Advice on setting operating point?

Discussion in 'Tube Audio' started by justinis, May 25, 2016.

  1. justinis

    justinis Active Member

    Messages:
    143
    Thanks GordonW and kward! I will start tweaking.
     
  2. justinis

    justinis Active Member

    Messages:
    143
    Alright, substantial progress has been made but I am in need of more advice. Hopefully someone will still be watching this thread!

    I completely built and biased the amp with 430V plates, 325V screens, 20mA cathode current per output tube with no input signal. However, the output of my rectifier is actually 495V, so I dropped this to 430V with several high-watt resistors (overall resistance: 650 ohms). I'm worried when I actually start driving the amp, the current through the large resistor(s) will make my plate voltage fluctuate. Also I don't know what the current draw will be at max load, but if it's 2 or 3 times the quiescent value I'm going to need very high-watt resistors, and my plate voltage will be way too low.

    I really don't want to pay for a different power transformer. I suppose my only option is to try to use high-wattage zener diodes? Or am I worrying too much about the 650 ohm resistor?
     
  3. Brice

    Brice Super Member

    Messages:
    1,111
    Location:
    Hoboken, NJ USA
    Do you use a tube rectifier?
     
  4. justinis

    justinis Active Member

    Messages:
    143
    I did. After some googling, it looks like the best way to handle this is a zener diode or diodes attached to the center tap of the power transformer HT: http://www.aikenamps.com/index.php/what-is-back-biasing/. I'm gonna head in that direction.
     
  5. Brice

    Brice Super Member

    Messages:
    1,111
    Location:
    Hoboken, NJ USA
    You may want to look at a rectifier with higher voltage drop? + resistor you may be ok.
     
  6. justinis

    justinis Active Member

    Messages:
    143
    Wow, that's an interesting idea. If this site is to be believed http://www.300guitars.com/articles/rectifier-tube-voltage-drop-chart/ I could get a 57V drop if I went with an 5R4. I'll have to look into its specs to see if it will work with my amp. Thanks for the suggestion!
     
  7. Brice

    Brice Super Member

    Messages:
    1,111
    Location:
    Hoboken, NJ USA
    No problemo.
    I also used a bucking box to lower the AC primary a bit. With the multiplying factor it does help quiet a bit.
     
  8. Dandy

    Dandy Super Member

    Messages:
    3,650
    Location:
    North West, UK
    Yes, I have had good results with a bucking arrangement plus a 5R4G rectifier. Better than a big resistor.
     
  9. Brice

    Brice Super Member

    Messages:
    1,111
    Location:
    Hoboken, NJ USA

Share This Page