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#1
Posted to rec.audio.tubes,uk.rec.audio
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6AU6 triode mode noise results
I have now made noise measurements on 28 6AU6 tubes, all connected with
plate and suppressor grounded and using the screen grid as anode. The cathode bias resistor is fully bypassed. Initial tests showed that noise was masked by hum from the heaters so for these tests 4xAA batteries were used for the heater supply. Noise measurements were made with the grid shorted to ground.The output noise in some tubes was so low it was difficult to measure accurately. In general the peak to peak measurement of the noise waveform on a scope connected to the output of a Ac millivoltmeter (20KHz bandwidth) was divided by 8 to get the rms value. The results we 1. Set of 5 Brimar EF94, and set of 4 Novosibirsk 6HN6 and 4 GE 6AU6W all measured 50uV rms noise 2. An RCA 6AU6A, 3 of Sylvania 6AU6. 1 Tungsolo 6AU6 and 1 Philco 6AU6 all measured 75uV rms noise. 3. 2 RCA 6AU6A measured 125uV rms noise. 4. GE 6AU6 measured 185uV rms noise. 5. 3 RCA 6AU6A, 1 Sylvania 6AU6, 1 Tungsol 6AU6 and 2 Motorola 6AU6 all measured 250uV rms noise. Conclusions. I measured the stage gain of one of the EF94 samples. The result was 25. This implies the best tubes are capable of an EIN of 2uV; not earth shattering but good enough for a mic pre with a 10:1 transformer at the input with a 150 ohm source. Nearly half the tubes tested achieved this noise level which is encouraging. Ian |
#3
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6AU6 triode mode noise results
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#4
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6AU6 triode mode noise results
Don Pearce wrote:
On Mon, 09 Jul 2007 22:46:22 +0100, wrote: I have now made noise measurements on 28 6AU6 tubes, all connected with plate and suppressor grounded and using the screen grid as anode. The cathode bias resistor is fully bypassed. Initial tests showed that noise was masked by hum from the heaters so for these tests 4xAA batteries were used for the heater supply. Noise measurements were made with the grid shorted to ground.The output noise in some tubes was so low it was difficult to measure accurately. In general the peak to peak measurement of the noise waveform on a scope connected to the output of a Ac millivoltmeter (20KHz bandwidth) was divided by 8 to get the rms value. The results we 1. Set of 5 Brimar EF94, and set of 4 Novosibirsk 6HN6 and 4 GE 6AU6W all measured 50uV rms noise 2. An RCA 6AU6A, 3 of Sylvania 6AU6. 1 Tungsolo 6AU6 and 1 Philco 6AU6 all measured 75uV rms noise. 3. 2 RCA 6AU6A measured 125uV rms noise. 4. GE 6AU6 measured 185uV rms noise. 5. 3 RCA 6AU6A, 1 Sylvania 6AU6, 1 Tungsol 6AU6 and 2 Motorola 6AU6 all measured 250uV rms noise. Conclusions. I measured the stage gain of one of the EF94 samples. The result was 25. This implies the best tubes are capable of an EIN of 2uV; not earth shattering but good enough for a mic pre with a 10:1 transformer at the input with a 150 ohm source. Nearly half the tubes tested achieved this noise level which is encouraging. Ian Is 150 ohms the optimum source through a 10:1 transformer? It is the typical source resistance of a microphone. Have you measured the noise level with a 15k grid resistor rather than a short? No, but the calculated equivalent input noise resistance is about 12K so with 15K across the grid I would expect about a 3dB increase in output noise. Ian d |
#5
Posted to rec.audio.tubes
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6AU6 triode mode noise results
Patrick Turner wrote:
wrote: I have now made noise measurements on 28 6AU6 tubes, all connected with plate and suppressor grounded and using the screen grid as anode. The cathode bias resistor is fully bypassed. Initial tests showed that noise was masked by hum from the heaters so for these tests 4xAA batteries were used for the heater supply. Noise measurements were made with the grid shorted to ground.The output noise in some tubes was so low it was difficult to measure accurately. In general the peak to peak measurement of the noise waveform on a scope connected to the output of a Ac millivoltmeter (20KHz bandwidth) was divided by 8 to get the rms value. The results we 1. Set of 5 Brimar EF94, and set of 4 Novosibirsk 6HN6 and 4 GE 6AU6W all measured 50uV rms noise 2. An RCA 6AU6A, 3 of Sylvania 6AU6. 1 Tungsolo 6AU6 and 1 Philco 6AU6 all measured 75uV rms noise. 3. 2 RCA 6AU6A measured 125uV rms noise. 4. GE 6AU6 measured 185uV rms noise. 5. 3 RCA 6AU6A, 1 Sylvania 6AU6, 1 Tungsol 6AU6 and 2 Motorola 6AU6 all measured 250uV rms noise. Conclusions. I measured the stage gain of one of the EF94 samples. The result was 25. This implies the best tubes are capable of an EIN of 2uV; not earth shattering but good enough for a mic pre with a 10:1 transformer at the input with a 150 ohm source. Nearly half the tubes tested achieved this noise level which is encouraging. So compared to 12AT7, 12AX7, 6DJ8, 6CG7, 12AU7 or most other real triodes the noise figures you have are about average. That is good news. I did not expect them to be better but it is good to know they are not really worse and they are a hell of a lot cheaper than any of the tubes you listed. 2uV isn't bad for MM or MC with a step up tranny, or a microphone with step up tranny, but with MM without a tranny and if the rated output was 2mV, then 2uV is only -60dB SNR, and barely good enough. Indeed, a tranny seems to be essential. Ian |
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