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Old 9th September 2018, 08:25 PM   #431
merlin el mago is offline merlin el mago
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Congrats Ale enjoy your work.
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Old 9th September 2018, 10:44 PM   #432
Hanze Khronye is offline Hanze Khronye  Australia
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Originally Posted by mogliaa View Post
One regulator can work. You can add a balance pot between filaments and SiC array for bias adjustment. That will compensate mismatch of valves and filaments

I just can't see how the pot will work with one regulator and a common CT filament supplying both triodes within the envelope.

Last edited by Hanze Khronye; 9th September 2018 at 10:56 PM.
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Old 10th September 2018, 02:28 AM   #433
jhstewart9 is offline jhstewart9  Canada
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Originally Posted by mogliaa View Post
How many volts do you need them to swing? Mind you the 3a5 is an excellent valve for low level signal and not to swing many volts. I measured about 0.2% for 10Vrms on a single ended driver test. It may sound really good though.

Not obviuos where the 10 mV was measured, at the input or the output? How much gain did the stage have? How did you measure D%?


Just curious. These preamps seem very different than what I recall, those required to amplify & properly shape the response curve of a magnetic pickup on vinyl. For that 12AX7 was the dominant choice. 12AY7 is purpose built for lower noise. EF86/Z729 would also work. And for very low noise, cascode was used, often done with high Gm RF triodes such as the 6BQ7/6BK7 family.with DC on the heaters.


This thread seems to be describing something for a very different requirement.
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Old 10th September 2018, 03:04 AM   #434
jhstewart9 is offline jhstewart9  Canada
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Originally Posted by andyjevans View Post
It all just gets more and more interesting!

I've been lazy and have just been listening to my 26 stage with SIC diode bias (not filament bias) for a few weeks. It sounded good right from the start. I keep meaning to change some of the diodes for a resistor, and then do the same in filament bias. I'll get round to it in the end!

I also want to try a 3a5 with SIC diode bias as a first stage for a PP amp. I had an idea something like this.

The front end of a PP amp if it were differential wud normally have a common impedance at the point where the cathodes are brought together. That is how balance & common mode rejection is obtained. THe common mode tolerance of the amplifier is also determined by what the common mode impedance is connected to the bottom end, common or a negative supply rail.



Diff Amps are very common in test equipment. An obvious application is the Differential Probe, a very useful tool when working with circuits where a common ground may not be a convenient measurement point. I use mine a lot on the front end of a virtual scope used in audio design. This particular model can withstand 750 volts offset while set to the X1000 mode.



The common impedance we find is normally a resistance or a bipolar transistor. That puts the cathode off ground for both AC & DC currents. Not a good place to be for a small DHT triode such as the 3A5.


But could be done, interference from the heater supply is a danger. A floating battery could be used to avoid that, but needs to be shielded.


Looks like could create more problems than it solves. Have you breadboarded a trial yet? Lots of factors to consider on the way to success!
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Old 10th September 2018, 11:07 AM   #435
Rod Coleman is offline Rod Coleman  United Kingdom
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Quote:
Originally Posted by mogliaa View Post
One regulator can work. You can add a balance pot between filaments and SiC array for bias adjustment. That will compensate mismatch of valves and filaments
Hi Ale!

Yes, you can use one regulator, and the heating and bias can be made to work.

But please consider the anode current in each filament. For push-pull, each filament will carry music-signal current flowing in anti-phase to the other triode.

When we share a heating supply the two filaments are connected together. Now, these anti-phase music-signals will be in conflict, and this collision is not helpful. The effect on the sound is similar to using a regulated-voltage to heat a DHT: The signal-voltage across the filament is crushed by the regulator's output capacitor & the low-impedance forced by the feedback of the regulator's closed-loop.

The most noticeable effect is on the stereo imaging. I am still trying to figure a way to measure this (suggestions welcome!) but it is very easy to hear it. I've been working with advanced loudspeaker cabinet designs with mitigation against diffraction effects (see Lynn Olson's pages) Running these with DHT amplification allows a stereophonic illusion that can expand well beyond the size of the listening room - and here the effect is most noticeable.
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Old 10th September 2018, 05:18 PM   #436
indra1 is offline indra1  Indonesia
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... I am still trying to figure a way to measure this (suggestions welcome!) but it is very easy to hear it. ...
What are the measurable quantities that you have found relevant? I have found very few published observation on correlation of spatial impression to measurement. However you may want to also include Harmonic Spectra distribution (using FFT software) and Phase of distortion residual (using twin T notch or DiAna) relative to fundamental into your consideration.


One worth mentioning is from the work of Nelson Pass stating that :
There was a consistent subjective observation that there was a difference not only with the level of second harmonic, but phase also. Negative-phase second harmonic tends to expand the perception of front-to-back space in the soundstage, separating instruments a bit. Positive phase does the opposite, putting things subjectively closer and "in your face." as reported on page 2 of Stereophile "Nelson Pass: Circuit Topology and the End of Science" interview. One of the image that describe the term "negative phase second harmonic" is

Click the image to open in full size.

as shown from First Watt F7 review thread.


Another I found closely related is an observation by Eduardo de Lima from page 9 of Whysingle-endedtubeamplifiers.pdf :
In this case the speaker alone has around 2% of distortion between 100 Hz and 1000 Hz with 2.83 Vrms in the input (that corresponds to 1W in an 8 ohm load). We can see this in figure 1. In figure 2 we connect it to a SE amplifier with an average 0.8% 2nd harmonic distortion at this output level. It is easy to see the distortion addition between 150 Hz and 300 Hz and subtraction around 900Hz. We could say that, as an average, the distortion has increased. Figure 3 shows the same set up with the other polarity. We can see that the whole 150 Hz till 300 Hz region has reduced distortion and that around 900 Hz we have an increase in it. The overall effect seems to be a reasonable decrease in distortion.
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Old 10th September 2018, 06:49 PM   #437
Rod Coleman is offline Rod Coleman  United Kingdom
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Hi Indra,

Thank you for looking those discussions out! I do expect there to be a phase-measurement component, since phase has the most obvious effect on the sense of space-perception.

I will enjoy chasing some of those articles and tools down. The effect I observe is not exactly a front-to-back image corruption; but I suspect the same kind of approach to measurement will be needed; not just reading off the THD.
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Old 10th September 2018, 08:14 PM   #438
indra1 is offline indra1  Indonesia
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... I suspect the same kind of approach to measurement will be needed; not just reading off the THD.
My impression is that for amplifiers, loss of spatial definition happens on all 3D axis, not constrained to a particular axis. However, I can not share much for non extensive experience and measuring gadgets available to me. THD is a lump sum, rather useful when measured by a microphone at listening position, quite uninformative for spatial impression correlation when measured as the output voltage of an amplifier. A harmonic spectrum distribution usually tell a bit more.
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