Dynaco A-410 transformer tube amp

Discussion in 'Tube Audio' started by schlagi914, Aug 10, 2004.

  1. schlagi914

    schlagi914 New Member


    has anybody ever seen this Dynaco Amp? The circuit seams to be leaned on the low output circuit for the A-410´s, but theres one more "entry-tube" besides the driver tubes and the power transformer has the following voltages: 5V 3A, 6,3VCT 4A and 810V 220MA that differs from the circuit.
    I don´t really know the tubes that fit in, except the EL84´s / 6BQ5 (perhaps 12ax7 as drivers as mentioned in the circuit?).
    If theres anybody aut there who could help me, I would be very glad.

    Thanks alot
  2. Thatch_Ear

    Thatch_Ear Addicted Member

    I saw one of these on ebay once. I think the transformers were for those that wanted to design their own amps. 810V means with a center tap you have 2X 405 V going to the rectifier, so the voltage would have to be stepped down quite a bit before going to the output transformers and to the plates of 6BQ5s (300V in the RCA RT manual).
    As a wag the opts probably were made to handle up to EL-34/6CA7s and if they came with the power transformer possibly 6550s as well.
    Ned would know, I am just guessing.
  3. Thatch_Ear

    Thatch_Ear Addicted Member

    You think this is a Dynaco chassis and schematic? With that kind of voltage I hope there is a choke down there in that power supply too, and yea, the caps are probably toast.

    And under chassis shot will be helpfull in seeing how many windings there are on the A-410s, and that was what I saw years ago on Ebay. If there are multiple primaries then this OPT could have been used for multiple tube types. That gives you lots of options for building something with them.

  4. Thatch_Ear

    Thatch_Ear Addicted Member

    Found info on the 410. You can use only tubes of the voltage of the 6BQ5, so considering the sockets 6BQ5s (probably) or 6AQ8s. If you rebuild you can use 6V6s.
    Link to site....lots of good info here guys, all the Dyna gear has schematics and that is just a bit of it. I hope you have Adobe. Look under Tube Data/Transformer Data.

  5. NOSValves

    NOSValves AK Subscriber Subscriber

    Geezz that Dynaco transformer catalog is cool imagine buying those transformers for $20 today LOL !!

  6. schlagi914

    schlagi914 New Member

    Thanks for your kind help!!!

    I just took some photos of the bottom of the amp. The big brown caps are 0.1mF at 1000V, the smaller ones 0.05mF at 500V.
    All caps seem to be replaced once, because they are all German ones and the Amp runs 110V.
    Also it´s shure that someone had much fun in constructing a ground network.
  7. schlagi914

    schlagi914 New Member

    the closer down right view :D
  8. schlagi914

    schlagi914 New Member

    the blue cap down right is 50mF at 450/550V

    this is the upper right view
  9. schlagi914

    schlagi914 New Member

    upper left view
  10. schlagi914

    schlagi914 New Member

    down left view.

    The big blue cap is a 100+100mF at 385V just soldered together (parallel?)

    If the pics aren´t good enough I could have some more.

  11. NOSValves

    NOSValves AK Subscriber Subscriber

    Man trying to figure out what tubes absolutely should be used in this amp from pictures is going to be rough. Without some kind of idea what voltages are present at the tubes.

    The 2 small signal tubes in the center are definitely phase splitters and of the dual triode type maybe 12AU7 or 12AX7. We needs to figure out if these front end tubes are wired for 6V heater or 12V heater. The one closest to the edge of the amp is a input amplifier tube. My bet is 12AX7 input amplifier one half for each channel and 12AU7 phase splitters.

    Now the output tubes are almost positively EL84 wired in UT but the UT taps are not directly connected but run through 10 Ohm resistor. Not sure of the bias setup it looks like all the cathodes are wired together and then the wire heads toward the front of the amp and I couldn't make out where it going. This is pin 3 of the output tubes physically trace it and let us know where it goes.

    I'd replace all the coupling caps and under chassis electrolytes for starters. Bring it up on a variac once the proper tubes are a known fact and see if the above chassis cans survive. I myself would just replace them.

    I bet this could be one great sounding little power amp once working up to snuff.

    This really doesn't look like a home built amp to me. If it was the builder was very good.

  12. Thatch_Ear

    Thatch_Ear Addicted Member

    I like that huge dropping resister with the slide. You can see both the OPTs B+ running from it. No chokes though and the PS is a bigun.
  13. NOSValves

    NOSValves AK Subscriber Subscriber

    Oh wow that does slide doesn't it ! Pretty cool never seen that before have you ?

  14. Thatch_Ear

    Thatch_Ear Addicted Member

    Yes and much larger. Here is a 100 watt, 100 ohm with a slide on the OPTs of a Fisher 400 for scale.
  15. NOSValves

    NOSValves AK Subscriber Subscriber

    You wouldn't happen to have a extra variable resistor like that would you ? I know someone that needs something like that for there power center channel on a EICO ST70. It calls for a 50 Ohm 50 watt Variable. Of coarse I would pay you whatever its worth and shipping.
  16. Thatch_Ear

    Thatch_Ear Addicted Member

    I don't think there is room for a 50 watt wire wound under the chassis of an Eico ST-70 but I can look next time I go to Ft. Worth.

    Here is a 75 watt. You can see that it is shorter but much wider than the 100 watt. The high wattage wire wounds are big and they are the only adjustable ones I can think of out side of a rheostat. The one I am holding here has no open wire on the side and you can't put a slide on it.
  17. NOSValves

    NOSValves AK Subscriber Subscriber


    Thanks for going to all that trouble ! I just gave the guy the Link provided above.

    Thanks again Craig

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