Results 1 to 19 of 19

Thread: 6bm8 Clipping question

  1. #1
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0

    6bm8 Clipping question

    In looking at a friends very low end looking SE 6BM8 amp that sounds bad, the sine wave coming out of pin 9 looks good. Looking at pin 3, the top of the wave is clipped way off. The resistors in the circuit measure ok and bypassing the coupling cap with a good one didn't help.
    The attached schematic isn't the actual circuit but similar. There is no gird stopper on pin 3 in the circuit that I'm working on. Any ideas on what's likely to cause this? Thanks
    Click image for larger version. 

Name:	6bm8 clip.png 
Views:	37 
Size:	44.5 KB 
ID:	49286

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,278
    Thumbs Up/Down
    Received: 56/0
    Given: 21/0
    Sounds to me as if the coupling cap is leaking.

    2 Not allowed! Not allowed!

  3. #3
    Supporting Member mozz's Avatar
    Join Date
    Oct 2015
    Location
    NEPA
    Posts
    564
    Thumbs Up/Down
    Received: 40/0
    Given: 27/0
    Bypassing the coupling cap is not gonna prove anything, unless the original was open, which isn't the usual failure mode. You have to take the old one out of the circuit, because it's probably leaky.

    2 Not allowed! Not allowed!

  4. #4
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Thanks. I replaced the cap and it made no change. Any other ideas? Thanks again!

    0 Not allowed! Not allowed!

  5. #5
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    10,056
    Thumbs Up/Down
    Received: 231/0
    Given: 432/0
    Can you post the DC voltages for the second 6bm8 stage for the pins shown on your pic.

    0 Not allowed! Not allowed!
    The voices in my head are idiots!

  6. #6
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Here are some voltages- thanks:
    edit: the pot isn't 22k. Also, the 22k Rk seems big so I tried a range of smaller values & it didn't help.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Crappy 6bm8 ckt.jpg 
Views:	31 
Size:	1.67 MB 
ID:	49317  

    0 Not allowed! Not allowed!

  7. #7
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,278
    Thumbs Up/Down
    Received: 56/0
    Given: 21/0
    I take it that you do not have another 6BM8 to try.

    If the signal looks good coming out of pin 9 & it looks bad at the 6BM8 grid, then something internal to the output tube may be bad & it is affecting the signal.

    You could try removing the cathode bypass capacitor.

    0 Not allowed! Not allowed!
    Last edited by Jazz P Bass; 06-15-2018 at 03:28 PM.

  8. #8
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,803
    Thumbs Up/Down
    Received: 42/0
    Given: 127/1
    Is that "4v with the arrow" the bias voltage? Measure across the 180R cathode resistor. Also, what's the AC signal that you are applying?

    If the signal coming off the triode's plate is OK, no need to look at that stage any further.

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  9. #9
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,253
    Thumbs Up/Down
    Received: 19/0
    Given: 2/0
    Does your output tube grid have a path to ground via the trem circuit?

    0 Not allowed! Not allowed!

  10. #10
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,278
    Thumbs Up/Down
    Received: 56/0
    Given: 21/0
    See if this Gibson schematic is close: GA5 T_ 6BM8.pdf

    0 Not allowed! Not allowed!

  11. #11
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,803
    Thumbs Up/Down
    Received: 42/0
    Given: 127/1
    The hand-drawn schematic shows 180R for the pentode's cathode resistor, but the two printed drawings show bigger values (680R and 750R). The valve data sheet I looked at suggested -16v bias for 200v plate-to-cathode. How close to these design values does this amp operate?

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  12. #12
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Jazz P Bass- I have tried 3 known good tubes with exactly the same results. I'll remove that bypass cap & give it a try.
    eschertron - The 4 v is the grid to cathode voltage. The voltage across 180R is also 4 v. The AC input signal to the amp is 110Hz at about 1/2 volt. I vary the input to the 6bm8 via the volume control and it clips at almost any audible volume. I'll check exactly when it clips. I'll also check at higher Hz.
    Mick Bailey: - I couldn't figure out the term ckt last night but I'm guessing it supplies a grid leak path to ground. I did clip a 1M to ground on the grid as a check and it made no change at all.
    Jazz P Bass - thanks for the schem, that will help me figure out the tremolo ckt.
    eschertron- The printed schematic isn't the actual circuit but similar. I used it as a pin reference for discussion. Sorry if it caused confusion. The hand drawn is the actual circuit.
    -4v bias is quite small compared to -16. I will muck with the pentode bias & see what happens but I wasn't thinking that would clip the input to the pentode.

    I don't understand blocking distortion & need to read up on it. Could that be involved?

    Thanks everybody. After trying your ideas, I think my next move will be to disconnect the trem and supply a grid leak at pin 3.

    0 Not allowed! Not allowed!

  13. #13
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,278
    Thumbs Up/Down
    Received: 56/0
    Given: 21/0
    That 110Hz is a tad lowish.
    Try 1K as an input frequency.

    0 Not allowed! Not allowed!

  14. #14
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,803
    Thumbs Up/Down
    Received: 42/0
    Given: 127/1
    +1 ^^^
    the lower the frequency - and 110Hz is more than a tad lowish - then the more easily blocking happens. So the test freq could be a factor.

    edit: I just saw "input 1/2v". What's the signal AC voltage on the plate of the driving triode? If it's more than about 2.5vac, you will be driving the pentode into grid limiting.

    I looked again at the data sheet for the 6BM8. By your numbers it's dissipating about 3.3W, close to the design of 3.5W. So my suspicions of a 'wrong value' cathode resistor are probably unfounded.

    0 Not allowed! Not allowed!
    Last edited by eschertron; 06-15-2018 at 09:03 PM.
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  15. #15
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    Thanks for the help everyone. I tried the above suggestions & didn't find anything new. Distortion was the same At 1k Hz as 110 Hz. Removing the bypass cap didn't change it. Adding a 1M grid leak and removing the tremolo circuit let the amp get a little louder before sounding bad. I'm done with this one! It's useful for one overdriven sound and that's it. Thanks again.

    0 Not allowed! Not allowed!

  16. #16
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,803
    Thumbs Up/Down
    Received: 42/0
    Given: 127/1
    With the caveat that there may yet be something in the amp that aught to be fixed, I've found that for my champion 600 reissue (and by extension all other SE designs) anything less than that at screaming for mercy distortion they sound kind of meh. Good for a lo-fi application like blues harp, garage rock, or Neil Young.

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  17. #17
    Old Timer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    1,767
    Thumbs Up/Down
    Received: 171/20
    Given: 24/5
    Quote Originally Posted by eschertron View Post
    With the caveat that there may yet be something in the amp that aught to be fixed, I've found that for my champion 600 reissue (and by extension all other SE designs) anything less than that at screaming for mercy distortion they sound kind of meh. Good for a lo-fi application like blues harp, garage rock, or Neil Young.
    They really shouldn't have a volume on them since they sound best at their loudest.

    nosaj

    1 Not allowed! Not allowed!

  18. #18
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    1,803
    Thumbs Up/Down
    Received: 42/0
    Given: 127/1
    Quote Originally Posted by nosaj View Post
    They really shouldn't have a volume on them since they sound best at their loudest.

    nosaj
    Oh, yeah! And lift the tone stack for extra 5F1-ish-ness!

    0 Not allowed! Not allowed!
    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas

  19. #19
    Member
    Join Date
    Sep 2009
    Location
    Santa Cruz
    Posts
    85
    Thumbs Up/Down
    Received: 2/0
    Given: 0/0
    I soldered the guitar to the cable. Soldered the cable to the grid. Bypassed all the tone and volume circuits, shorted the power switch so it's always on, soldered the plug to the wall socket, removed the pilot light so it didn't suck power and now it sounds great!

    2 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Eric Barbour's 6BM8 One-Tube Reverb Imagined...
    By dchang0 in forum Theory & Design
    Replies: 56
    Last Post: 09-12-2016, 07:53 PM
  2. Push-pull amp using 6BM8/ECL82 tubes
    By ZehQuait in forum Build Your Amp
    Replies: 2
    Last Post: 11-12-2013, 05:31 PM
  3. 6BM8 tubes... what to use them for!
    By Bruce / Mission Amps in forum Theory & Design
    Replies: 81
    Last Post: 01-29-2013, 03:01 AM
  4. 5F1 Champ w/ 6BM8 One-Tube Reverb Pics
    By dchang0 in forum Build Your Amp
    Replies: 1
    Last Post: 12-01-2010, 02:09 AM
  5. Schematic for Guyatone GA-430?? (6BM8 reverb circuits?)
    By cgarai in forum Schematic Requests
    Replies: 2
    Last Post: 10-14-2010, 06:35 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
  •