[High_Q]

?  What is a typical life of a tube?  I have 5670W tubes.

 

[nikongod]

The lifespan of a tube depends on many factors of useage. Generaly: several years of "a few hours a day" can be expexted from small signal tubes operated within their suggested limits.
 
http://www.head-fi.org/forum/thread/558758/hd800-distortion-at-low-impedance-setting
 
Does it distort when you use the amp the way the designer designed it to be used? Try the middle or high setting. 

 

[High_Q]

^The designer actually said to use HD650 at lowest impedance setting because of the damping, and it should have sufficient power to drive the the phones.  HD650 does sound more controlled at lower impedance setting.  That was for my HD650 though, I've notice HD800 distorts at low impedance setting.  I know the both phones have same impedance, and also the same sensitivity.   I have noticed that one of my IEM would distort(I use tube because it sounds best with it).  I just need to know the life span and how tube reacts when it is about to die.

 

[High_Q]

In Elekit’s opinion, the gain of TU-882R/AS is designed with perfection. 
Here is the remark about the impedance and the design of TU-882:

"Matching the impedance is important only to convey the power of the amplifier to the headset in most effective manner.

However, having the best sound quality is another story.
Matching the impedance could cause distortion and low damping. 
You need to see the balance of the impedance matching and good sound quality.
As the said headphone, Senn HD650, seems to convey the power very effectively, it no longer has to be connected to high impedance. 

Connect it to low impedance and get the best sound out of it. Low distortion and high damping.

As you know, the vacuum tube amplifier has very low damping, 5 or 10 at most, compared to a semiconductor amplifier having 100-200 damping. 

Connecting it to low impedance could lead to high damping and low distortion, a better sound quality, which must be something that all users want to achieve. "

 

[High_Q]

Maybe HD800 does not convey the power very effectively as stated in the bold.  I don't know what that means though.

 
Quote:

In Elekit’s opinion, the gain of TU-882R/AS is designed with perfection. 
Here is the remark about the impedance and the design of TU-882:

"Matching the impedance is important only to convey the power of the amplifier to the headset in most effective manner.

However, having the best sound quality is another story.
Matching the impedance could cause distortion and low damping. 
You need to see the balance of the impedance matching and good sound quality.
As the said headphone, Senn HD650, seems to convey the power very effectively, it no longer has to be connected to high impedance. 

Connect it to low impedance and get the best sound out of it. Low distortion and high damping.

As you know, the vacuum tube amplifier has very low damping, 5 or 10 at most, compared to a semiconductor amplifier having 100-200 damping. 

Connecting it to low impedance could lead to high damping and low distortion, a better sound quality, which must be something that all users want to achieve. "

 

 

[nikongod]

I'd be curious to see an actual measurement of the output impedance of the amp on the various load settings. The range given as output impedance looks more like reccomended load to me, which is a common misprint. Do you think its at all reasonable that the output impedance of the amp on any given setting varies by a factor of 5 or more? I certainly dont. 
 
Bust out the ACvoltmeter, a potentiometer, ohm meter, and a 1khz test tone and see what you got.
 
Thanks to the IEC Standard which calls for 120 ohm output impedance OR zero ohms at the MFR's choice for driving headphones what works best is actually anyone's guess and blanket statements cant really be made. Damping factor is not as important to single driver spealers/headphones as it is to multiple driver speakers/headphones. Feel free to ignore it in single driver applications. 
 
What you are hearing as detail may be the transformers ringing due to being so underloaded. Im not sure if Id hear it with HD650s, but the 800's could show it for what it is. How do the leading edges of a 10Khz square wave look on the various load settings with a 300ohm load?
 
Anyways, as tubes age transconductance decreases which results in less voltage swing. Using such a large stepdown on the transformer you need every volt you can get, and you are getting fewer. Use the middle tap, you will still have plenty of damping factor, less ringing, and stress the tubes less. If the low-impedance setting were the only one to use would the MFR have spent the extra money on the 2 extra transformer taps?
 
As an aside is there a schematic for the amp online? It looks like a 2-stage RC coupled common cathode - common cathode "series feed" SET amp, but maybe there is some cool I could steal.