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    VTA70 Driver LO Gain Resistor Substitutions

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    heyraz

    Posts : 58
    Join date : 2011-10-26

    VTA70 Driver LO Gain Resistor Substitutions

    Post by heyraz on Sat Nov 19, 2011 12:16 am

    I'm a little confused. On the VTA70 Driver Board parts list I received for the Lo Gain substitutions, R11-R14 have a stated value of 33k 2W but the Mouser Part # is for a 36K resistor. R19-R22 have a stated value of 30k 2W but the Mouser Part # is for a 33K resistor. Which is correct? Are there any other typos you're aware of that I may have overlooked?
    I'm also unsure of the type of resistor composition to use in the different situations. Wirewound, metal film, metal oxide, or carbon composition. I'm aware that wirewound's are quietest but can be inductive causing instability problems in some situations. What's the best way to go? Rich

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by tubes4hifi on Sat Nov 19, 2011 6:27 pm

    either values will work fine, I guess I need to update and/or remove the part numbers.
    I've been supplying 33K and 30K for about a year now, metal oxide.

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by baddog1946 on Sat Nov 19, 2011 10:45 pm

    you might want to check out "rescalc" this is an easy to use resistor calculation program it solves resistor problems and explains a few things you are asking about. Very cool little program

    http://www.pmillett.com/rescalc.htm

    heyraz

    Posts : 58
    Join date : 2011-10-26

    Rescalc

    Post by heyraz on Sun Nov 20, 2011 2:02 am

    Baddog,
    Thanks for the link, it makes calculating parallel and series resistors a breeze. I remember doing that longhand when I was a kid. Unfortunately, I'm not well versed in potential dividers or sources and sinks yet, so I was hoping for more of an explanation with regard to the Hi Gain to Lo Gain resistor substitutions.
    I'm also a little confused about where to use (what application) the different types of resistors. I've read that wirewound resistors are the best choice for noise, followed by metal film, metal oxide, carbon film, and lastly , carbon comps. I am not interested in the Carbon Comp "Mojo" and would like to build a low noise amp using (mostly) metal film resistors with wirewound resistors where they would have the most impact. I understand they are known to be inductive in some cases causing instability and can open when they fail.
    Any thoughts where it would be best to place wirewound resistors?
    Thanks.
    Rich

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by PeterCapo on Sun Nov 20, 2011 11:58 am

    I am not sure where the "best" place would be to place exactly what kind of resistor, but I have a feeling that much of it would be subjective, anyway.

    The only technical limitation I am aware of (of course, there may be others) is not to use metal-type resistors as grid stoppers, that is, in series with tube grids such as you would find with the EL34 power tubes in the original Stereo 70 design. Some folks recommend only carbon film or carbon comp as grid stoppers, but I am not sure if this is because of the inductance or simply because there is metal in the resistor that could cause issues when used as a grid stopper.

    In any case, if there are places in a circuit where you would want to use a wire wound type, there are non-inductive versions available, such as "Mills" resistors. If you beat the bushes you may be able to find other non-inductive wire-wounds. Mouser and Digikey may have some, and some audiophile parts vendors like Michael Percy might have others. I haven't looked in a while.

    tubes4hifi
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    Join date : 2008-11-30

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by tubes4hifi on Sun Nov 20, 2011 5:35 pm

    I've never really studied the inductive character of WW resistors, but as normal practice I use metal films for just about everything in the signal path other than sticking to carbon films for grid resistors. I only use WW for large current, small value resistors are needed in a power supply,
    where the inductance actually helps rather than a potential problem.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by PeterCapo on Sun Nov 20, 2011 9:46 pm

    Roy, do you use carbon for just grid stoppers or do you also use carbon for grid leak positions? I thought I understood that metal resistors are only a concern with grid stoppers? And what is the issue again, that the metal resistor types form an antenna along with the tube's grid which invites HF pickup or some kind of ultrasonic oscillation or something like that?

    Thanks.
    Peter

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: VTA70 Driver LO Gain Resistor Substitutions

    Post by tubes4hifi on Mon Nov 21, 2011 10:31 pm

    I use carbon film for grid stopper resistor only, for grid leak I use metal film, the grid leak resistor is simply tied to ground,
    and is normally a very high value like 470K to 1M. The grid stopper resistor is usually 1K or less, and is used :
    - to help prevent high frequency parasitic oscillation in the tube itself
    - to help prevent RF from getting into the input stage
    - to limit grid current if/when the tube is driven into the positive grid region, to prevent "blocking" distortion
    For those who aren't familiar with some of these terms, here is a good drawing showing the various parts of a typical amp stage:



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