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-   -   Got the build itch again. 80w kt88 (http://www.diyaudio.com/forums/tubes-valves/122032-got-build-itch-again-80w-kt88.html)

itsmejto 26th April 2008 12:35 AM

Got the build itch again. 80w kt88
 
Stumbled on a stripped dynaco st-70 chassis.
Decided to put it to good use.
Should deliver about 80 watts a channel if my last one was anything to judge by.
Using a Welbourne pcb (slightly modified) with 12ax7's and 6u8a's
Plate voltage arround 600vdc at 400ma
Yes, tube life will be limited, but it's for Ebay when done.

http://i78.photobucket.com/albums/j1...o/DSCN0729.jpg
http://i78.photobucket.com/albums/j1...o/DSCN0730.jpg
http://i78.photobucket.com/albums/j1.../st70board.jpg
http://i78.photobucket.com/albums/j1...t70chassis.jpg

Brett 26th April 2008 12:50 AM

Re: Got the build itch again. 80w kt88
 
Quote:

Originally posted by itsmejto

Plate voltage arround 600vdc at 400ma
Yes, tube life will be limited, but it's for Ebay when done.

600V @ 100mA/tube will give them a very shot life.

Jeb-D. 26th April 2008 12:54 AM

Quote:

Yes, tube life will be limited, but it's for Ebay when done.
Note to self: don't buy KT-88 amp from ebay :D

Brett 26th April 2008 01:01 AM

Quote:

Originally posted by Jeb-D.
Note to self: don't buy KT-88 amp from ebay :D
I was thinking the same thing myself........

Ty_Bower 26th April 2008 02:38 AM

Re: Got the build itch again. 80w kt88
 
Quote:

Originally posted by itsmejto
...600vdc at 400ma
Uh, is that kind of power transformer gonna fit in the hole?

tubelab.com 26th April 2008 02:46 AM

Quote:

Plate voltage arround 600vdc at 400ma
Lets see, 60 watts of dissipation per tube, 240 watts total in the output tubes. The whole amp will melt if the rectifier tube doesn't blow up first! I have been known to bend some tube specs a bit (OK a lot) but I haven't found a KT88 that can deal with 60 watts. The best are EH KT88's and they glow dimly at 45 watts, brightly at 50 watts. They wouldn't last long (a few hours maybe) at 60 watts.

Power transformer? No, you are going to need a nuclear reactor.

sorenj07 26th April 2008 03:40 AM

If I had a quad of KT88's and wanted to get 80W out of them, I'd probably build monoblocks and transformer couple a 6V6 or something and run into class AB2. Might be feasible then...

I'm interested in how long this amp would run after being switched on. 10 seconds? 30? Unless of course there was a typo in the first post..

HollowState 26th April 2008 04:45 AM

I have a pair of mono blocks that run NOS GE 6550A's in ultralinear with at least 610 volts at 60ma per tube. I can get 100 watts rms out down to 60 Hz with the Acrosound TO-600 4K P-P load. 100 ma is too much and not necessary. However, I would be very afraid to try this with todays garbage 6550/KT88 tubes.

itsmejto 26th April 2008 03:16 PM

oh my, such negativity.
Guess none of you ever saw a Harmon Kardon Citation II then.
Granted it has superb transformers, though it does run as standard at 65wrms per channel with old rca 6550's.
The last one I had was biased at 85ma and sounded both loud and clean, with the original 6550's from 30 years ago. I still use them on a daily basis in my home system and they havn't burned up yet.
As for the rectifier burning up, check out the specs on a 5au4, it may surprise some of you. (1400v @ over 1a peak) but granted it would be a weak spot pushed so hard I may have to use schottkies.

I'll be using EI-kt90's which should handle the extra load, check out the specs on em, they are not toys. the kt88's are just a set of test tubes to make the layout look better. As for the power tansformer, it's an Antec 800va (5-1/4" dia. x 4" tall) toroidal mounted on it's edge and should fit fine.

Jeb-D. 26th April 2008 04:48 PM

Quote:

I'll be using EI-kt90's which should handle the extra load, check out the specs on em, they are not toys. the kt88's are just a set of test tubes to make the layout look better.
You should check out the kt90-EH. The EI's are said to be great, but there is a large amount of defective ones floating around. I think the kt90-EH look better than kt88. I'm curious how they sound.


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