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Old 18th October 2007, 01:34 AM   #1
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Unhappy chinese tube amp

Howdy people, I am a diy virgin so please be gentle. I always wanted a tube amp like a mac275 etc but nice ones are a small fortune. So while surfing ebay one day I am lured bya chinese amp. I couldn't help myself. The place was going out of business in hong kong so I bought four. One week later they showed up. They look very nice but two of them came with no tubes. They should be 4 KT88 2 12AT7 and 2 12AU7. I know nothing. I have read that after going through the amps to make sure they are made correctly that tube rolling is good for 5% improvement. Anyway my question is what tubes would you guys recomend. And are tube numbers the same. Like does a KT88 have an equivalent with a different number? Oh the amps are music angel Kt88, mengyue mini and a meng mini headphone amp. Thanks Tom
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Old 18th October 2007, 01:57 AM   #2
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12AT7 and 12AU7 are common tubes, and should be easy to find. Unused old stock tubes will cost a lot. New production tubes are cheap and plentiful. You may find JAN (joint army-navy) substitutes for most of the 12A?7 series. For example, the JAN 5814 is interchangeable with the 12AU7.

You can probably substitute the 6550 for the KT88. New production KT88 will probably cost between $100 and $200 for a set of four. I've heard nothing but good reports on the New Sensor "Genalex" re-issues:
http://newsensor.com/ProductHighLight.aspx?ProId=1149

Old stock KT88 (GEC or original Genalex Gold Lion) will cost more than a small fortune. Old stock GE6550A are also supposed to be very good, and any would-be owners will have to pay dearly for them.
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Old 18th October 2007, 02:02 AM   #3
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Default Selection

Hi Tom:
The choices on tube selection for your amp are numerous..fortunately.
There is two basic choices, new or "Old", the old stuff is very good as compared to the new stuff, The prices are jacked up according to the percieved quality of the old stuff. The more common old stuff is priced very reasonable. The lifetimes rated in hours of operation is very high (EG. 30,000 Hrs.) , so a 'worn out' tube is rather unlikely.
The new stuff is primarily made overseas with Russia and the baltic states being the leader in production. The reason for this is the old USSR had and still uses tube gear in applications where in the US would have been replaced with solid state long ago. Even some U.S. companies have relocated to these factories in said locales as they have the machining and experience building tubes.
The old adage still applies "You get what you pay for"
Me.. I would get the "old" stuff as they have a proven track record.
_______________________________________Rick....... .
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Old 19th October 2007, 09:25 PM   #4
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Thanks for the info. Another question. Do I have to have the bias readjusted when I install the replacement tubes or am I good to go?
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Old 20th October 2007, 06:16 AM   #5
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I just bought some of the new Tung Sol 6550s cryo treated and they are very good. I was shocked how good. I didn't think they'd hold their own against my old stock tubes but they do and are in fact better than some of them...

6550 Link

They also do the Genelex KT88s:

KT88 Link
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Old 20th October 2007, 06:46 AM   #6
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The continuing saga of my chinese amp purchase. Well the two big KT88's came with no tubes. So I moved on to the mengyue mini el84 15w per amp. It blew the fuse at power up. Argh! So I'm 0 for 3. Then I hooked up the mengyue mini headphone amp and it actually works! Sounds pretty good too. So much for the great deal.
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Old 20th October 2007, 06:41 PM   #7
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Well all is not a loss.. you have the amps.. and they can be fixed and made to work. You have two options... do it yourself with help from everyone here or look for someone here or locally to do it for you.
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Old 20th October 2007, 06:54 PM   #8
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Quote:
Originally posted by Cycline3
Well all is not a loss.. you have the amps.. and they can be fixed and made to work. You have two options... do it yourself with help from everyone here or look for someone here or locally to do it for you.

I think you will find that where you live is a hotbed of DIYAudio activity !!!!!

Ask. I'm sure you will get help.

Good luck.

Andy
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Old 20th October 2007, 08:44 PM   #9
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Ok with all of that encouragement, I'm going in! I'll open her up right now and see what I can find. I finally got ahold of the guy who sold them and he's gonna refund me $220. for the missing tubes. So I think I'll try some of those cryogenicly treated ones I've seen on ebay.[QUOTE]
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Old 20th October 2007, 09:26 PM   #10
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Hhmm, well I see nothing obvious but then I know nothing. The only thing I noticed was there is nothing connected to the ground terminal on the power port.
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