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    VTA70 Driver LO Gain Resistor Substitutions

    heyraz
    heyraz


    Posts : 59
    Join date : 2011-10-26

    VTA70 Driver LO Gain Resistor Substitutions Empty VTA70 Driver LO Gain Resistor Substitutions

    Post by heyraz 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
    Roy Mottram
    Roy Mottram
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    Posts : 1839
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    Post by Roy Mottram 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
    baddog1946


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

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    Post by baddog1946 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
    heyraz


    Posts : 59
    Join date : 2011-10-26

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    Post by heyraz 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
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    Post by Guest Sun Nov 20, 2011 11:58 am

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 1:38 pm; edited 1 time in total
    Roy Mottram
    Roy Mottram
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    Post by Roy Mottram 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.
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    Post by Guest Sun Nov 20, 2011 9:46 pm

    .


    Last edited by PeterCapo on Wed Dec 02, 2020 1:38 pm; edited 1 time in total
    Roy Mottram
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    Post by Roy Mottram 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:


    VTA70 Driver LO Gain Resistor Substitutions Tube_stage

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