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    6 ohm speaker on 4,8 ohm ST35.

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    Stevedel

    Posts : 19
    Join date : 2012-02-15

    6 ohm speaker on 4,8 ohm ST35.

    Post by Stevedel on Thu Oct 01, 2015 5:42 am

    A couple of years ago I built a Dynaco ST35 out of various parts. I think that it is the sweetest sounding amp that I own. I had been using it with a pair of old Technics SB6oooA 6 ohm speakers. I think that I was using the 8 ohm tap. The ST35 also has a 4 ohm tap.

    After about a year of use. one of the Mullard "reissue" EL84 tubes started making popping sounds. EICO tube tester verified a bad tube.

    I have tried other EL84s and the Mullard reissues sound best to me. I'd love to have some NOS Mullards but...

    Should I have been using the 4 ohm tap, or should I try to squeeze in a pair of 6 ohm Edcore output transformers. I have a pair of Edcores with 4, 8, and 16 ohm taps, for a different ST35, using a chassis and board from Triode. The are much larger and heavier and the 6 ohm version uses the same core and end bells. The only difference is a slightly lower input impedance than it's 4, 8 and 16 ohm brother.

    Should I order a 6 ohm pair of transformers? I can use the 4 ohm taps on the Triode ST35s and just chalk it up as a bad tube. No damage was done to the amp, as it powers a pair of Rectilinier Mini IIIs just fine with a set of JJs. I'd like to order another set of Mullards but don't want to damage that wonderful little amp.

    Thanks so much.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by PeterCapo on Thu Oct 01, 2015 8:35 am

    I would not be inclined to think it's an issue with the transformers.  The Stereo 35 always ran the EL84s kind of hard.  Maximum plate dissipation rating for the reissue Mullard EL84 is 12 watts, and some folks say current production power tubes like this one don't last as long as the tubes of yesteryear.  It may be that your amp just burned out the tube because of the level of plate dissipation.  If this is the case, then I suppose the situation could be made worse if the tubes are not closely matched and you don't have the individual bias adjustments. Or, maybe the tube just failed for some other reason.

    It would probably be a good idea to lower the B+ a bit, which takes a little juggling, because lowering the B+ also affects the bias circuit that in turn will also need an adjustment.  Factors to consider include your how AC mains voltage runs.

    peterh

    Posts : 681
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by peterh on Thu Oct 01, 2015 2:01 pm

    As for tubes, any EL84 would do just fine. ST35 are pressing the tubes, installing EFB will
    decrease the tube loading AND in addition give more power AND less distorsion.

    Installing EFB is way cheaper and with much better result that switching transformers.

    Source for EFB :
    http://tronola.com/html/daves_store.html
    scroll down to "ST-35 EFB Power Supply Board"



    Last edited by peterh on Fri Oct 02, 2015 2:49 am; edited 1 time in total

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by PeterCapo on Thu Oct 01, 2015 7:54 pm

    EFB is a fine idea, but keep in mind that it still needs matched power tubes.  I think I saw an article about EFB with individual bias adjustment for each power tube, but I don't believe it's available (yet) as a drop-in kit like the standard, or basic, EFB.

    If not using EFB, some EL84s are more robust than others.  Several have the 12 watt plate dissipation limit, but others are rated for 14 watts such as: Sovtek EL84M (not the regular Sovtek EL84), NOS Russian military equivalents of the 7189, Electro Harmonix EL84, and possibly the new Tung Sol EL84.

    Stevedel

    Posts : 19
    Join date : 2012-02-15

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by Stevedel on Wed Oct 14, 2015 11:06 pm

    I think that it was just a bad tube. I installed a matched quad of Sovtek EL-84Ms (7189 equlivent), balanced everything with the amps bias control, moved it into the living-room setup with a pair of Rectilinear III Highboys and basked in a wide-open glorious sound.

    Granted, the Rectilinears normally ask for a more powerful amp, but they haven't yet complained.

    tubes4hifi
    Admin

    Posts : 1292
    Join date : 2008-11-30

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by tubes4hifi on Wed Oct 14, 2015 11:23 pm

    also keep in mind that speaker "impedance" such as a rating of 6 or 8 ohms is a "nominal" value, not even an average value.
    99% of all speakers have their lowest impedance below 300Hz, and it raises gradually at all higher frequencies.
    Also 99% of all speakers have a very large bump in the low end, usually somewhere between 30 to 70Hz where the impedance is typically around 25 to 45 ohms.
    Here's an example . . .

    Stevedel

    Posts : 19
    Join date : 2012-02-15

    Re: 6 ohm speaker on 4,8 ohm ST35.

    Post by Stevedel on Thu Oct 15, 2015 1:26 am

    Thanks, tubesforhifi. I'm presently studying two, fairly recent basic tube design books. I now regret dropping my junior college basic electronics class. Of course, that was 48 years ago but who's counting.

    I should probably order a couple more books from you.

    Regards, Steve

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