VTA Upgrade to Dynaco ST-70 problem.

Discussion in 'Tube Audio' started by W0ALA, Sep 11, 2017.

  1. W0ALA

    W0ALA AK Subscriber Subscriber

    Messages:
    17
    Yes I have swapped them and even installed a new matched set of EH 6SN7.
     
    Last edited: Sep 13, 2017
  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    32,444
    Location:
    Southern NJ
    Hm, B+ is low according to the schematic. Its calling for 430. Being 50v low to start with, the other voltages would also be off. That actually may be the root of the problem. Methinks it would be worth checking your main power supply voltages. If the rectifier is tired or if something else is causing the low voltage none of this is going to work as expected.

    The inverter plates would be off R19 and R21, and whatever the equivalent is on the other channel. R11 is the plate resistor for the voltage amp stage. That one seems OK. Schematic calls for 140, 137 is plenty close enough.
     
  3. W0ALA

    W0ALA AK Subscriber Subscriber

    Messages:
    17
    Well i think I may have found the problem. The LM334 for the right channel gave up the ghost tonight. Btw the voltages coming out of the rectifier is 465vdc at the B+ input to the board is 407vdc when the line voltage is 120vac. I have 2 new lm334s on the way. I had ask Roy if he thought that was the source of my problem but the voltages were so close he didn't think so. I noticed when checking R19 and R20 they were way different and then the right channel just died. :-(

    Thanks again for everyones comments and guidance. I will see what happens after the replacement.
     
  4. hogwylde09

    hogwylde09 Vintage Tube Hi-Fi Rules Subscriber

    Messages:
    59
    Location:
    Jasper, Tx
    I would change out that 5 watt wire wound resistor at the quad cap lug 3-4 to a carbon comp 3 watt.
    The important thing here is that you want around 380vdc on the driver board, and usually 2.2K is about the right amount to get that.
    Try it first, then adjust if necessary. You will need to have all tubes installed and biased before making the voltage check.
     
  5. W0ALA

    W0ALA AK Subscriber Subscriber

    Messages:
    17
    Well I just wanted to thank all of you for your input and guidance. The LM334 was bad and the new one fixed the problem. I did put the original 2.2k resistor back in between the 3rd and 4th lugs of the can cap and the amp sounds amazing like it should of from the get go. I do like the Mundof caps but I have several others I plan on trying out over time. Thanks again guys!
     

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