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    Preamp Tube Channel Balance

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    mazeeff

    Posts : 116
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    Location : Grand Rapids, Michigan, USA

    Preamp Tube Channel Balance

    Post by mazeeff on Wed Dec 02, 2015 8:40 am

    I recently acquired a rebuilt and fully calibrated Hickok 6000 tube tester. Like most of you, I have acquired a very large number of various preamp and power tubes. I also never seem to throw a tube out, even though they are bad! I decided to go through my entire collection on the Hickok, and was a little shocked at the results. Out of 25-30 12au7s and 12ax7's, I have only three Mullard CV4003's that are perfectly balanced. The rest are off by as much as 20%, with a few complexly dead channels as well. The Hickok runs a B+ of 190v, which is less than these tubes see in a VTA-70. Is there a way to test the L/R balance of a 12AU7 preamp tube in a VTA low gain board, while in-circuit? Does a 20% channel difference in a 12au7, result in a 20% difference at the outputs, or does the amp somehow correct for this? I also tested 20 EL34s, and saw a 10-20% variation on those as well. My brand new Shuguang's consistently tested 15% higher than the same tube after 1 years use. Thanks,

    Mike

    corndog71

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    Re: Preamp Tube Channel Balance

    Post by corndog71 on Wed Dec 02, 2015 9:49 am

    Not familiar with that tube tester but I'm wondering when you say 20% what exactly are you measuring?  Transconductance?  The measurement you really need at least for the the VA (center) position is gain.  I think that gets measured differently.  I don't know.  NOS tubes can be a crap shoot.


    Last edited by corndog71 on Wed Dec 02, 2015 4:49 pm; edited 1 time in total

    mazeeff

    Posts : 116
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Preamp Tube Channel Balance

    Post by mazeeff on Wed Dec 02, 2015 10:24 am

    I am measuring "Mutual Conductance" on the Hickok 6000. As an example, I have a RCA 12au7 that measures 2200 micromhos for Triode #1, and 2000 for Triode #2.

    Mike

    peterh

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    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Preamp Tube Channel Balance

    Post by peterh on Wed Dec 02, 2015 10:49 am

    mazeeff wrote:I recently acquired a rebuilt and fully calibrated Hickok 6000 tube tester. Like most of you, I have acquired a very large number of various preamp and power tubes. I also never seem to throw a tube out, even though they are bad! I decided to go through my entire collection on the Hickok, and was a little shocked at the results. Out of 25-30 12au7s and 12ax7's, I have only three Mullard CV4003's that are perfectly balanced. The rest are off by as much as 20%, with a few complexly dead channels as well. The Hickok runs a B+ of 190v, which is less than these tubes see in a VTA-70. Is there a way to test the L/R balance of a 12AU7 preamp tube in a VTA low gain board, while in-circuit? Does a 20% channel difference in a 12au7, result in a 20% difference at the outputs, or does the amp somehow correct for this? I also tested 20 EL34s, and saw a 10-20% variation on those as well. My brand new Shuguang's consistently tested 15% higher than the same tube after 1 years use. Thanks,

    Mike
    Tubes, as mechanical devices, made by hand, is not perfect devices. In fact even at it's hayday
    the spread between devices was significant. That's why amplifiers uses feedback that cancels the
    effect of non-perfect devices.
    You ask if a 20% difference in parameters will result in 20% of sound volume ? No, the answer is that
    the circuit used is forgiving, and will be close to immune of these differences. Other designs
    might not be that immune.
    About powertubes, here it is a little more important, thus the need for matching powertubes
    ( close to the conditions they will be used ) to avoid saturating OT with unbalanced tubes
    and overloading the feedback chain when two tubes have large difference in Gm.

    It depends on the design of the amp, yesterdays designers was aware of this.


    mazeeff

    Posts : 116
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Preamp Tube Channel Balance

    Post by mazeeff on Wed Dec 02, 2015 12:24 pm

    Thanks. If I focus on the three 12au7s used on the low gain board, where is it more important to have balanced triodes within a single tube. Preamp or phase splitters?

    Mike

    peterh

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    Join date : 2012-12-25
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    Re: Preamp Tube Channel Balance

    Post by peterh on Wed Dec 02, 2015 2:18 pm

    I guess Bob will fill in here, but as both positions seems fairly well designed +-10% should be
    more then enough.

    The preamp has local nfb, and the splitters are balanced with a current source. On top of
    that nfb will furher reduce inbalance.


    sKiZo

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    Join date : 2013-04-01
    Location : Michigan USA

    Re: Preamp Tube Channel Balance

    Post by sKiZo on Wed Dec 02, 2015 3:54 pm

    I use the most sensitive test equipment ever devised ... my ears.

    Granted, a center driver with perfectly matched triodes, as well as matched tubes in the sides, should get you perfect balance, but even then, expect some inconsistencies in the room (and source) to throw things off. That's why I went with individual volume pots for each channel on my custom build ...



    You'd be surprised how small a tweak it can take to make your music pOp ...

    mazeeff

    Posts : 116
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Preamp Tube Channel Balance

    Post by mazeeff on Wed Dec 02, 2015 4:09 pm

    In my case, I do not have control over Left/Right balance. I am also a little confused as to how the nfb circuit can help restore balance to a slightly (+/- 10%) unbalanced preamp tube.

    Mike

    peterh

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    Join date : 2012-12-25
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    Re: Preamp Tube Channel Balance

    Post by peterh on Wed Dec 02, 2015 4:29 pm

    mazeeff wrote:In my case, I do not have control over Left/Right balance. I am also a little confused as to how the nfb circuit can help restore balance to a slightly (+/- 10%) unbalanced preamp tube.

    Mike
    The short version is that :
    - an open-loop amp ( has no NFBB) is very dependent on it's component, both as "raw" amplification and
    frequency behaviour.
    - NFB will make the amp more or less independent of the tube as the amplification is (most) a
    product of Rin / Rout  ( see https://en.wikipedia.org/wiki/Negative_feedback_amplifier )

    A good verbal description is at http://www.learnabout-electronics.org/Amplifiers/amplifiers31.php

    The VTA has NFB is several ways, both "local" and "global". Thus the effect of +-10% of a tube
    does not really matter.

    mazeeff

    Posts : 116
    Join date : 2014-01-06
    Age : 61
    Location : Grand Rapids, Michigan, USA

    Re: Preamp Tube Channel Balance

    Post by mazeeff on Wed Dec 02, 2015 4:48 pm

    Got it. The links helped a lot. Thanks,

    Mike

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