Dynakit 70 in the house

Discussion in 'Shock Brother's DIY Amps' started by peteb, Aug 1, 2020.

  1. peteb

    peteb Friend of Leo's

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    2E57787E-049A-4E34-AB8B-30D775314603.jpeg It only took 31 years, she’s mine.


    Now what do I do with it?


    All I really want to do is see if the tubes have the right voltages and then put some signal thru it.



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    Last edited: Aug 1, 2020
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  2. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Cool.
     
  3. String Tree

    String Tree Doctor of Teleocity Ad Free Member

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    Turn up some Deep Purple!
     
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  4. rangercaster

    rangercaster Poster Extraordinaire

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    What speakers are you running ???
     
  5. awasson

    awasson Poster Extraordinaire Gold Supporter

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    Such a nice amplifier. Just a Classic. I’ve got something in my office my dad built that’s really similar to that and of the same era. I’m running a pair of Advent speakers with it.


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  6. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I bet you enjoy it!
    I have a mk iv that I found at the dump years ago, minor repairs, and it sounds great. I used to use it for practice, running an mp3 player in, and one of my EV floor monitors out. Plenty of power, and sounds fantastic!
     
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  7. muscmp

    muscmp Tele-Afflicted

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    i have two st70s that i've refurbed. used cap can mentioned above and replaced electrolytics including the bias cap. i run them in mono with mcintosh mx110 as the front into two jbl 4311s.

    play music!
     
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  8. peteb

    peteb Friend of Leo's

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    I’m checking things out slowly and carefully.

    The quad of old Millard EL34s are drawing current, around 100%. MPD in fixed bias, with no red plating.

    I only have dummy loads hooked up to the outputs so far, so no idea of noises or hums etc.


    The plate voltage on the EL34s is 390, with -30 fixed bias.

    Should that -30 V be more like -40 V?




    Here is the problem going forward.


    All four grids of the two pre amp tubes (this is only the power amp) have high voltage 300+ DC.


    You can see on the schematic that the first plate connects directly to the next grid with no coupling cap. There would then be high voltage DC on the grid.


    How can that work?
     
  9. David Barnett

    David Barnett Doctor of Teleocity

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    Bias for the Stereo-70 is easy, start with the manufacturer's recommendation, measured 1.56V across the 15.6 ohm cathode resistors. They gave you handy test points right there on the octal sockets at the front of the amp.

    After everything is up and running feel free to play with that, but it's a convenient starting place while you're troubleshooting.
     
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  10. elpico

    elpico Tele-Afflicted

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    Direct coupling one tube's grid to the previous plate happens all the time. See any Marshall/vox tone stack driver for example. That said, I don't think the input grid should have high voltage. Are you sure you're not measuring the screen grid?
     
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  11. peteb

    peteb Friend of Leo's

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    David,

    Now I get it.

    I thought the 1.56 V was the pre amp tube bias, because 1.5 V is a typical pre amp tube bias, and the preamp tubes are on the PCB which I know has the bias circuitry, and I now see the front panel says pre amp power by the 1.56 V.

    But, yes, pin5 of each power tube has a wire going to one corner of the PCB. Maybe that is the signal from the preamp tubes? But either way, turning the screws definitely changes the fixed bias on the power tubes, one screw per pair.


    Elpico,

    Thanks for confirming high voltage DC can be on the grids. The first grid has it too, the schematic shows the high voltage DC going to plate 1 also connects to grid 1 thru a resistor. No current flow thru the grid means no voltage drop.


    I adjusted the bias from -30 V which I believe it was purposely set at. The extent of the range leaves the bias at -35 volts which is still not cold enough. One EL34 still idles at 104% MPD.


    They run these hot. 1.56 V at the bias test point means 50 mA per EL 34.


    One of my EL34’s is closest to 50 mA. It is at 52 mA, 410 on the plate for 85% of the MPD of 25 W.



    Once the bias was lowered all the down, I connected a test speaker to the right side, dummy load still on the left.

    It made amp sounds, clicks from the input, no hum (although there is about 45 VAC on the chassis) so I put a guitar signal into the input of the power amp.

    One 7199 appears non op. I put the left 7199 on the right side and signal got thru to the speaker.



    I was surprised, the guitar had to be turned up high to get thru at all, and then it was less than 1 Watt output and extremely clean and cold sounding. That makes sense because it needs the preamp to boost the signal.


    Now I will look at the pre amp. I assumed that there are no tubes in the pre amp because the box has no vents.


    Just now looking closer there are some vents on the back on one end.
     
  12. David Barnett

    David Barnett Doctor of Teleocity

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    There are four 12AX7As and a 12X4 in the preamp. If you're lucky the 12AX7As will all be Dynaco/Telefunken tubes.

    The sockets on the front of the Stereo-70, in addition to providing a bias test point for the amplifier, were there to provide power for a pair of the original Dynaco PAM-1 mono preamps, which did not have internal power supplies. The stereo PAS-2 preamp you have connects normally and does not require the external power source from the amp.
     
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  13. David Barnett

    David Barnett Doctor of Teleocity

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    Also I would recommend replacing the selenium diodes with silicon.
     
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  14. peteb

    peteb Friend of Leo's

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    The pre amp functions, or at least the right channel does.

    I played some guitar through the pre amp and the power amp.



    Now that I have heard it, I have no interest in using it as a guitar amp or as a stereo.


    Next thing is figure out how to part with it.




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  15. SAguitar

    SAguitar Tele-Meister

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    There is some mad scientist beauty in there! Love it.
     
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  16. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    Maybe 10 years ago, a friend (amp tech), had one of those 70's under his bench, said his dad gave $5 for it at a yard sale. It was full of Mullard tubes that all but one tested good.
    He sold it on the bay for $100 less tubes, as is!
     
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  17. peteb

    peteb Friend of Leo's

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    It is posted in the classified

    $150 OBO with tubes

    Edit:

    A TDPRI member wants it as the stereo amp it is, that is best over all.
     
    Last edited: Aug 6, 2020 at 8:17 PM
  18. David Barnett

    David Barnett Doctor of Teleocity

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    Yep, those are Telefunkens.
     
  19. David Barnett

    David Barnett Doctor of Teleocity

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    What is your objection to it, as a stereo?
     
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