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Questions about vacuum tube data sheets

  1. Jun 15, 2016 #1
    Here is a typical data sheet for a commonly used 6l6GC tube.

    Under typical characteristics (in this data sheet) a value of Ra and Ra-a are given.

    I assume Ra-a means Za-a and it could also be written as Zout. Is this a correct assumption?

    On a related issue, I assume the following. Output transformers for tube amps are of two types. One for single ended and one for push-pull.

    Does Zout change for amps with more than two output tubes in push-pull?


  2. jcsd
  3. Jun 15, 2016 #2


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    These are the assumed anode loads in the "typical" setup. If your setup has a different value, you cannot rely on the "typical" values.
  4. Jun 15, 2016 #3
    Hi Svein,

    To be clear my first question was only about symbols used in data sheets and their meaning.

    So..do the symbols Ra-a and Za-a and Zout have the same meaning? I assume they do.

    Back in the day when we actually spoke English none of this was an issue...lol as can be seen on this RCA data sheet of the same tube number. The easy to understand term load resistance was used and stated based on plate voltage. I guess it was also assumed that one knew this value represented the output impedance of the tube based on some standard frequency.


    Many people today use the RCA data sheets with the misguided idea that tubes of the same number and of current manufacturer have the same exact values and properties.


  5. Jun 15, 2016 #4


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    Just curious, since you probably do a lot more tube equipment restores than I. Have you found that new, current manufactured tubes caused a need to modify any part of their related circuits? Also (maybe too soon to tell), do you think the quality of these new tubes are better or worse than the old one?

    Luckily for me I have a fairly good stock of old tubes to draw upon.
  6. Jun 15, 2016 #5
    Hi Don,
    I have a very good supplier of tubes who can supply both new tubes and new old stock. I have been buying from Eddie at Eurotubes for a while now and I no longer have issues with tubes. They are hooked up with the JJ tube factory and I use JJ tubes where ever I can. I rarely have occasion to turn on my TV-10 Navy tube tester any longer.

    Of the dozen or so tube manufactures left in the world my experience has been they all make good and bad tubes. In most cases it is less about brand than how well the tubes are tested.

    I don't think the new tubes are made as well as the old RCA tubes for example. The last couple of years that RCA made tubes were not as good as years prior to the end of the production. There was no reason to maintain the plant and the staff at the highest level if you plan to shut it down.

    Everything cost more today so tubes are made to fit the limited demand. I assume the guitar amp market is where most of the audio tubes go too now days.

    I have found little reason to modify circuits as a result of lower quality new tubes. I pick certain brands for certain applications. A JJ GZ34 rectifier tube is closer to the original than a Solvtek. The Solvtek will put out about 15 Volts more than a JJ. This will cause the rectifier to have less sag and act a little more like a solid state diode rectifier which some don't like and some do. All guitar players are crazy, this includes me and perhaps especially me...lol

    I sometimes use new old stock but I find that it is rarely worth the money in guitar amps. For sure a NOS RCA 7025 is a better tube than a JJ ECC83 S but at over a hundred dollars for the RCA I have detuned my ears...lol There are certain places, like in very high end microphones, where buying the best tube you can get is logical. A couple of hundred dollars for a tube is a small part of the cost of a five to ten thousand dollar microphone.

    The bottom line from my perspective is to find a good tube vendor who can and will supply you what you need 99% of the time.

    Eddie gives me pretty close to exactly what I ask for and test every tube he sells me to include pre amp and inverter tubes. All the power tubes I get from them in matched quad sets run within two to three mA. 5mA is the excepted standard and few production amps hold to that standard now days.

    Tubes have always been a problem from day one till now, they cost too much, there too hard to make, they don't live a long and happy life, they use too much power, and produce too much heat. Having said that, I don't think they will be going away anytime soon.


  7. Jun 16, 2016 #6


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    After reading the data sheet and graphs on the link you gave, here is my interpretation:

    Ra is the internal effective Plate Resistance of the tube. This is usually shown for a triode used in a gain stage so you can calculate the actual in-circuit voltage gain. It is the slope of the Plate Characteristic Curve.

    Ra-a is the primary impedance of the output transformer used for nominal Push-Pull operation.
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