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KT88 Tubes

I am new to tube amps having acquired a Harmon Kardon Citation II about a year ago. A little while ago I started experiencing one of the channels dropping out and an occasional loud pop. Today I noticed one of the KT88 tubes glowing purple where the rest glow orange so I have assumed that the tube has gone bad. As I bought this amp used, I have no idea of the age of the tubes. So I have a couple of questions. First, should I replace just the two tubes on the bad channel with a matched pair, or should I replace all 4 tubes with all 4 being matched. Second, any recommendations for good quality KT88 tubes? Any other thoughts or ideas would be appreciated. 

russbargmann
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viridian's avatar
viridian
5,250 posts
 

Before buying more tubes, exchange the tubes in the right channel for the left. If the bad tube remains bad, it is a tube issue, however if the tube that you switch into the bad position red plates, it is an issue with the amp. Good luck!

IMHO, replace all output tubes if it is a tube issue.

audition__audio
1,647 posts
 

Good advice from viridian. Always replace the entire set of power tubes unless you had an early failure in the set. If this amp is in original form internally you will need to replace caps at the minimum.

jasonbourne71's avatar
jasonbourne71
1,390 posts
 

You can use 6550's in the Citation II. They are generally cheaper than KT88's. That's what I use in my pair of Dynaco Mk III 's.

brandonhifi's avatar
brandonhifi
71 posts
 

Having tested nearly every quad-matched KT88 tube currently available on the market, none have approached the level of detail retrieval, holographic imaging, and tonal purity delivered by the Ray Tubes Select KT88 from Apos. It stands in a class of its own. Highest recommendation. https://apos.audio/products/ray-tubes-kt88-select-vacuum-tube?_pos=4&_sid=6e37ebf16&_ss=r&variant=45395661160684

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