More Insane Numbers

[kt88pppamp]

I just completed the second block of my PPP rev 2 amplifiers.





I am measuring at 1k 0.15% THD at 140W RMS power. The other block is measuring about 0.25%. Changing around tubes influences these numbers I am finding. The 12AX7s are junkers that test good I pulled from a trashed oscilloscope. The KT90 tubes are EH as well as the 12BH7 driver.

Has anyone else heard of this company? They with balanced technology claim similar numbers with their 65 watt balanced units.

http://www.sacthailand.com/AmpNewGlowMasterKT88.html

It was these people that actually inspired me to try for the low THD numbers.

[Geek]

I smell WAY too much NFB for my liking to get those numbers

Aside that, how's it sound?

[kt88pppamp]

The feedback is 14-15dB.

Open loop it is about 0.7%ish at full power.

Making the feedback 9 or 10 dB brings the THD up to 0.3%ish, but the sensitivity is too high with that low amount.

The amps sound very clean and punchy but they do not seem much louder than the MKIII's, other than the bass hits much harder.

[kt88pppamp]

I am having a small issue.

One of the first block's tubes are arcing. I cannot tell which one it is. I tried upping the screen stoppers from 100 ohms to 820 but it still seems to be doing it. I first found out because intermittently heard tears and pops in my left speaker. I have a feeling I have a dud KT90. I still can get it replaced cause its under warranty. Any way of telling which tube is the offender?

Also, is 550V too high for the plate/screen in UL mode? The datasheet says I'm well within the ratings. Do those things just tend to fail at higher voltages?

[Geek]

The feedback is 14-15dB.

Open loop it is about 0.7%ish at full power.

Making the feedback 9 or 10 dB brings the THD up to 0.3%ish, but the sensitivity is too high with that low amount.

The amps sound very clean and punchy but they do not seem much louder than the MKIII's, other than the bass hits much harder.

Rockin'!

Arcing tube = dying tube. Replce the pair.

Cheers!

[kt88pppamp]

It turns out the KT90 tubes are fine (after trying a different quad).

It was one of the stupid driver tubes that caused the loud tearing/popping sound . I observed the effect on the scope and after a pop, the signal loaded down for a second or two than recovered. As time went on, the event repeated more frequently. Did anyone have a similar experience? What is going on in the preamp tube that makes it do that?

To determine whether it was the 12AX7 or 12BH7, I will try each in a different amp.

That book in the photo belonged one of my electronics professors. It was his when he was an undergraduate from the 1959-1960 school year. He graduated in 1963 (I think). What do you think of the book?

[Cubdriver]

That book in the photo belonged one of my electronics professors. It was his when he was an undergraduate from the 1959-1960 school year. He graduated in 1963 (I think). What do you think of the book?

I rather like it - snagged a copy of it from a used book store down in Florida a few months back while on a trip. Have been slowly meandering through it in my spare (so called because I have so little of it) time.

-Pat

[kt88pppamp]

Textbooks from that vintage remind me of how dumbed down education has become. Even someone like me, who is an intellectual that strongly values education, has trouble understanding the way it is written. I am almost 30 and that book was written for an audience of 18 or 19 year old engineers in training. What an 18 or 19 year old could understand in the 1950s seems like light years from what they can now.

I have a feeling a large reason for that was that fewer groups of persons had access to education and many chose not to attend college at all. With a narrower audience of students, textbook publishers assumed better college preparedness.

Modern higher education emphasizes exams, quizzes, testing, and "cookbook style" labs. Today's realities demand an flexible education model that revolves around critical thinking. With that, our very diverse group of students will become better motivated because they actually will feel challenged. Being trained to "think outside the box" will also sustain America's innovative spirit and culture, a critical trait that will allow us to compete globally. It also made America great in the first place.

For those hopeless cases that choose to have fun in college rather than learn, simply boot them out. If they party and binge drink, give them lengthy jail terms or work programs coupled with intense counseling/accountability training. ADHD is no excuse! It is overdiagnosed because of TV, junk food, and the permissive parenting epidemic.