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Original Message

RE: KT 120 Tubes in Yaqin MC-100B Amp

Posted by casouza on March 28, 2011 at 21:43:41:

I suggest that you buy an inexpensive infrared thermometer and measure the power transformer's temperature with the KT88s after warmup. The power transformer is the one near the power switch.
Then install the KT-120s. If the transformer temperature goes up more than 20 degrees F or 10 Centigrade, I suggest that you remove the KT120s.
A 10 degree C rise in external temperature may not look like much, but the internal temperature rise will be much higher, probably 40-50 C.
Assuming a "normal" temperature of 50 C, with KT120s the transformer hot spot may reach 100 degrees C or more.
I copied the following from a paper on transformers:
Modern transformers make use of thermally upgraded paper that has been chemically treated to improve the stability of cellulose structure. The rated hot spot temperature for this kind of paper is 110°C and it can be seen that an increase of 7°C will double the aging acceleration factor.
For older transformer built with normal Kraft paper, the rated hot spot temperature is either 95°C according to IEEE or 97°C according to IEC.
Google "transformer hot spot", there are lots of interesting articles.

In summary, it is a risky idea,