AC15TVH issues...

Discussion in 'Amp Tech Center' started by Daz, Oct 13, 2019.

  1. Daz

    Daz TDPRI Member

    Sep 21, 2004
    Vancouver, BC
    I have an AC15 Heritage Handwired head that I've owned for, oh, maybe 10 years or so. Last night it crapped out on me. Everything lights up and all, but the sound is very weak and fizzy/distorted. Just prior to this, I noticed some very faint motor boating.

    I removed the chassis and fired it up again. It appears that one of the EL84s (V6) is red plating, and it's noticeably hotter to the touch than V5 after I shut things down. I swapped in another pair of power tubes and get the same result.

    Where should I start looking? Bad screen resistor? I'm no tech wizard, but around 10-15 years ago I built maybe a dozen tube amps. None of them Voxes. :)

    Here's a schematic:
  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Definitely suspect a screen grid resistor. Check the bias circuit, too. If the redplating does not follow one tube, then concentrate on the socket that exhibits the redplating.
  3. gusfinley

    gusfinley Tele-Afflicted

    Jun 11, 2014
    SLC, UT
    I'd lean toward a screen grid resistor as well, if it's only affecting one tube.

    Amps of this age are about due for service due to the life-span of the electrolytic capacitors. Best to get the electrolytic caps replaced every 10-15 years. A lot of amps I have built are about due for service!
    Last edited: Oct 14, 2019
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