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KT120 vs KT88 in triode mode SE.

84.211.111.210
Posted on October 16, 2013 at 14:54:37
Balle Clorin
Audiophile

Posts: 781
Joined: July 30, 2003



I just wanted to share my experience after going from KT88 to KT120 in my triode connected single ended amp. I thought the KT120 soundes nice and combines well with my driver stage (notice the harmonic distortion cancellation??)

My simple PC measurment with REW confirms that the distortion is lower with KT120 ( some people have stated the opposite), in my case at least. YMMV. I can also turn volume higher befor it clips. Not 300B class, but nice sound anyway. Lifetime will be longer than for KT88 at 30w dissipation.

Beste regards
Balle

 

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"My simple PC measurment with REW confirms that the distortion is lower with KT120 " No it doesn't, posted on October 16, 2013 at 20:02:21
Chip647
Audiophile

Posts: 2806
Location: The South
Joined: December 24, 2012
 You found conditions where you varied the load to the KT120 that showed lower, the same, and higher distortion compared to only one operating condition of the KT88.

You can get more power out of a Single Ended tube ONLY if you increase the standing bias, otherwise you will get the same power for a given level of distortion for a given bias. Your "more KT120 power" is only with a ton of asymmetrical wave distortion at 30.8 watts plate dissipation. Also, you are cutting off the power measurement at 4 watts.

 

Well hre's more data. Consistent lower distortion at same watt,tap and load with KT120, posted on October 17, 2013 at 11:58:33
Balle Clorin
Audiophile

Posts: 781
Joined: July 30, 2003



Yes I am aware that the same operating point is not supposed to give any more power for "free". However in every measurment case the the distorion rises in FFT spectrum,oscilloscope view and VAC measured at the load when going from KT120 to KT88. ( small difference in Ia and gain may occur, but both KT120 KT88 operate at 80 mA+/- 1mA.)

The Power = 1/8*(Vmax-Vmin)*(Imax-Imin) for the tube operation. So I suspect that a change in the Ia vs Va-k curves also effefect product of the "Power Square". Less spacing to and different shape of the "Vg=-80" curve can give both higher Ia and less Va = less power, and at the same time increased distorion.

The curves show that the KT120 at every watt level has lower distortions given the same tap and same load. The curves are cut off at beginning visible clipping on the oscilliscope.

Red and blue curve is KT120 at 4 and 8 ohm tap, dashed lines for KT88 types are always above at higher distortion

As I said this is simple measuremt with simpe equipment, the Fluke and Tektronix and Audio Precision apparatus is missing. However the trends are consistent in the data.

Best regards
Baalle Clorin

 

Here is where graphs don't really tell the story., posted on October 17, 2013 at 19:53:42
Chip647
Audiophile

Posts: 2806
Location: The South
Joined: December 24, 2012
 You are splitting hairs. The difference in sound quality between 0.5% distortion and 1.0% distortion is not as large as it would appear in actual listening through speakers. Instead of a resistor, use and actual speaker and take a look at the same measurements, it will make you puke. ;-)

 

RE: Here is where graphs don't really tell the story., posted on October 18, 2013 at 08:58:06
Balle Clorin
Audiophile

Posts: 781
Joined: July 30, 2003
 Well I do not want to start a debate here but i did that too. And also measured distortion from the speakers. Stilll looks quite good. Anyway with 95 db speakers I oprate at about 0.5 watt output and distortion is negible , bot it is nice to know that I am not listening to excess distortion. The point was to show that in my amp KT120 is a better choice than KT88....Good to have technical support in addition to the ears

 

LT Spice model for KT120, posted on December 25, 2013 at 05:04:55
Balle Clorin
Audiophile

Posts: 781
Joined: July 30, 2003


















To whom it may concern... FRom automaticas Sofia tracer plots
* KT120So2k8 LTSpice model
* Modified Koren model (8 parameters): mean fit error 1.664mA
* Traced by erik 25/12/2013 using Curve Captor v0.9.1
* from sofia tracer
* kt120so2 LTSpice model
.subckt kt120so2k8 P G K
Bp P K I=(0.09616726061m)*uramp(V(P,K)*ln(1.0+(-0.1983471212)+exp((0.5067247971)+(0.5067247971)*((28.86336585)+(2.153515075m)*V(G,K))*V(G,K)/sqrt((-0.1596450898)**2+(V(P,K)-(-9.567928489))**2)))/(0.5067247971))**(1.391876027)
Cgk G K 10.2p ; 0.2p added
Cpk P K 9.3p ; 0.5p added
Cgp G P 29.5p ; 0.5p added
Rpk P K 1G ; to avoid floating nodes
d3 G K dx1
.model dx1 d(is=1n rs=2k cjo=1pf N=1.5 tt=1n)
.ends


I used this method
http://oleate.free.fr/___/pong/Doc/2%20Doc%20Pr%E9pa/9.%20Tipe/Doc/Spice/HOW%20TO%20BUILD%20YOUR%20OWN%20TRIODE%20SPICE%20MODEL.doc

with this tool
http://es.sourceforge.jp/projects/sfnet_curvecaptor/downloads/curvecaptor/0.9.1/curvecaptor-0.9.1-winxp.zip/


 

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