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Old 9th January 2013, 12:43 PM   #31
TheGimp is offline TheGimp  United States
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By using a Twin T filter I am no longer overloading the front end of the sound card, and I see a drop in 2nd harmonic of about 10dB. The rest of the harmonics are about the same.

I expect this is representative of the tube performance.

I measured the response of the twin T filter with a 8.24V p-p signal at various frequencies, and used the values to make a correction table (1).

The mean offset (excluding values close to the notch) is 2.44dB so I add that as a correction factor to measurements I make to get the true value of the harmonics(2).

I think I can improve the noise level slightly when I make a housing for the Low Distortion Oscillator and Twin T filter as currently everything is spread out on the bench.

Here is a typical 6JC6A (NOS RCA).
Attached Images
File Type: jpg Corrected THD Spreadsheet.JPG (46.7 KB, 219 views)
File Type: jpg 6JC6A-No5_Thd.JPG (60.2 KB, 211 views)
File Type: jpg 6JC6A_No-5.JPG (108.0 KB, 207 views)
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Old 9th January 2013, 04:55 PM   #32
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At 0.5vrms out 22ma 186 v with a ccs i got 0.018% from a 6j9p or 0.066% swinging 187vp-p, admitedly at 28ma/208v (5.8w)

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Old 9th January 2013, 05:52 PM   #33
TheGimp is offline TheGimp  United States
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The 6JC6A was tested at 200V, 3mA 2.8Vrms (8.24Vp-p) using a gyrator to hold the anode voltage. Current was adjusted by a potentiometer in the cathode (fully bypassed).

I'll try to test the 6J9P tonight if I get time.
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Old 10th January 2013, 12:48 PM   #34
TheGimp is offline TheGimp  United States
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I tried 8 6J9P-E last night. These were from two suppliers, but all the same manufacturer varying from 1977 to 1987. All were in the same blue/yellow/black marked boxes with data sheets wrapped around the tubes.

All 6J9P were triode strapped with Gyrator set to 220V at the anode.

Eight were tried at 3mA, with an average 2nd harmonic of -83.2dB, worst -77dB, best -93dB. All show the same trend in harmonics. The best is shown as FFT 1.

At 1mA both 2nd Harmonic distortion and noise floor were worse (2), but harmonics above 2nd were better.

At 5mA the noise floor improved, but higher harmonics are increasing (3).

At 7.5mA, the trend continues, but second harmonic is starting to decrease. This is probably a tipping point. (4)

At 10mA, third harmonic exceeds the level of second harmonic at 3mA. (5)

Back to 3mA and 8Vrms out. The signal is starting to overload the op amp input to the twin-T filter. Above 8Vrms harmonics look like clipping. (6)

Finally I tested a 6CJ6 at 3ma for comparison. (7) It is interesting in that second harmonic is much greater than the 6J9P, but beyond that harmonics are much lower. This is probably why the 6CJ6 is preferred even though the overall thd measurement is higher.

At low current (1mA) the 6J9P looks similar, but has a higher noise floor. This is where I've been running 6J9Ps in the past. A good compromise that is worth listening to might be between 1 and 3mA.
Attached Images
File Type: jpg 6J9P_3mA.JPG (151.4 KB, 185 views)
File Type: jpg 6J9P_1mA.JPG (154.0 KB, 175 views)
File Type: jpg 6J9P_5mA.JPG (153.6 KB, 16 views)
File Type: jpg 6J9P_7-5mA.JPG (152.2 KB, 15 views)
File Type: jpg 6J9P_10mA.JPG (158.4 KB, 15 views)
File Type: jpg 6J9P_3mA_8vrms.JPG (153.5 KB, 11 views)
File Type: jpg 6CJ6_3mA.JPG (152.8 KB, 16 views)
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Old 12th January 2013, 03:49 PM   #35
TheGimp is offline TheGimp  United States
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I'm putting the two LD Oscillators and a the Twin T filter in a chassis to help shield them and clean up the wiring.

In order to isolate the boards from the magnetic field of the transformers I've oriented them as as shown in the attached photos.

It occurred to me that since the two transformers are nearly identical (same part number from the same mfg), it might be possible to cancel some of the B field by wiring the primaries anti-phase. If so, which transformer orientation will help reduce the field the most?
Attached Images
File Type: jpg TwinT_Layout_a.JPG (151.7 KB, 48 views)
File Type: jpg TwinT_Layout_b.JPG (159.2 KB, 60 views)
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Old 14th January 2013, 02:26 PM   #36
TheGimp is offline TheGimp  United States
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The block diagram shows the test set I’ve build. It has the two Low Distortion Oscillators selected by a three pole double throw switch (have to switch both power connections plus the output), followed by an adjustable attenuator and a buffer. The buffered output drives the UUT.

The output of the UUD goes through a buffered step attenuator with selections for 0dB, -10dB, and -30dB.

This drives the Twin T filter, which is followed by a divide by two and buffered by a differential output buffer. The divide by two compensates for the differential driver to the sound card.

FFT 1 is a 6J9P at 2.5mA and 220Va driven to 2.8Vrms out (8.12Vp-p). From it we can see that the measured fundamental is at -0.32dBfs ref to 2.8Vrms. So I have a measurement error of 0.32dB that will need to be added to the measured value.

Does this also get added to all harmonics as a correction factor?

Comparing FFT 1 and FFT 2 we see about a 12dB difference in second harmonic and about a 10dB difference in subsequent harmonics. This is attributed to overloading the input of the sound card, and is one of the main reasons for building the test-set. The greater difference in second and subsequent harmonics is in part due to the Twin –T filter which was measured previously at -0.26dB.

FFT3 is the same 2.8Vrms but with a -10dB attenuator switched in.

FFT4 is the same 2.8Vrms out with the -30dB attenuator switched in, and bias point adjusted to 180V.

To calculate distortion of FFT5 (45Vrms) it is necessary to measure all harmonics and adjust them for the attenuator (-30dB), test-set and Sound-Card errors (-0.26dB).

Taking the first five harmonics I get -75, -96, -108, -120, -128dB
Correcting for the attenuator I get -75+31=-44, -96+30=-66, -108+30=-78, -120+30=-90, -128+30=-96.

The soundcard error of -0.26dB should be added, however I have not expanded the plots enough to get that much resolution so I ‘m leaving it off for now but to nit-pick it should be added in.

The fundamental is calculated by adding the gain over 2.8Vrms in dB. 45/2.8=16.07, converting to dB I get 24.12dB..

So now the harmonics need to be adjusted by 24.12 to get their relative value to the fundamental, giving -68.12, -90.12, -102.12, 114.12, -120.12

From this I get 0.039%thd.

Is this correct? It seems low to me from what I expected at 45Vrms from a triode strapped 6J9P.
Attached Images
File Type: jpg Twin-T Block Diagram.JPG (19.3 KB, 45 views)
File Type: jpg 6J9P 2-8vpp.JPG (159.2 KB, 33 views)
File Type: jpg 6J9P Twin-T-0atten.JPG (159.2 KB, 16 views)
File Type: jpg 6J9P Twin-T-10dB.JPG (152.7 KB, 12 views)
File Type: jpg 6J9P Twin-T-30dB.JPG (155.5 KB, 8 views)
File Type: jpg 6J9P Twin-T-30dB_45Vrms.JPG (153.0 KB, 12 views)

Last edited by TheGimp; 14th January 2013 at 02:51 PM. Reason: Math error in calculating gain over 2.8vref
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