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    Grid Resistors

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    jwb474

    Posts : 8
    Join date : 2014-07-13

    Grid Resistors

    Post by jwb474 on Wed Sep 10, 2014 1:34 am

    I was reading an old audio express article from David Gillespie on Techniques to Maximize Power Tube Life. He recommends putting a 100ohm resistor in series with the interlinear taps to the upper grid. Out of all of my dynaco units and Mods to them I have never seen this mod done. Is there any reason NOT to do this mod if it does no harm, but may prevent the instability he talks about in the article?

    Jim McShane

    Posts : 154
    Join date : 2011-10-19
    Location : South Suburban Chicago

    Re: Grid Resistors

    Post by Jim McShane on Wed Sep 10, 2014 8:27 am

    I think you were referring to the screen grid resistors, and using them with an ultralinear configuration.

    It was not done back in the day for cost reasons. More expensive gear did use them (Citation II amps for instance). I agree with Dave, I definitely think you should use "screen stoppers", but I prefer a slightly large value - around 270 Ohms or so. Guitar amps often use larger values to protect the screens, 1K/5 watt resistors are not uncommon.

    There's no downside other than cost and the work involved to install them. Do it!

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Grid Resistors

    Post by Bob Latino on Wed Sep 10, 2014 8:56 am

    jwb474 wrote:I was reading an old audio express article from David Gillespie on Techniques to Maximize Power Tube Life. He recommends putting a 100ohm resistor in series with the interlinear taps to the upper grid. Out of all of my dynaco units and Mods to them I have never seen this mod done. Is there any reason NOT to do this mod if it does no harm, but may prevent the instability he talks about in the article?

    If you read that article carefully, you will see that Mr. Gillespie does say that the ultralinear output stage as in figure 3B is "acceptably stable only when operated from a power supply with characteristics as shown in Figs. 1A or 1B."

    The original Dynaco Mark II, Mark III, ST-70 and Mark IV and all the VTA amps use a power supply as found in figure 1A. This is a classic type of power supply that remains stable under all normal conditions of operation of the amp. The original Dynaco tube amps did not use screen resistors because in the opinion of David Hafler and Ed Laurent who started Dynaco in 1955, they were not necessary when operated from a tube rectifier/choke input power supply (figure 1A). I agree with them.

    If others would like to read the entire article, it is here > Gillespie article on tube life

    If others have another opinion or would like to post their thoughts about this, feel free to give your opinion ..

    Bob

    jwb474

    Posts : 8
    Join date : 2014-07-13

    Re: Grid Resistors

    Post by jwb474 on Thu Sep 11, 2014 3:01 am

    Thanks for the response to my question. On the stock MK3 that I bought the VTA drivers assemblies for I will not put in the screen resistor, but for the MK3 project coming up in which I have bought KT120s and 150s (expensive) and experimenting with power supplies, I think I will put them in.


    PeterCapo

    Posts : 380
    Join date : 2008-12-05

    Re: Grid Resistors

    Post by PeterCapo on Fri Dec 04, 2015 10:12 pm

    As I understand Dave Gillespie's article, he recommends the screen stability resistors for vintage amps such as the Dynacos where their power supplies have been "modernized," which I take to typically mean increased capacitance.

    Should the screen stability resistors be carbon comp as is sometimes suggested for grid stop resistors?

    peterh

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

    Re: Grid Resistors

    Post by peterh on Sat Dec 05, 2015 3:05 am

    PeterCapo wrote:As I understand Dave Gillespie's article, he recommends the screen stability resistors for vintage amps such as the Dynacos where their power supplies have been "modernized," which I take to typically mean increased capacitance.

    Should the screen stability resistors be carbon comp as is sometimes suggested for grid stop resistors?
    Yes. It's imperative that they are non-inductive.

    Tubes4ever

    Posts : 59
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Grid Resistors

    Post by Tubes4ever on Sat Dec 05, 2015 12:58 pm

    Does anyone know where to find a copy of the article "Why Power Tubes Arc" that is mentioned in the David Gillespie article? Sounds like it would be interesting to read.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: Grid Resistors

    Post by tubes4hifi on Sat Dec 05, 2015 2:33 pm

    the article clearly states that the article you are asking about is in AudioXpress Nov 2004 ("Why Power Tubes Arc")
    here's a link to their website . . . http://audioxpress.com/page/audioXpress-Subscription-Services.html

    Tubes4ever

    Posts : 59
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Grid Resistors

    Post by Tubes4ever on Sat Dec 05, 2015 7:12 pm

    Thanks.  Yes that is obvious.  I was having a senior moment and kept typing audioExpress.  Kinda messes up the search results.

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