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    Rating for EL-34 grid stoppers?

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    Downhome Upstate

    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    Rating for EL-34 grid stoppers?

    Post by Downhome Upstate on Wed May 28, 2014 7:31 am

    I'm wiring a bare ST-70 chassis, and need to know the power rating of the 1K0 grid stopper resistors on pin 5 of the output tube sockets. 1/4 watt or 1/2 watt?  The current in the control grid circuit should be minimal to non-existent, right?  If I'm using carbon composition grid stoppers, should I go up one value (from 1/4 to 1/2 watt) to reduce the noise inherent in carbon comp types?

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Rating for EL-34 grid stoppers?

    Post by PeterCapo on Wed May 28, 2014 8:08 am

    I believe the originals were 1/2W.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Rating for EL-34 grid stoppers?

    Post by sailor on Wed May 28, 2014 12:47 pm

    If you want less noise I would use Metal film 1% 1/2 watt or at least carbon film 5% 1/2 watt. You can pull a schematic for the original ST 70 in the manual section above.

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Rating for EL-34 grid stoppers?

    Post by sailor on Wed May 28, 2014 12:49 pm

    Actually above,
    Sticky: Free Dynaco tube schematics

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Rating for EL-34 grid stoppers?

    Post by PeterCapo on Wed May 28, 2014 2:43 pm

    Some folks feel metal film is not the best choice for grid stoppers, including some designers – Joe Curcio and Gregg van der Sluys come to mind, though there are probably others, too.

    Anecdotally, even if they don’t start picking up radio signals, metal film – or even carbon film supposedly due to the spiral cut of the resistance element – might cause a "shrillness" to the sound.  Carbon comp is said to be the best choice as grid stopper resistors to avoid effects of "parasitic oscillation" and related concerns.  I used carbon comp for the grid stoppers in my new Stereo 70 and the amp sounds great.

    That being said, I know that some amps nevertheless come with metal film everywhere, so it would probably be fair to say the jury might be out on this, but in any case the resistor leads should be kept as short as possible.

    Jim McShane

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

    Re: Rating for EL-34 grid stoppers?

    Post by Jim McShane on Wed May 28, 2014 6:14 pm

    I prefer to use carbon composition (CC) resistors as grid stoppers, as they have essentially zero inductance (just a tiny bit of lead and stray inductance). The drawbacks to carbon comps really don't cause trouble when they are used as grid stoppers. The noise is not an issue since the current in a grid circuit is vanishingly small which means the "microarcing" noise CC resistors generate is pretty much nonexistent. Also, the drift issue is not a concern, as grid stoppers exact value is not critical - anywhere close works fine.

    That all said, metal and carbon film resistors can work fine in that spot in nearly every case. The spiraling does cause them to have more inductance than a CC resistor, but the low resistance values used mean not too many spirals are cut so the added inductance is quite low.


    Last edited by Jim McShane on Wed May 28, 2014 7:15 pm; edited 1 time in total (Reason for editing : Correct verbiage)

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Rating for EL-34 grid stoppers?

    Post by sailor on Wed May 28, 2014 7:04 pm

    At least we agree on the 1/2 watt.LOL Very Happy  I guess I am biased against carbon because I have been replacing noisy carbon resistors with film for the last 40 years. It started when I modified my first Dynaco PAS 2 in 1976. They may have a better tone when installed but let's compare them at 5 or ten years from now. But at the end of the day  to each there own . The most important thing is to enjoy the music.
    Oh and to answer the original question. You want to go up in Watt rating for the same kind of resistor to lower the noise because the PPM rating is based on temp. and a larger resistor can dissipate the heat better and run cooler therefor quieter. However the original was 1/2 watt and that is what I would use. As you said there isn't much load on that resistor to begin with so there is really little heat to dissipate.
    PS Just read Jim's post and I think he nailed it. Great post Jim.

    Downhome Upstate

    Posts : 37
    Join date : 2014-03-21
    Location : Greeley, PA

    OP responds

    Post by Downhome Upstate on Wed May 28, 2014 7:50 pm

    Thanks for the replies. Jumping the gun a little, I ordered a small handful of 1/4W and 1/2W Ohmite 'Little Demon' carbon comp types.

    I'll try one or the other, and attach the body as close as possible to the lug on pin 5 without actually making contact. I understand that such close placement (i.e., short lead length) helps in the parasitic oscillation minimization department.

    Onward and upward.

    P.S. Hi, Jim. Long time, no correspond.  Very Happy  Thanks for your input, and be well.

    Mike Pinsky

    Jim McShane

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

    Re: Rating for EL-34 grid stoppers?

    Post by Jim McShane on Wed May 28, 2014 8:49 pm

    Downhome Upstate wrote:Thanks for the replies. Jumping the gun a little, I ordered a small handful of 1/4W and 1/2W Ohmite 'Little Demon' carbon comp types.

    I'll try one or the other, and attach the body as close as possible to the lug on pin 5 without actually making contact. I understand that such close placement (i.e., short lead length) helps in the parasitic oscillation minimization department.

    Onward and upward.

    P.S. Hi, Jim. Long time, no correspond.  Very Happy  Thanks for your input, and be well.

    Mike Pinsky

    Hi Mike!! Great to hear from you.

    Solder those .5 watters in there just the way you described and they'll do a fine job for you! Best of luck with your project!!

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Rating for EL-34 grid stoppers?

    Post by zx on Thu May 29, 2014 4:56 am

    Can Anyone say why theres no grid stoppers on the 6AN8.... stock MK3 board....?
    Was as Most say .....Cash?
    thanks








    thanks for the site Bob........

    Wieslaw Lipowski

    Posts : 6
    Join date : 2009-05-21

    Re: Rating for EL-34 grid stoppers?

    Post by Wieslaw Lipowski on Thu May 29, 2014 9:25 am

    Look at the tube sockets, between pin 6 and 5.

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Rating for EL-34 grid stoppers?

    Post by zx on Thu May 29, 2014 8:42 pm

    Any help.......
    Mcintosh not so much...never like the VTL amps.................12at7....not for me...better than a 12aX
    on the grid stopper on the 6AN8. input tube in the MK3s.......yes, No, maybe................dose the 6AN8 need a.............grid stopper??
    why not.....?





    thanks for thje site Bob...........................have fun with tubes................

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: Rating for EL-34 grid stoppers?

    Post by sailor on Fri May 30, 2014 4:42 pm

    I assume you are talking about the 6AN8 input side and not the output. If this is the case it is because it is a pentode first tube and in most cases it is not necessary to use one on a pentode. However your triode tubes in many cases do need a grid resistor.

    zx

    Posts : 194
    Join date : 2011-08-05

    Re: Rating for EL-34 grid stoppers?

    Post by zx on Fri May 30, 2014 6:22 pm

    Thanks for the info.....i have put a 250ohm 1/4 watt... it sounds better i think...

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