2. amps-preamps
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Tube amp question

I am a novice tube amp owner. I have had my Black Ice Audio Fusion F22 for not quite a year. I want to understand plate voltage and how it impacts tubes or vice versa. I want to roll the tubes to the KT150's which Black Ice has said can be used without issue. The plate voltage of the amp is 500v. In other posts, someone said that KT150's require a minimum of 600v at the plate. Since Black Ice says their amp can use them, does this rating depend on the circuit? I have some basic understanding of electronic circuits and I googled plate voltage but I'm still not clear on what it is exactly. Can someone knowledgeable with tube amps help me understand this? Thanks!

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Showing 1 response by gs5556

gs5556
1,218 posts
 

If you use the KT150, I would bias it to 80ma. Based on the amp power specs for the EL34 and 6550, the bias settings of the tubes and 500V at the plate, the amp operates in Class A/B and the output transformer is about 6500 ohms in Ultralinear. The manual also shows the ammeter's upper limit as 80ma.

The EL34 operates at about 5 watts Class A, and A/B to its rated power. The 6550 runs about 12 watts Class A. 

The KT150 at 80 ma with a 6500 ohm xfmr at 500V will operate in Class A for 20 watts and max out in Class A/B at about 75 watts. The bias point is a little less than 60% of the tube dissipation which is on the cool side but within the sweet spot envelope for Class A/B. The tube should work fine. But... save your money. The KT120 operates identically biased at 80ma and runs 70% dissipation, which is the upper limit for Class A/B operation.

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