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Are (immediately from the start or over time) the tubes that come with a tube amplifier (say the popular KT88 in Single Ended mode or Push Pull mode) replaceable with say the larger KT120 or KT150 any other tube from the "KT" series?

Q1) In other words are the KT88, KT100, KT120 and KT150 Tubes safely interchangable generally speaking in a well built tube amplifier?

Q2) are there any downsides (from a musical quality point of view) why one should NOT want to upgrade to a larger KT tube? In other words is the newly desiged KT150 generally a better (sounding/lifespan) tube than the KT120 tube?

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are the KT88, KT100, KT120 and KT150 Tubes safely interchangable generally speaking in a well built tube amplifier?

The KT120 and KT150 are both pentode devices and the KT88 and KT100 are described as beam tetrodes so no, I wouldn't think these can be easily interchanged even though the data sheets have the same "pin-out". Without delving into all the detail (which you of course can do) there may be several technical differences that may or may not make interchangeability easy or difficult.

On the Non-Equivalence of Beam-Tetrodes vs Pentodes appears to be a very useful site that might give you reasons to interchange those devices: -

enter image description here

The Svetlana data sheet for the KT88 says it's an exact replacement for the KT100 so I guess these can be interchanged freely.

are there any downsides (from a musical quality point of view) why one should NOT want to upgrade to a larger KT tube? In other words is the newly desiged KT150 generally a better (sounding/lifespan) tube than the KT120 tube?

I can only comment on the data sheets and these, as usual do not state anything about musical quality.

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  • \$\begingroup\$ Thanks very much! Eventhough it is not The Answer to my questions I am nevertheless more content after reading your answer. Pentode and tetrode sound different mechanism... I will read more into that thanks! "There may be several technical differences that may or may not make interchangeability easy or difficult" Meanwhile lets give some one else the room to give another answer and clear our doubts. \$\endgroup\$
    – Sam
    Dec 14 '16 at 14:58
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    \$\begingroup\$ Waiting for an old guy then LOL \$\endgroup\$
    – Andy aka
    Dec 14 '16 at 15:00
  • \$\begingroup\$ @Sam doesn't look like an old guy turned up to answer so did you get answers to what you felt I didn't answer? \$\endgroup\$
    – Andy aka
    Mar 4 '18 at 16:50

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