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Old 13th September 2012, 06:11 PM   #21
Charm is offline Charm  Russian Federation
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Klausmobile Tube Tester Files

Headphone amplifier SRPP
1, 2, 3, 4, 5, 6

Last edited by Charm; 13th September 2012 at 06:23 PM.
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Old 13th September 2012, 07:44 PM   #22
disco is offline disco  Netherlands
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Thank you, that is a good read.

"Total distortion less than 0.003%, noise of the soundcard is -130 dB, so
the spectrogram is not visible. Peak value of the signal at the output of the
soundcard is 1 V.
"
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Old 13th September 2012, 09:30 PM   #23
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Hi Disco!
Your cathode bypass cap on the output stage is way yoo small. You have to calculate it from the actual resistance at the cathode. That would be something like 750 ohm in parallel with ri/mu. I would use 470 or 1000 uF. The value you have used spoils the dampingfactor at low frequences. That could explain some of the loudness and boominess. Also try to put a resistor on top of the driver instead of SRPP. That might give you a little more swing.
Thorsten
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Old 14th September 2012, 05:41 PM   #24
disco is offline disco  Netherlands
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Thorsten, thanks for the input. I followed your suggestion but the peculiar reproduction is not caused by the bypass cap. You are right indeed:

Click the image to open in full size.Click the image to open in full size.

To obtain full gain, good amplitude response and to minimize the effects on phase and transient response, the feedback voltage over the cathode must be shunted by a bypass capacitor of low reactance up to 1 Hz. This cap forms a LPF with the output resistance at the cathode, which consists of Rk (the bias resistor) parallel to rk (the cathode resistance).

I haven't actually measured the relationship between Ug and Ua but from the many graphics it would come down to a mu of 2.5 or something close for my operating point. So, filling in the formulae rk = 1200+600/3.5 = 514 ohm, parallel to 750 ohm results in 305 ohm. Ck = 1/6.28x305 = 522 uF.
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Old 21st October 2012, 04:17 PM   #25
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Quote:
Originally Posted by disco View Post
Thank you, that is a good read.
Thank You.
Admin MusicAngel.
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Old 21st October 2012, 04:42 PM   #26
rongon is offline rongon  United States
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Quote:
Originally Posted by mogliaa View Post
To my surprise as I was looking to use this valve in an OTL headphone amp, found it was not very linear:

THD benchmark | Bartola Valves

Close to the top chart of the worst ones am afraid, however this is just a THD test...
Cheers,
Ale
That Bartola blog doesn't mention how the tubes were tested. It could be that the 6N6P was measured with a low plate current, since it really does like to see at least 15mA per triode. With 6N6P, I think distortion would be higher with 10mA or less.

There seems to be all sorts of contradictory data on 6N6P distortion characteristics. Some say it has high THD, many others say low.

?
--
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Old 21st October 2012, 05:08 PM   #27
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by rongon View Post
That Bartola blog doesn't mention how the tubes were tested. It could be that the 6N6P was measured with a low plate current, since it really does like to see at least 15mA per triode. With 6N6P, I think distortion would be higher with 10mA or less.

There seems to be all sorts of contradictory data on 6N6P distortion characteristics. Some say it has high THD, many others say low.

?
--
Hey Rongon,
Here are my tests:

Valve Ia [ma] Va[V] Vg [V] THD
6N6P 27.5 271 -10 0.11%
6N6P-I 13 305 -15 0.13%

6N6P test point was well over Pa so should retake tests....
Enjoy
Ale
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Old 21st October 2012, 05:16 PM   #28
rongon is offline rongon  United States
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Quote:
Originally Posted by mogliaa View Post
Hey Rongon,
Here are my tests:

Valve Ia [ma] Va[V] Vg [V] THD
6N6P 27.5 271 -10 0.11%
6N6P-I 13 305 -15 0.13%

6N6P test point was well over Pa so should retake tests....
Enjoy
Ale
Do you remember how you loaded the tube for testing? Plate load (resistor, transformer, active?), cathode load, cathode resistor with bypass cap, not bypassed, etc? Just curious...

I usually use 6N6P at more like Va = 150V, Ia = 15mA, Vg = -5V. I wonder if that would change the distortion measurements by much...

Thanks!
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Old 21st October 2012, 05:19 PM   #29
mogliaa is offline mogliaa  United Kingdom
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all tests were done with cascoded DN2540 CCS load at 22dBu to reflect tube distortion. Obviously test wasn't done on the best operating point so probably not a good reference here for you
ale
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Old 21st October 2012, 05:54 PM   #30
rongon is offline rongon  United States
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Still, a good data point. Thanks again.
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