Plate current swing when signal applied

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by HH1978, Jun 14, 2019.

  1. HH1978

    HH1978 Member

    Messages:
    85
    Joined:
    Jun 9, 2019
    Hello,

    This is probably a dumb question, but...

    I just replaced the power tubes in my JTM 45 (KT66) and biased it at 36ma plate current using a bias probe, then controlling with the bias test point (1ohm resistor from cathode). The reading at the test point is slightly higher, which seems logical since screen current must be added.
    The plate voltage is 460V, the screen voltage is 463V.

    Anyway, with the bias probe still installed, I applied a guitar signal. I was expecting the plate current readings to oscillate above and below the idle point. But it seems to only go above, with 75ma peak.
    Is that normal? Since the average plate dissipation is what is important for tube safety, are the tubes safe in this case?
    I also observed how the plate voltage changes with signal, the lowest reading I got was 406V.
    0,075A x 406V = 30,45W which is 122% of plate dissipation (or worst if peak current doesn't occur at the same time than highest voltage drop).

    Last thing I did was to put the bias probe on my Blackface Vibrolux, to see if the behavior of the tubes was similar, and indeed it is. That amp has used the same power tubes with no problem for 2 years, so I suppose it is normal.

    What am I missing?
     
    Steppin' Wolfe likes this.
  2. Steppin' Wolfe

    Steppin' Wolfe Member

    Messages:
    4,124
    Joined:
    Jul 19, 2007
    It is normal. Current draw increase as signal is applied in fixed biased circuits.
    Cathode biased circuits act differently.
     
  3. teemuk

    teemuk Member

    Messages:
    2,920
    Joined:
    Apr 21, 2009
    I think you are meaning class-A versus Class-B or AB bias.
     
    Steppin' Wolfe likes this.
  4. Steppin' Wolfe

    Steppin' Wolfe Member

    Messages:
    4,124
    Joined:
    Jul 19, 2007
    Thanks for that, teemuk. I did simplify it...and was incorrect in doing so. I for one have not encountered...knowingly....a class A fixed biased amp. And it was early in the morning...as it is today. So, I went back to Aiken.....he explains in a way I can’t. And I believe that my statement is accurate other than leaving out those other two classes of fixed bias, neh??

    https://www.aikenamps.com/the-last-word-on-biasing
     
  5. HotBluePlates

    HotBluePlates Member

    Messages:
    6,174
    Joined:
    Mar 27, 2017
    Lots of potential for confusion here.

    Does your bias probe (the method you seem to have used for monitoring current while playing) only measure d.c. milliamps? If yes, it is almost certainly not seeing the peak a.c. (alternating current) while you're playing. It might be detecting the average current while you're playing, but since you're not playing sine waves who knows what the conversion factor is from that average current to the peak alternating current will be.

    That said... When you play, average current through the tubes will rise (little-to-none for clean Class A, some-to-a-whole-lot for Class AB depending on power output). But, some amount of this "current when applying signal" is transferred via the output transformer from the tubes to the speaker(s).

    Current swing * Voltage swing while you're playing (or driving the output section with a signal) is called "Power Input" (from the power supply to the tubes). The power transferred from the tubes to the speaker is "Power Output" (obviously). Power Input - Power Output = Power Dissipation at the tube plates. That is, of all the power delivered from the power supply to the tubes, some amount is not sent out the output transformer to the speaker, and that remaining power is left for the tube to waste as heat.

    So you can't find out anything useful about how hot the tube plates run with signal without first making sure you're measuring in a way that provides meaningful results. And unless also know the power being delivered to the speaker (or dummy load) so it can be subtracted from the "power input" being supplied to the tubes.
     
    Last edited: Jun 15, 2019

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice