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One channel dead?

Discussion in 'Tube Audio' started by valco, Nov 7, 2018.

  1. valco

    valco Active Member

    Messages:
    108
    So I did two mods on my Dynaco ST70 and now the right channel is silent. Any help woud be appreciated!

    I replaced the rectifier tube with UF4007 diodes. and used a 10w/50ohm resistor on the output of the diodes to bring the voltage down a bit. I also added a CL90 on the AC wall line, before it hits the fuse.

    I also added two .22uf input caps to remove with any voltage that may be on the input when using various preamps.

    The voltages all over the amp are great. Except for the right channel. The two right channel EL34s both read only about 100vDC on pins 3 and 4. They should be around 410vdc each.

    For the diode mod, I just put a diode from pin 4 and from pin 6 to one end of a 10w/50ohm resistor. Then ran the other end of the resistor to pin 8. The voltage reads as it should.

    For the input caps, I followed audioregenesis's instructions:

    1. Remove the two 470K resistors currently at the input jacks. If you do not have replacements, remove them carefully, as you will reuse them in step 3.
    2. Remove the two wires currently connected between eyelets 7 & 17, and their respective input jack. Install two .22 uF 100 volt caps of your choice in their place. The voltage rating of the caps should be kept low to keep the size small.
    3. Reconnect one of the removed 470K resistors directly between eyelets 10 & 17 for the Right channel, and the other between eyelets 7 & 8 for the Left channel.
    4. Install two 1 meg 1/4 watt resistors in the positions originally occupied by the two 470K resistors in the original build.
    This procedure effectively places an appropriate coupling cap in series with each input of the ST-70, so that any low level DC that might be part of the input signal is blocked, and can no longer induce any LF instability.
     

     

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  2. 6DZ7

    6DZ7 Super Member

    Messages:
    2,178
    Check for correct voltage on the B+ line to the tranny. Bad solder? Broken wire?
     
  3. valco

    valco Active Member

    Messages:
    108
    Thanks. The quad cap reads perfect voltages.
     
  4. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    And pins 3 and 4 for the left EL34 read about 410VDC while the right ones read about 100VDC? There should be a red wire from both output transformers that connect to node "C" of the power supply (choke/6.8KΩ/lug #1 of quad cap). It's all the same node - should read the same.

    Is the red wire from lug #1 of the quad cap to the right channel OPT intact?
     
    Last edited: Nov 7, 2018
  5. 6DZ7

    6DZ7 Super Member

    Messages:
    2,178
    Since the voltage at both tubes is equal and low it suggests the input line has a problem. It could be a funky solder connection that might read good voltage but the wire has bad flow and high resistance. Or, it could be the strands in the wire are broken and only a few are carrying voltage. Or it could be the connection of the wire inside the cover where it attaches to the windings is broken from flexing/movement.... You might have to disconnect the wire and read the resistance through the tranny to see if the B+ wire is broken.
     
  6. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,584
    Location:
    Middletown,New York
    i would check the input cap and resistor connections, also the Mono / stereo switch, try switching to mono to see if anything changes
     

     

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  7. valco

    valco Active Member

    Messages:
    108
    Dah! Thank you all! Found it. It was that red wire from the transformer that connects to the quadcap. If I wiggle it, the sound comes back. Resoldered and its all good now.
     
  8. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    Go Team Dynaco!
     
  9. valco

    valco Active Member

    Messages:
    108
    Yes! TEAM DYNACO!
    You guys are the best. Thanks!

    Also built a bucking transformer. That in combo with the CL90, the amp is now running at its correct voltages and a much cooler temperature too. I’m done messing with this amp now.

    Except maybe some new power tubes....
     
  10. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    Which driver circuit do you have in the amp, and what kind of sound quality do you like?
     
    Last edited: Nov 7, 2018
  11. valco

    valco Active Member

    Messages:
    108
    The ST70 is totally stock values. I rebuilt the entire thing with new PCBs, caps, resistors. Using original transformers.

    Paired with a rebuilt to stock PAS-2. Except I removed the tone controls.

    I built my own tone controls using the LM1036 IC. Which are removable with the flip of a switch. They do a great job and don’t mess up the sound at all. I use them to just bump up the bass and treble a bit.

    Turntable is a Rek-o-Kut L34 Rodine Jr w/ an ESL S-1000 tonearm and a Shure M3D.

    All into a pair of recapped/redoped KLH Model 6 speakers.

    I really enjoy the sound. Its wonderful on older recordings like vintage jazz and classical stuff. Some modern stuff needs a little bit of help to my ears to reach a bit lower and higher. That’s why I have the tone controls. No extreme adjustments. They really open up some modern recordings.

    I’m gonna order some KT-77s pretty soon to try out.

    Amazingly, this entire system was given to me by a friend of a friend for free. The original owner (the father) passed away. I promised the family it would be in good hands, and that I would do some restoring to keep it running far into the future.
     

     

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  12. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    The tone controls on the preamp make the choice of a different tube a bit less critical. KT77 or 6CA7 could be a change from EL34 sound.

    Sounds like a nice setup you have there.
     
  13. valco

    valco Active Member

    Messages:
    108
    What’s your setup, petercapo? Do you use the stock driver, or something else?

    I know those VTA boards are popular. I’ve never heard them myself.
     
  14. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    I was using what is essentially an https://www.dynakitparts.com/ Stereo 70 that I built some time around 2013. It just sounded great. It has the original Dynaco driver circuit, and it sounded so good that I felt no need to switch to a different driver circuit. My Stereo 70 held its own very nicely compared with my Cary SLI-80 Signature, Parasound Halo A21 and McIntosh MC275 MkIV, and sounded better than amps I heard at local dealers, for instance, Octave Audio and Prima Luna. The original Dynaco circuits still do what they have always done, that being, and with a realistic assessment, they get you much or most of the way to the sonic quality of some highfalutin amplifiers for a lot less money.

    Regardless, you just gotta love the way the original Dynacos sound. They are smooth and non-fatiguing while still delivering on sonic goodies like imaging, space, rich tone, natural timbre, detail, presence, etc. And, with the right tubes, rolloff at the frequency extremes is mitigated.

    I also rebuilt a PAS-2, and it sounds great: http://audiokarma.org/forums/index....input-impedance-solid-state-power-amp.834063/

    I am now slowly rebuilding an original Stereo 70.
     
    Last edited: Nov 7, 2018
  15. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,797
    Location:
    Southern NJ
    I'd still love to see a full shoot-out of the various Dyna driver boards as compared to the stock circuit. The results might be interesting.
     
    stanko likes this.
  16. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    On as level a playing field as possible, yes it would be interesting, at least in terms of the sonic preference of the listeners, which would inevitably vary by taste.

    A technical comparison would have been possible all along, at least on paper, if the various driver boards generally had their specs openly published. Actual bench testing by a disinterested party would be best, of course.

    At least we have the recent landmark testing studies by Dave Gillespie and George Ronnenkamp http://www.audioregenesis.com/ that corroborate the original Dynaco specs as well as the decades-old independent reviews of the original Stereo 70. The original Dynacos have been an open book all along. Not so much for the different driver boards.

    Reason for edit: clarification.
     
    Last edited: Nov 9, 2018

     

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  17. stanko

    stanko Hoarder Subscriber

    Messages:
    790
    Location:
    Oshkosh, WI
    I'm with you.
     
  18. petercapo

    petercapo AK Subscriber Subscriber

    Messages:
    903
    A comparison like that would need to take great pains to overcome the prejudice, if that were even possible. IMO, it is to the detriment of the audio community that the original Dynacos have a "perception problem" with some. Perhaps many, actually.
     
    Last edited: Nov 8, 2018
  19. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,797
    Location:
    Southern NJ
    I don't mean so much a subjective listening shoot-out, but a test gear report. I get that data isn't all of the story but its the only objective means of comparing things. I'm just curious what exactly the test gear has to say changes when a different topology driver is installed.

    Subjective tests are impossible to prove anything with. Ask 10 people, get 10 answers, and they are all correct since its just an opinion.
     
    petercapo likes this.
  20. c_dk

    c_dk Addicted Member

    Messages:
    6,248
    Location:
    West Michigan
    What would be very interesting is a series of bench tests collaborating the perceived sound differences between various tubes used in a chosen reference am or preamp.

    A $200.00 vintage masterpiece, that receives all the accolades, compared to a mundane $10.00 current production tube should be different enough.

    There has to be some sort of measurable in circuit difference.....correlating the measured differences to the gushing verbiage could be tough.....
     
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