KT77 OPT impedance

dgta · 29th September 2011, 05:46 AM

Looking at the Genalex KT77 datasheet and for PP UL, it shows Va=500V with Rload =5.5k, then Va=600V with Rload=9k. That's quite a jump.

Question to those who have worked extensively with this tube: can I interpolate? Let's say I have a transformer with 7.2k primary. Can I feed it 550V and be where the tube works best?

artosalo · 29th September 2011, 08:03 AM

According to my experience in general, the UL connected output stage is not so critical to load resistance. Genalex data is obviously given at optimum point. With lower load resistance the maximum available output power (before clipping occurs) is increased and the distortion level slightly increased too. In case of higer load impedance the situation is opposite. Also the bias adjustment has effect the this situation.

It is also important to notice that the distortion values given at Genalex data are without global negative feedback. So even with a small ( some 10...12 dB) feedback the distortion values are very low.

In reality, the load impedance reflected to output tubes by the speaker load varies between, say 3k...25 k ohms.

So 7.2 k transformer is very suitable for KT77 with 550 V supply voltage.

29th September 2011, 05:46 AM	#1
dgta diyAudio Member Join Date: May 2011	KT77 OPT impedance Looking at the Genalex KT77 datasheet and for PP UL, it shows Va=500V with Rload =5.5k, then Va=600V with Rload=9k. That's quite a jump. Question to those who have worked extensively with this tube: can I interpolate? Let's say I have a transformer with 7.2k primary. Can I feed it 550V and be where the tube works best?

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29th September 2011, 08:03 AM	#2
artosalo diyAudio Member Join Date: Feb 2010	According to my experience in general, the UL connected output stage is not so critical to load resistance. Genalex data is obviously given at optimum point. With lower load resistance the maximum available output power (before clipping occurs) is increased and the distortion level slightly increased too. In case of higer load impedance the situation is opposite. Also the bias adjustment has effect the this situation. It is also important to notice that the distortion values given at Genalex data are without global negative feedback. So even with a small ( some 10...12 dB) feedback the distortion values are very low. In reality, the load impedance reflected to output tubes by the speaker load varies between, say 3k...25 k ohms. So 7.2 k transformer is very suitable for KT77 with 550 V supply voltage.

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