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PostPosted: 06 Oct 2011, 20:00 
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Joined: 07 Sep 2011, 20:55
Posts: 24
Location: Longmont, Colorado
poty wrote:
allenb wrote:
If I may ask, what's your impression of your ForeWatt's sound after more listening time?

At the time I'm using the same in principle (SRPP), but different in some countings tube preamp. Despite my initial feelings about "abusing" :) the layout initially designed to have rather opposite outcome (current vs voltage) I like it more and more. I'm going to build an integrated full (stereo) amplifier combined the ForeWatt and OddBlock principles, taking good from synergetic effect and using some convenients like remote control, control bus, automatic balancing, safety circuits...


Well, if you're going to be integrating a forewatt and oddblock you must be very pleased with the sound.

Allen


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PostPosted: 09 Oct 2011, 12:00 
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Joined: 09 Oct 2011, 03:07
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Hi,

I would like some figures for this circuit before I build. How much current does the 12AU7 draw? Also can anyone know the voltage on the grids?

Best wishes
David


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PostPosted: 10 Oct 2011, 06:31 
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Joined: 24 Oct 2010, 07:05
Posts: 312
First of all - the curcuit uses ECC802S, not 12AU7 (I know several circuits which use the 12AU7 in the SRPP though).
Considering the initial values of parts each channel draws around 4,3mA, so the voltage on the lower tube's grid is 2V; on the top tube's grid is also 2V to cathode, but the cathode itself is ideally half the 215V or 107.5V to the ground.
I can't imagine how the values will help you build the preamp?


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PostPosted: 10 Oct 2011, 08:05 
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Joined: 09 Oct 2011, 03:07
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Hi,


Thank you for the information. You are right it it will not help in building the project but it is nice to measure the values after it is built. It give me confidence if the values I measure approximate to the calculated values. I am actually planning to use a 6189 tube, supposedly a milltary version of the 12 AU7. I have a number of these lying around.

Thanks again.

David


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PostPosted: 11 Oct 2011, 21:19 
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Joined: 04 Jun 2008, 20:59
Posts: 4310
Location: Arizona, USA
Hi, Yes I do specify the JJ ECC802s as it is quiet and seems to have the best sound quality (please don't bash me for that as everyone has a favorite sound). The 12AU7 can be used, but I don't recommend any. I have tried several and I didn't like the sound. As for voltages etc. The tubes depending on brand will draw about 5 ma for each channel. The voltage at the mind point ought to be half the B+ applied to the top anode. In practice is will be different a bit. With matched sections in the same tube it can be close. I find the JJ tubes to be fairly good at matching even though it is not requested. For best channel balance you should however ask for matched tubes. The actual voltage at the midpoint can be off by as much as 10-20% and not make a significant difference. Much more (positive) can put the cathode to heater voltage above the really safe mark. I realize that often it is listed as up to 200 volts. Fine, I have had them (several) brands arc internally at less than that. Use the value of 100 and you will always be safe. Grid voltage on the lower tube is essentially zero. On the upper tube it will be about 1.5 volts different from the cathode. The cathode voltage on the lower tube will be in the range of 1.5 to 2.2 volts depending on brand. BTW a good sub for the ECC802S is the JJ gold pin ECC82S.

Good listening
Bruce

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PostPosted: 12 Oct 2011, 02:18 
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Joined: 24 Oct 2010, 07:05
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gofar99 wrote:
The tubes ... will draw about 5 ma for each channel. ...
Grid voltage on the lower tube is essentially zero. On the upper tube it will be about 1.5 volts different from the cathode. The cathode voltage on the lower tube will be in the range of 1.5 to 2.2 volts depending on brand.

While I don't have the actual device on hands it seems there is a mistake. As far as the tubes are connected in serial the current through them must be equal. Having equal values cathode resistors makes the grid-to-cathode voltage equal too (well, in the range of 1% deviations from the precision of the resistors' values). Knowing the current and the voltage:
5mA * 470 Ohm = 2.35V +/- 0.02V
not the 1.5V you have mentioned in any way.
My calculations of the currents and the voltages was based on the marked values on the schematic and (as soon as the 270V is the approximate value) should be used with some precision in view.


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PostPosted: 12 Oct 2011, 12:43 
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Location: Arizona, USA
Hi, You are correct that the current drop should be the same (thus the voltages across the resistors). :thumbsup: I was writing faster than my brain was thinking. :shock: The range though does depend on the tube brand and type. It took me a while to realize that all 12AU7s (really any kind of tube) are not on spec nor on the same set of specs and that similar tubes like the ECC82 and ECC802S were also not the same. It is one of those areas that is often overlooked. This makes tube gear fun. It is also why I specify tube type and brands in projects where the actual performance is important. When you see that in any of the projects I cook up, it almost always means that actual test data was different from what the formulas and sims indicated.

Good Listening
Bruce

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PostPosted: 15 Oct 2011, 07:00 
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Hi,

I never ceased to be amazed by the HiFi and guitar amp communities (including my myself) who build amplifiers with vaccum tubes. They are terrible electronic devices. The specs published have such a wide variation from tube to tube, whatever the brand or suffix is applied to the name, as to be almost meaningless by modern standards. Electrolytic capacitors are similar, often with +20/-80 designations for capacitance. It is simply the nature of the beast. Not only that but tubes and electrolytic capicitors change their specs in the circuit with time. Transconductance falls or rises. Capcitance falls or rises. Film capacitors can be measured when soldered into place, five years later they measure exactly the same. If the tolerance is quoted as 5% it is 5% and remains so. Electrolytic capacitors cannot hope to acheive this, whatever their brand. Oscon capcitors may give some hope if they ever become available in a high voltage and high capactance package.

Don't get me wrong I like tubes but we have to realise that they are very poor electronic devices and have never been high precision technology. My rant is now over, almost! Why do people still build and design projects with Vacuum tube rectifiers?

Best wishes
David


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PostPosted: 15 Oct 2011, 11:39 
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Joined: 24 Oct 2010, 07:05
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David,
Answer to your only asked question is simple - because they (tube rectifiers) much more softer in work which produces narrower and less in amplitude parasitic spectrum. The fact eases filtering significantly. The vacuum tubes also don't have the reverse current, which in high voltages could add to the rectifier noise.
The rest of your thoughts should be treated accordingly for other active circuits too - there is no ideal active part.


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PostPosted: 15 Oct 2011, 12:03 
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Location: US Pacific Northwest
I apologize to everyone for the off topic reply but I feel compelled.

David;

Frankly I find your assertion that vacuum tubes are "terrible electronic devices" way off the mark. With 30 years of design and build experience under my belt (using everything from old tubes to the most modern Si and GaAs integrated circuits) my experiences have taught me otherwise. Tubes are elegant in their simplicity, stable over long periods, and tube to tube variation (normalized for age) is well with +-10% in almost all instances (in my experience). Additionally the negative side electrolytic capacitor variability problems of old have been virtually eliminated with modern assembly techniques. Almost every main line manufacturer now quotes their electrolytic values at +/- 10% or better. And most guarantee it. Further, variation within lots is typically half this value. Considering that we built all vacuum tube radar systems with lifetime overall performance variability of less than 5% and with longterm phase stability of less than -50dB, I would hardly call these "terrible"! And finally, one of the biggest advantages of tubes is that they experience what in industry is referred to as "graceful performance degradation". In solid state electronics you have to greatly complicate your designs to get this characteristic, with tubes you get it largely for free.

As for vacuum tube rectifiers, every one of my high voltage designs, when built by me, has been within 5% of my design point. Further the noise generated by vacuum tube rectifiers is far lower than that generated by solid state rectifiers. If you're having trouble with your power supply design process, perhaps we can help. ;)

Now my rant is over. I suggest if anyone wants to discuss this further, we take it to the "DIY Electronics and Components / Parts" forum.

Thank you.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog


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