it is by switching tubes from the good channel to the defective channel, with the exception of the 12ax7 tube (in the middle, in the front).

You can simply grab the tubes and pull them out slowly, and put them back in the other channel, just make sure you unplugged the amp and wait a minute before pulling the tubes. I'd start with the 12au7 tubes, 2 at a time (switch the two on the left with the two on the right). If the problem changes to the other channel, you've found the defective tube pair. If not, swap the big ones (2 at a time). Same story, just take it slow.

When this doesn't work, it probably is the 12ax7 in the front, they are pretty cheap and every guitar/amp shop should have them in stock (just like the 12au7's), make sure you don't get the cheapest chinese tube. And if you have to replace a 12au7, make sure you get a matched pair and replace two at a time, as they have to amplify symetrically. The KT88 tubes are a bit expensive, i hope it's not one of them .
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Old 5th November 2008, 03:52 PM   #3
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by erikb
Well, i don't think it will be that hard, you probably have to replace a tube (or two). You could even find out which tube it is by switching tubes from the good channel to the defective channel, with the exception of the 12ax7 tube (in the middle, in the front).

You can simply grab the tubes and pull them out slowly, and put them back in the other channel, just make sure you unplugged the amp and wait a minute before pulling the tubes. I'd start with the 12au7 tubes, 2 at a time (switch the two on the left with the two on the right). If the problem changes to the other channel, you've found the defective tube pair. If not, swap the big ones (2 at a time). Same story, just take it slow.

When this doesn't work, it probably is the 12ax7 in the front, they are pretty cheap and every guitar/amp shop should have them in stock (just like the 12au7's), make sure you don't get the cheapest chinese tube. And if you have to replace a 12au7, make sure you get a matched pair and replace two at a time, as they have to amplify symetrically. The KT88 tubes are a bit expensive, i hope it's not one of them .

This is a pretty good way to render both channels inoperative, particularly if one of the defective tubes is an output tube. It is far better to try some inexpensive or used/known good replacement tubes rather than swapping potentially bad tubes into the working channel. I can't tell you the countless times former clients did this - leading to significant additional damage and a much bigger repair bill.

Normally you would do some work to identify other failed components before replacing any tubes.
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Old 5th November 2008, 05:54 PM   #4
erikb is offline erikb  Netherlands
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Oh, i never had any problems with it and i've done it a few times. But i guess you are right, destroying more of your amp is a possibility and could be expensive.
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Old 5th November 2008, 10:18 PM   #5
hwong is offline hwong  Canada
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Thanks for the help. I'll try to replace the tube tonight and test them.

I do have an extra set (come with the amp) of 12AU7 and 12AX7. The one that are in the amp now are JJ tubes.
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Old 6th November 2008, 01:39 PM   #6
cwujek is offline cwujek  United States
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Default Re: Tube amp repair

Quote:
Originally posted by hwong
I disconnected the left speaker from the amp and there are still music coming out from the right speaker.
Surprised no one mentioned it, but this is a big no no. Never run a tube amp without some form of load on the output. If your output tube/transformer weren't bad, you could have a problem now. Hope you find a solution.
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Old 7th November 2008, 03:16 AM   #7
hwong is offline hwong  Canada
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Finally have time tonight to test the amp with the set of output tube that came with the amp and everything seemed to be working. So that mean the problem is with the output tube. Will play the amp a bit more longer to make sure I have not accidentally broke something else.

Thanks
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