The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all Tubes4hifi.com products and all Dynakitparts.com products


    ST70 Back to Life

    Share

    Drugolf

    Posts : 25
    Join date : 2014-12-29

    ST70 Back to Life

    Post by Drugolf on Sat Jan 10, 2015 1:02 pm

    Hello all! My first post here since acquiring my various Dynaco pieces and I thought I would quickly share with you much more experienced and smarter folks my Dynaco ST70 restoration experience. (note not Dynakit)

    I have a little thread on this over at AK but since we are talking Dynaco....

    Picked up the unit several months ago in basically original condition as it was packed away in a box in the late 80's until I opened it. great condition.

    Initially brought it back to life slowly via variac at my techs place and used it a bit before I started getting various popping and distortion.
    Ended up that several of the tubes had or were failing including one of the EL34's, both 7199, and the GZ34. Suddenly my investment was increasing to say the least as I wanted to try and stick with Old stock type tubes to keep this baby as original as possible. The existing EL34's are XF2 Mullards, the rectifier was an Amperex and the 7199's were whatever got the RCA brand. Doug at Dougs tunes came though for me on a matching EL34 and I went with a Japanese NOS mullard from Brent Jessie for the Rectifier.

    In the meantime after reading all the great stuff on here as well as all the various other resources which are wonderfully available out there via the interweb, I went ahead and replaced the cap can (with increased values), all the caps up top, and used the Dynakitparts Bias kit to replace the S rectifier. All these parts I got from Kevin at DKP's and a big thanks to him for answering my newbie questions along the way. This was my first foray into working on electronics (I do some cap work on speakers) so I had plenty dumb questions along the way. I also went ahead and did the 7199-6GH8A conversion with the socket adpaters thinking while this was not "original" it sure made sense since it would be easy to go back to 7199's if desired. Dave G's Base line testing was a great reference.

    I also did the rectifier diode mod per the sticky here.

    Originally I thought I would have my local tech do this work, but he convinced me to go for it myself and save some $ since I had so many tubes to buy. Once done though I took it to him to check my work and test so we could bring it on line correctly. Viola! it worked great! At least until he went ahead and did a clipping test to see how strong things were. It unfortunately exposed that I have another EL34 that is weakening. Argh. It still sounds pretty good and biased fine so I guess I will go ahead and use it while I search for a replacement to match up with the replacement.

    The one thing that caught my eye was a pretty brightly glowing Rectifier tube. Is this normal?

    I have not hooked it up at home yet and plan on doing so today and will post my findings and maybe some photos. I have a restored to original Pas2 preamp I will be using with it and both Klipsch Heresy's or Altec Valencia B's are in the house, ready willing and able. Very Happy

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: ST70 Back to Life

    Post by nerpissad on Mon Jan 12, 2015 5:14 am

    Drugolf wrote:The one thing that caught my eye was a pretty brightly glowing Rectifier tube.  Is this normal?
    Don't really know how brightly your rectifier is glowing, but the rectifier is the tube that glows the most of all the tubes in an ST70. Since the tech you brought it to didn't notice I assume that it's ok, though.

    lorne

    Posts : 4
    Join date : 2015-01-10
    Location : Sendai, Japan

    Re: ST70 Back to Life

    Post by lorne on Mon Jan 12, 2015 10:21 am

    Dugolf ... Good on ya for a brave entry into the vacuum world and a subsequent success!

    I have been in vacuum land for many years, but I am still in the dark. I hope my recent rebuild turns out as successful as yours.

    How does it sound?

    ... Lorne

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST70 Back to Life

    Post by sKiZo on Mon Jan 12, 2015 2:41 pm

    You think a 5AR4 gets a good glow going in an ST70, you should see it in an ST120 ...



    That's a Chinese tube, but I also get a cheery glow from an old stock Mullard ... Downright scary it is ... I've since gone over to a GZ37 ...



    Drugolf

    Posts : 25
    Join date : 2014-12-29

    Re: ST70 Back to Life

    Post by Drugolf on Tue Jan 13, 2015 11:07 am

    Thanks guys. The tube was bad. For now I locally got a Grove Tube until I can get the old stock replacement.
    I am not quite done apparently. Now that the rectifier is good to go, I was able to try and dial in the bias. I am not able to get one of the sides up above 1.25. I know one of those tubes is getting weak so sure enough when I swapped the tubes side-to-side the low bias followed the tubes. Now I am waiting until I can find a match to the other single mullard I had just got.
    A couple small things I could use some feedback on though are:
    1. When I dont have the output cables plugged into the front jacks, I get a hum from one channel once the tubes warm up. With the cables plugged in, no hum. ?
    2. Switching the unit off I get a fairly loud crack. Is that typical/okay?

    nerpissad

    Posts : 58
    Join date : 2012-09-16
    Age : 43
    Location : amsterdam

    Re: ST70 Back to Life

    Post by nerpissad on Tue Jan 13, 2015 12:12 pm

    I'm not able to answer your first question, but I think when it comes to number 2 it is important to turn off all equipment in the right sequence.
    This is how I do it:
    Stop the source from playing music and turn down the volume on the preamp. Then I switch off the ST70. After that, I turn off preamp and turntable and/or DAC.
    This is basically the reverse order of when turning everything on Wink

    peterh

    Posts : 641
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 Back to Life

    Post by peterh on Tue Jan 13, 2015 1:21 pm

    Drugolf wrote:Thanks guys.  The tube was bad.  For now I locally got a Grove Tube until I can get the old stock replacement.
    I am not quite done apparently.  Now that the rectifier is good to go, I was able to try and dial in the bias.  I am not able to get one of the sides up above 1.25.  I know one of those tubes is getting weak so sure enough when I swapped the tubes side-to-side the low bias followed the tubes.  Now I am waiting until I can find a match to the other single mullard I had just got.
    A couple small things I could use some feedback on though are:
    1. When I dont have the output cables plugged into the front jacks, I get a hum from one channel once the tubes warm up.  With the cables plugged in, no hum.  ?
    2.  Switching the unit off I get a fairly loud crack.  Is that typical/okay?
    1/ any open output might pick up electric fields from the surroundings. Inside there is lot's of
    low energy fields emanation from the power lines in the house

    2/ yes unless you have one of :
    - solid state relay that breaks in the zero-crossing
    - some sort of arrestor around the circuit switch ( an RC-device across the switch )
    the amplitude/noise might be lower if inputs are connected to a low impedance source
    cabling inside the unit may pick up more noise if not routed properly
    Breaking both pre-amp and st70 with the same breaker will make more noise

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: ST70 Back to Life

    Post by sKiZo on Tue Jan 13, 2015 2:16 pm

    Sounds like the RCA cable is providing a common ground between the amp and pre. Might want to check the RCA's for shorts to the case? Maybe missing an insulator or something.

    Easiest way to tell might be to temporarily remove the RCA jack from the chassis and see if you still get hum with no cable attached.

    Drugolf

    Posts : 25
    Join date : 2014-12-29

    Re: ST70 Back to Life

    Post by Drugolf on Tue Jan 13, 2015 8:31 pm

    Here is another oddity.  I am using my Pat5 as the pre.  When I turn the volume all the way down, I still hear the source music. ? That does not happen when t4h Pat5 is being used with my ST400.

    Sponsored content

    Re: ST70 Back to Life

    Post by Sponsored content Today at 4:45 am


      Current date/time is Mon Sep 26, 2016 4:45 am