Page 2 of 2 FirstFirst 12
Results 36 to 60 of 60
Like Tree13Likes

Thread: Bias problems---

  1. #36
    Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    76
    Did anyone notice the strange "inverse operation" of V2-B and V4-B in the schematic? I wouldn't trust this sloppy diagram.
    Dave H and nickb like this.

  2. #37
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    More testing today = with all tubes removed

    from the el84 area and after , to the OT and jacks , all continuity is ok

    plates at 360v

    2 test points are not right at pin3 - should be 9-12v according to scem but i get=
    v6 5mv
    v7 0 v
    v8 5mv
    v9 0 v

    grounds check good , all speaker jacks getting to ground

    R84 tests around 0 ohms in circuit and no matter where the half pwr/ full pwr / standby switch is set = its going right to ground , both ends . But it tests 100k out of circuit??

    if something was wrong with C47 or C48 , could that cause the mayhem ??

  3. #38
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Quote Originally Posted by Valvehead View Post

    R84 tests around 0 ohms in circuit and no matter where the half pwr/ full pwr / standby switch is set = its going right to ground , both ends . But it tests 100k out of circuit??

    if something was wrong with C47 or C48 , could that cause the mayhem ??
    If you disconnect what I recon is a quick connector SW9. Still 0 Ohms to ground? If yes, disconnect SW8. Still 0 Ohms to ground?

  4. #39
    Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    76
    Quote Originally Posted by Valvehead View Post
    More testing today = with all tubes removed

    from the el84 area and after , to the OT and jacks , all continuity is ok

    plates at 360v

    2 test points are not right at pin3 - should be 9-12v according to scem but i get=
    v6 5mv
    v7 0 v
    v8 5mv
    v9 0 v

    grounds check good , all speaker jacks getting to ground

    if something was wrong with C47 or C48 , could that cause the mayhem ??

    With all tubes removed there is no current through R83/85 and TP10/TP11 should have 0 V.

    I did not read all the posts, but can you exclude a conductive V6 socket? If you disconnect the 2k2 grid stopper resistor directly at the V6 socket, what voltage do you measure at open pin 2 (tubes pulled)?

  5. #40
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Quote Originally Posted by Helmholtz View Post
    With all tubes removed there is no current through R83/85 and TP10/TP11 should have 0 V.

    I did not read all the posts, but can you exclude a conductive V6 socket? If you disconnect the 2k2 grid stopper resistor directly at the V6 socket, what voltage do you measure at open pin 2 (tubes pulled)?
    Answer in reply #18. 0V when grid stopper resistors are lifted.

  6. #41
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by doombass View Post
    If you disconnect what I recon is a quick connector SW9. Still 0 Ohms to ground? If yes, disconnect SW8. Still 0 Ohms to ground?
    the standby / half / full power is on 1 , 3 way toggle switch . 2 wires go to the trans and one wire goes to "tx8" on the pcb. With all 3 wires removed that resistor still goes to ground ... both ends

  7. #42
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by Helmholtz View Post
    With all tubes removed there is no current through R83/85 and TP10/TP11 should have 0 V.

    I did not read all the posts, but can you exclude a conductive V6 socket? If you disconnect the 2k2 grid stopper resistor directly at the V6 socket, what voltage do you measure at open pin 2 (tubes pulled)?
    i just clipped that 2.2k out and i get 0v to about 1.5mv...the meter just kind of goes up and down in that range

  8. #43
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Quote Originally Posted by Valvehead View Post
    the standby / half / full power is on 1 , 3 way toggle switch . 2 wires go to the trans and one wire goes to "tx8" on the pcb. With all 3 wires removed that resistor still goes to ground ... both ends
    Oh, I meant the 4 tubes/2 tubes-switch which bypasses R84 to ground. In the 4 tubes setting the switch is actually supposed to short out R84 to ground.

  9. #44
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by doombass View Post
    Oh, I meant the 4 tubes/2 tubes-switch which bypasses R84 to ground. In the 4 tubes setting the switch is actually supposed to short out R84 to ground.
    yes- we are talking about the same switch--r84 is going to ground on both ends , in 2 tube mode , 4 tube and even on standby

    i pulled the 3 wires off it and it made no difference

  10. #45
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    But if we are talking about SW4_A there would be only two wires, on pin 8 and pin 9 with pin 7 not connected. Correct?

  11. #46
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by doombass View Post
    But if we are talking about SW4_A there would be only two wires, on pin 8 and pin 9 with pin 7 not connected. Correct?
    ok--sorry i was referring to the front panel switch . There are 2 switches . On the back of the amp is the 2 tubes/ 4 tubes toggle ( sw4)...and the front of the amp there is the "half power / full / standby switch..so i was looking at the front one .. SW 2

    ill take some more measurements later tonight --the confusion was on my part as usual....

  12. #47
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Quote Originally Posted by Valvehead View Post
    ok--sorry i was referring to the front panel switch . There are 2 switches . On the back of the amp is the 2 tubes/ 4 tubes toggle ( sw4)...and the front of the amp there is the "half power / full / standby switch..so i was looking at the front one .. SW 2

    ill take some more measurements later tonight --the confusion was on my part as usual....
    Yes, and you will most likely find readings of R84 to come out at the approx 100k it is supposed to be once you have flipped the switch to 2 tubes operation. Then we're back at the strange stray current in the path/junction of C45/R43/R76/R78.

  13. #48
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by doombass View Post
    Yes, and you will most likely find readings of R84 to come out at the approx 100k it is supposed to be once you have flipped the switch to 2 tubes operation. Then we're back at the strange stray current in the path/junction of C45/R43/R76/R78.
    yes R84 is fine.

    I took some readings -measuring at pin2 , V6

    sw4 2 tube mode / sw2 half power mode = amp starts around 118v and slowwwly...slowly drops down to around 22v......then all of a sudden it jumps up to 90v...then starts slowly dropping again. Im talkng about 1 volt per minute.

    sw4- 2 tube / sw2 Full pwr mode = starts at 200v and creeps up to 214v

    sw4 - 4 tube mode / sw2 Full pwr = 214v

    sw4 4 tube mode / sw2 half pwr = 214v that drops slowly down to 104v...then slowly up to 107v..then back down to 104v

    does this sound like a cap creeping up and down / discharging ??

    V8 / V9 never changed voltage during this , still around 0v at pin 2

  14. #49
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,278
    SW2 lowers B+ by using a different transformer tap.
    SW4 adds a high resistance into the cathode of two tubes to basically disable them.

    I can see no reason why either of these is causing your problem, which is DC voltage on two of your tube grids. You need to figure out where it's coming from. If it were me, I'd start unsoldering things connected to those tube pins until the voltage went away to try and determine where it's coming from. It may even require unsoldering socket pins and checking the voltage on the trace to see if the socket is somehow "leaking". Look for moisture. Look for solder blobs. Look for debris stuck under the socket, Look for a part lead shorting a trace, etc.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  15. #50
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Yes, one way to go could be to lift C45, R43, ,R76 and R78 ends towards their common junction point which in a healthy amp sets it free from everything else. If you still have that high voltage you clearly have a leaking pcb or something else shorting the trace on that junction. Then measure for continuity to other parts. I'd say for example "C" feeding the PI plates would be a logical start. If you find anything there you might initially want to isolate for example the B and C supplies from each other at the PSU end instead of unsoldering the sockets to narrow down which one is the culprit.
    Last edited by doombass; 04-18-2018 at 09:00 AM.

  16. #51
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    I did some testing tonight-- the following readings are with C45 and C46 removed from the amp pcb. I have it propped up so i can pull more parts easily now . No tubes in amp .

    So , at V5a plate 340v ---- V5b plate 340v

    the joint of C45 + R73 is 196v
    the joint of C46 + R74 is 0v

    r73/74 are going to ground like they are supposed to on other end.

    joint of R71 and V5a plate is 340v
    joint of R72 and V5b plate is 340v

    point C is 344v

    the switches are in full power and 4 tube mode

    im not seeing any solder blobs or anything ..

  17. #52
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Quote Originally Posted by Valvehead View Post
    I did some testing tonight-- the following readings are with C45 and C46 removed from the amp pcb. I have it propped up so i can pull more parts easily now . No tubes in amp .

    So , at V5a plate 340v ---- V5b plate 340v

    the joint of C45 + R73 is 196v
    the joint of C46 + R74 is 0v

    r73/74 are going to ground like they are supposed to on other end.

    joint of R71 and V5a plate is 340v
    joint of R72 and V5b plate is 340v

    point C is 344v

    the switches are in full power and 4 tube mode

    im not seeing any solder blobs or anything ..
    This confirms your previous measurements. That high voltage is still present. Now you need to begin checking why you have that voltage (post #49 and #50).

  18. #53
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Made some progress !! Got rid of that high v at the grids. It must have been a trace or the pcb ... I have some parts " flying " off the pcb now = C45 / R73 / 76 / 78 --going directly to the pin at v6.

    Now im measuring -2mv to +3 mv at all grids ..and the meter is just jumping up and down constantly

    the pcb is grounded where it needs to be and no tubes are in yet

    so without tubes im needing around 20mv or so yes?
    Last edited by Valvehead; 04-20-2018 at 03:05 AM.

  19. #54
    Junior Member
    Join Date
    Mar 2018
    Location
    Sweden
    Posts
    10
    Nice! I may be wrong but without tubes and healthy C45 and C46 I would'nt expect you to see much of any DC voltage at all on the power tube grids. Possibly your meter might detect a small amount of AC ripple from the DC supply depending on the efficiency of the filtering?

  20. #55
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Ok, here is where Im at. Amp up and running--no red plating,,passes signal fine .

    All tubes are in , but my grids arent settling down . So its in full pwr and 4 tube mode , at V6- V9 grids my meter is jumping around from 5mv up to 16mv...its all over the place on all tubes , but it settles on 12mv for about 2 seconds then moves up and down , fairly quickly . I re-checked my ground and tried a different meter .
    Any ideas ?

    When it cools down ill try a diff set of el84s i have

    Also that bad trace i bypassed runs directly under C3 (radial cap 33u/450v )...i wonder if that is going bad or leaking...

  21. #56
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,278
    You aren't running signal through the amp when you check grid voltage are you?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  22. #57
    Old Timer
    Join Date
    Feb 2007
    Posts
    1,275
    Quote Originally Posted by The Dude View Post
    You aren't running signal through the amp when you check grid voltage are you?
    nothing plugged in... controls at 0

  23. #58
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    5,278
    IMO, if it's just mV, it's not anything to worry about anyway, unless you hear noise.
    Helmholtz likes this.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  24. #59
    Senior Member TimmyP1955's Avatar
    Join Date
    Mar 2007
    Location
    Indianapolis
    Posts
    346
    When the power tubes are in, make sure the amp is connected to a load - without one it might oscillate.

  25. #60
    Senior Member SoulFetish's Avatar
    Join Date
    Jan 2016
    Location
    Massachusetts
    Posts
    603
    Quote Originally Posted by Valvehead View Post
    Ok, here is where Im at. Amp up and running--no red plating,,passes signal fine .

    All tubes are in , but my grids arent settling down . So its in full pwr and 4 tube mode , at V6- V9 grids my meter is jumping around from 5mv up to 16mv...its all over the place on all tubes , but it settles on 12mv for about 2 seconds then moves up and down , fairly quickly . I re-checked my ground and tried a different meter .
    Any ideas ?

    When it cools down ill try a diff set of el84s i have

    Also that bad trace i bypassed runs directly under C3 (radial cap 33u/450v )...i wonder if that is going bad or leaking...
    leaking electrolyte can absolutely cause instability/conduct voltages and could have been (could be) the cause of HV leaking on the grids. Often this can be tough to diagnose with radial caps. I've seen what looks like a complete breakdown on the underside of the caps with no apparent bulging or visual indication when looking over the top of the PCB. It can be easily obscured by silicone or a densely populated board
    g1 likes this.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

Page 2 of 2 FirstFirst 12

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Bias Problems in a New Build
    By Just Mike in forum Maintenance, Troubleshooting & Repair
    Replies: 18
    Last Post: 09-01-2014, 01:23 AM
  2. Bias circuit problems...
    By Colonel_Sanders in forum Maintenance, Troubleshooting & Repair
    Replies: 6
    Last Post: 06-18-2008, 10:05 AM
  3. TSL100 Bias Problems
    By bmd in forum Maintenance, Troubleshooting & Repair
    Replies: 2
    Last Post: 05-30-2008, 01:09 AM
  4. Bias Problems With Marshall DSL100
    By wakeskater in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 11-18-2007, 11:43 PM
  5. Torres bias kit problems
    By strat100 in forum Maintenance, Troubleshooting & Repair
    Replies: 11
    Last Post: 07-19-2007, 11:44 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •