1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

No negative grid bias (5e3 Startup) high voltages

Discussion in 'Shock Brother's DIY Amps' started by pinchegil, Oct 26, 2020.

  1. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    Started this boothill amp 5e3 at beginning of covid as my first amp, had wait issues with getting transformers and got side tracked by a couple of champs. I have now resumed this build.
    Following Rob's startup sequence I have stalled on Negative grid bias, I have no negative DC on pins 6 of each socket (pre grid stopper?). In addition to that the voltages seem high compared to what I have found on the net. I have checked and rechecked continuity according to the layout so now I am officially stuck

    Transformer is Classic Tone 40-18021
    With rectifier and v1 (12AY7), v2(12AX7) tubes (no power tubes in place) using fluke 87 and a variac @ 115v
    I have taken readings using a Bendex 5y3gt and an JAN 5y3wgta bot had about the same readings

    5.3 volts at rectifier heater
    6.7 tube heaters
    367 at the high voltage
    No negative voltage at pins 6 of power tube sockets :(


    v1
    pin:
    1. 155
    3. 2.6
    6. 159.4
    8. 2.6

    v2
    pin:
    1. 185.1
    3. 195
    6.246.3
    8. 70.6

    B+ 471
    B+1 471
    B+2 442
    B+3 316

    5e3.jpg
     
  2. Paul G.

    Paul G. Friend of Leo's

    Posts:
    3,476
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Negative grid voltage applies to fixed bias setups. Every single tube in your amp is cathode biased.

    The purpose of bias is to keep the grid negative RELATIVE TO THE CATHODE. This is accomplished one of 2 ways, applying negative voltage to the grids OR having the cathode positive relative to ground, which makes the grid (at ground) more negative.

    You should be seeing about +20VDC at the cathodes (pin 8) with the tubes inserted. If you see positive voltage there, you're OK.

    Post your voltages.
     
  3. Paul G.

    Paul G. Friend of Leo's

    Posts:
    3,476
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Your voltages are high because you don't have power tubes in.

    If you have a bulb limiter, you're probably ready to start it up with tubes in (and speaker or dummy load). If it seems ok, then let it settle a bit, then plug into the wall and measure all of your voltages.
     
    pinchegil and secretsoundz like this.
  4. Paul G.

    Paul G. Friend of Leo's

    Posts:
    3,476
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Resistors only drop voltages if there is current flowing through the circuit. No current, no voltage drop.
     
  5. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    Thanks!!
    New voltages @123v (wall) with 8 ohm dummy load
    v1 (12ay7)
    pin1 102.1
    pin3. 1.8
    pin6 105.8

    v2(12AU7) (Electroharmonix 12ax7 won't fit in socket sleeve)
    pin1 123
    pin3 1.3
    pin6 165.3
    pin7 19.6
    pin8 48.6

    v3
    pin4 293
    pin8 18

    v4
    pin4 293.9
    pin8 18
     
  6. Paul G.

    Paul G. Friend of Leo's

    Posts:
    3,476
    Joined:
    Mar 17, 2003
    Location:
    Rhode Island
    Better. Pin 4 are your screens on the 6v6s. What are your plates reading (pin 3)?

    Also -- a 12AU7 doesn't have enough gain for the PI, so the amp may not have the volume or drive a 12AX7 will. What do you mean it won't fit in the sleeve?

    Typical 5E3 numbers.

    V1
    pin1 -- 125
    pin3&8 -- 2
    pin6 -- 125

    V2
    pin1 -- 160
    pin3 -- 1.5
    pin6 -- 200
    pin1 -- 19
    pin8 -- 53

    V3 & V4
    pin3 -- 385
    pin4 -- 360
    pin8 -- 25
     
    pinchegil likes this.
  7. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    v3 v4 plates are 332.5 the EHX 12AX7 Gold the glass is too thick to fit in the shield base of the socket. I researched and found others had the same issue. I can change out the socket but I think Ill order a JJ (they fit) or bite the bullet on a NOS and call it good
     
    Last edited: Oct 26, 2020
  8. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,502
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Really? I've order probably 300 EH 12ax7s over the last few years and haven't found any amp they won't fit into. It's more likely the socket shield is slightly out of round, so you can squish it just a bit if it looks more ovalish to you.

    Did you put in the power tubes and see if it's rockin'?
     
    pinchegil likes this.
  9. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    The shield is round I caliperd?? em. It is just small and the tube is just fat. No problems I have an ugly amp project with a larger socket shield. Added the power tubes, voltages look good with a dummy load but tomorrow Im going to pull a 12ax7 out of my peavey 30, attach a weber 12a125-a to it and see what she'll do.
     
    Paul G. likes this.
  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    5,945
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Yep, I’ve had one of those wide bottle tubes. EHX or a TAD version or something. I used mine in a tabletop amp with no sleeves or shields.

    If you didn’t already pull the trigger, JJ ax7s are not the best loved sound-wise. The 'unfat' Reflektor brands seem more popular — TS or especially the 'Svetlana' IME...
     
    pinchegil likes this.
  11. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    yeah I pulled the trigger on a couple of NOS 12AX7s RCA and Sylvania since everything else is NOS. I found a local source liquidating and got a pretty good deal. I'll let everyone know how it works out this weekend
     
    King Fan likes this.
  12. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    ok not knowing anything about a 5e3 before i built and this might seem beginner but when I plug the speaker in one output it has almost no volume, when I plug it into the other better have your ear plugs in, using a weber 12a125a very impressed but still puzzled over why the selection of the output jack is so critical. The quiet output jack is a 3 wire self grounding while the loud jack is just two wire. Sound wise it rocks, so much so that my wife stated she hadn't heard that amp before
     
  13. pinchegil

    pinchegil Tele-Meister Platinum Supporter

    Age:
    58
    Posts:
    259
    Joined:
    May 25, 2020
    Location:
    Roxborough Park, CO
    Ok, after re-reading RobRobs explanation i get it!
     
    dan40 likes this.
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,138
    Joined:
    Dec 29, 2012
    Location:
    United States
    The main speaker jack is shorted by its switch for output transformer protection. If you plug into the aux speaker jack and leave the main speaker jack empty the OT is shorted.
     
    Paul G. and pinchegil like this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.