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    Help with a Dynaco ST-70

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    darkhawk22

    Posts : 2
    Join date : 2016-04-25
    Location : Acworth, GA

    Help with a Dynaco ST-70

    Post by darkhawk22 on Mon Jan 09, 2017 10:15 am

    I need some help with my Dynaco ST-70

    History:

    I've been listening blissfully to my ST-70 for the past year or so with no issues. I turn it on in the morning around 6 when working from home and off around 3 or so. It is being ran from the pre-amp output of my Pioneer SX-980 and powering my HPM-150's.

    When i purchased the Amp (was sold to me as tested and working) I cleaned it up, replaced the main cap and the rectifier (it was making a buzzing noise) and maintain the bias at approx. 1.54. My output tubes are Winged C EL-34's.

    Issues:

    Recently the amp. had an issue where the front left output tube started to run hot. There was a popping through the tubes so I looked up and noticed the issue. I powered it down and let it cool. I swapped the front and back left tubes and powered it back up. The left channel sounded weak but no red plating or running hot so I powered it down and swapped the output tubes to the right channel and vice versa. Let it cool and powered it back up. Same issue. Turned it off and swapped the 7199's. This seemed to fix the problem. It worked for a few days and then the weak channel issue was back. When i went to check the bias and put the multi-meter ground wire to the Amp housing the weak channel issue went away. (Perhaps a grounding issue?). Later on the amp wanted to run hot again and blew a fuse. So I powered it off and purchased a new Rectifier Mullard GZ-34.

    Now:

    I installed the Mullard GZ-34 Rectifier, checked the bias and set to 1.54 approx. and ran the amp for about 20 mins. the weak left channel was there at first and then went away as I touched the multi-meter lead to the housing. After a while the front left output tube went hot again and the amp blew a fuse Note: these left tubes are the ones that were originally on the right. So now i'm not sure what to do. Any help appreciated!





    Peter W.

    Posts : 204
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Help with a Dynaco ST-70

    Post by Peter W. on Mon Jan 09, 2017 10:26 am

    The first thing that comes to mind is that you may need a new set of both driver and output tubes. 9 hours per day for say.... 4 days per week is 1872 hours - against no clue as to how many hours were on the tubes prior to you getting the amp.

    And, I will ask my usual questions:

    a) where are you? If you are anywhere near Philadelphia, PA, I have the capacity for an actual quality-test of your present tube complement.
    b) If not, is there anyone near you with a good tube tester?

    As to 7199s, I have more than a life-time supply, even with three amps in the inventory that use them. If this turns out to be the issue, let me know.

    The point of testing the tubes is that you want to remove that issue from consideration before looking for component or mechanical faults. And, if there is such a fault, merely replacing the tubes will not effect a cure.

    peterh

    Posts : 718
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Help with a Dynaco ST-70

    Post by peterh on Mon Jan 09, 2017 11:08 am

    You need a fresh quad of output tubes.
    Don't mess with the 7199, don't fool around with anything else, just replace the EL34 to a
    MATCHED QUAD ( or two matched pairs) and adjust bias accordingly.


    darkhawk22

    Posts : 2
    Join date : 2016-04-25
    Location : Acworth, GA

    Re: Help with a Dynaco ST-70

    Post by darkhawk22 on Mon Jan 09, 2017 11:24 am

    gotcha ok i'll go for a new set of El-34's. I'm looking at Mullards, JJ 6Ca7's, or Electro-Harmonix big bottle. Any recommendations or is there really that much of a difference?

    peterh

    Posts : 718
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Help with a Dynaco ST-70

    Post by peterh on Mon Jan 09, 2017 12:23 pm

    darkhawk22 wrote:gotcha ok i'll go for a new set of El-34's.  I'm looking at Mullards, JJ 6Ca7's, or Electro-Harmonix big bottle.  Any recommendations or is there really that much of a difference?
    Make sure it's a matched set ( matched at working voltages) and that
    the seller gives warrantee. Don't go for "cheapest in town" go for
    a supplier you trust.

    Mr. Jim McShane is a vendor i would trust, he is a member of this board.


    Last edited by peterh on Mon Jan 09, 2017 2:20 pm; edited 1 time in total (Reason for editing : Gettin name correct)

    PeterCapo

    Posts : 400
    Join date : 2008-12-05

    Re: Help with a Dynaco ST-70

    Post by PeterCapo on Mon Jan 09, 2017 2:22 pm

    darkhawk22 wrote:gotcha ok i'll go for a new set of El-34's.  I'm looking at Mullards, JJ 6Ca7's, or Electro-Harmonix big bottle.  Any recommendations or is there really that much of a difference?

    There is a difference.  What kind of sound do you like?  In my Stereo 70...

    I found that the Electro Harmonix EL34 has a more lively sound - even punchy - with more top end and low cost. From what I have read, the same might apply to the EH 6CA7 (but costs more) while it is also said to sound less like a traditional EL34.

    I have also read comments that suggest the Svetlana (NOT Winged C) EL34 also has top end.

    The Gold Lion KT77 is the best I have heard in my Stereo 70 - depth, imaging, focus, top end, dynamic, etc., but is very expensive.

    The Mullard EL34 reissue has a bit more top end than the Tung Sol EL34B but not as much top end as the EH or Gold Lion. The Mullard EL34 reissue has the relaxed EL34 sound but the bass is not very strong. However, it is spacious, images nicely and has very good focus.

    I'd like to say try some Chinese tubes, but the concern I have with them is not their sound quality but the quality of their PINS. Specifically, most octal base Chinese tubes I have owned, and many that I have seen images of on-line, have pins that are SPLAYED. The splayed pins make it more difficult to get them into the sockets, and I have found that the socket contacts then become deformed, which could result in problems if you switch to different tubes that have straight pins. So, Chinese EL34 might be reasonable to try-out IF you have a way to straighten the pins before installing them. Another problem is the soldering on the pins. If there is bulbous solder on the end of the pins, the solder should be reflowed.

    I like the JJ E34L. I had a quad from around 2003 - rich, sweet tone, palpable imaging, though the top end is kind-of rolled-off. The concern here is the number of problem reports I read about JJ in on-line forums. Reliability issues should be able to be mitigated if you can find a seller that screens them properly.

    I tried a quad of JJ KT77 once and found that there was detail, top end and all that. But, they were blurry and there was a metallic glaze over the face of the sound. I found it pretty dreadful, to tell you the truth. Still, a lot of people swear by these tubes. I don't know, maybe the quad I had was defective.

    I have read that the JJ 6CA7 is a warm sounding tube with bottom end emphasis.

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