Need some help, 5E3 build

Discussion in 'Shock Brother's DIY Amps' started by Paul G., Apr 25, 2019.

  1. Paul G.

    Paul G. Friend of Leo's

    Posts:
    2,564
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Finished up the build, go to test voltages, all pretty good -- a little high, but probably my wall voltage --except for pin 2 on the 12AX7. Should be 160, getting 281. There is a 100k Ohm resistor supposed to be dropping from 252 (I get 285) to 160 Vdc, but it just drops from 285 to 281.

    With the pin end disconnected, I get the same drop over the resistor, so I say -- aha! I'm not drawing any current. Checked the socket, and swapped out the tube - no difference. Tried a different 100k Ohm resistor - no difference. Put in a 1Meg resistor for S&G, voltage barely dropped.

    The Pin 6 with a 56k dropping resistor from the same spot works fine dropping to 210Vdc (s.b 198, pretty good). Likewise the voltages for the 12AY7, all dropping from the same place (preamp filter cap).

    Amp make a light hum, and tapping on 2 & 6 on the 12AX7 makes healthy pops, but not getting much out of 12AY7. Plugged in, signal is very low, rattley and distorted.

    Not my first amp, but this is beyond me. Triple checked circuit, joints all good. Freaking out.

    Voltages (Voltage in parentheses are target):

    B+ at pin 8 rectifier - 383 (386)
    V4 (6v6): Pin 4 - 340 (342): Pin 8 - 23 (22)
    V3 (6v6): Pin 4 - 348 (342): Pin 8 - 22 (22)
    V2 (12AX7) Pin 1 - 281 (160): Pin 3 - 5.2 (1.4): Pin 6 - 210 (198): Pin 7 - 19 (19): Pin 8 - 51 (53)
    V1 (12AY7) Pin 1 - 132 (125): Pin 6 - 135 (125): Pin 8 - 2.2 (1.9)

    It's just weird to me that the same voltage from the filter cap drops nice-nice everywhere but there.

    Any ideas?
     
  2. muswell_hillbilly

    muswell_hillbilly Tele-Meister

    Posts:
    118
    Joined:
    Apr 8, 2019
    Location:
    Greenfield, WI
    Do you have another 12AX7 you can try in that position?

    Edit: Sorry - I see now that you already said you swapped out the tube :(
     
  3. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    38
    Posts:
    949
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    V2 cathode resistor too high a value, biases the triode too cold. Very little current flows, very little voltage drop across the plate load resistor. The resistor that is connected to pin 3. See how high the voltage is? Without more info, that's my guess.
     
    King Fan, Paul-T, D'tar and 2 others like this.
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    2,843
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    What @Snfoilhat said - measure resistance from pin 3 to ground. You could have accidentally put the wrong resistor there - don't worry, we've all done it.
     
  5. Paul G.

    Paul G. Friend of Leo's

    Posts:
    2,564
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    That was what I felt, not enough current going through,due to low bias so not enough current to drop voltage. I had some wine with supper, so no more work tonight, I'll give it a go in the morning.
     
  6. Paul G.

    Paul G. Friend of Leo's

    Posts:
    2,564
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    That was it. When I bolted the board into the chassis I must have nudged the 1.5K bias resistor -- even though it looked like it was still making contact, it wasn't. Fixed that, and everything worked perfectly. Can't crank it at 11:30, but all the controls work, got good volume, nice and sweet.

    Thanks for helping steer me in the right direction.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  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.