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Old 7th June 2005, 06:34 PM   #1
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Default KT88 or 6550?

I am going to try building this VTV amp:http://www.plitron.com/pages/Products/Audio/vtvkt88.htm
and was wondering whether i should use kt88s or 6550s. I see that they are regarded as being interchangeable and was interested in opinions as regards to how they sound and suitability as to this circuit. Also i would love to get some opinions regarding this amp. This will be my first push pull project and i`m trying to keep it relatively simple.
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Old 7th June 2005, 09:27 PM   #2
PRR is offline PRR  United States
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mclink.it shows essentially identical curves:



I'd even say they are the SAME in triode and UL. Slight difference in Pentode is similar to the difference between two tubes of the same type. Note that these are curve-traces, not data-book sales-charts, and the pictures show what appear to be old-stock tubes.

And while 1950s-1960s KT88 and 6550 came from different factories, current-production 6550 and KT88 are very likely to be the SAME guts in different bottles and pin-outs (6550's G3 is loose from the cathode).

Anyway: between ultra-linear, cathode resistors, and significant overall NFB, if you hear any difference between different big-Beam tubes in that VTV plan then you probably have (a) defective tube(s).
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Old 7th June 2005, 09:50 PM   #3
PRR is offline PRR  United States
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Not quite identical:
Click the image to open in full size.

6550 seems to be 10% hotter. This is of course within the range of variation between two tubes of the same type and the same factory vintage. For two tubes of "different types", it makes you think they may not be different at all.
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Old 7th June 2005, 10:27 PM   #4
PRR is offline PRR  United States
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One fan's history of the big bottles with LOTS of pictures:

http://www.jogis-roehrenbude.de/EL34.../6L6-Story.htm --- 6L6, grandfather of all beam power tubes
http://www.jogis-roehrenbude.de/EL34...ory-Seite3.htm - 6550 and KT88
http://www.google.com/language_tools?hl=en -- Google will translate (badly) German to english
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