Results 1 to 7 of 7

Thread: Mismatched power tubes

  1. #1
    Member
    Join Date
    Nov 2017
    Posts
    23
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0

    Mismatched power tubes

    Hi all,

    I just finished working on a Fender Blues Deluxe Reissue (new filter caps, rebiasing to factory spec). I'm sending a 1khz sine wave into the power amp input, and this is what I get on the scope:

    Click image for larger version. 

Name:	IMG_0434-1.jpg 
Views:	44 
Size:	927.7 KB 
ID:	54230

    One side of the waveform is clipping much sooner than the other. If I flip the power tubes, the top of the wave clips early instead of the bottom.

    Is this a case of mismatched tubes? Is this expected in tube amps? Is this considered normal operation, or is there something else I should be looking at?

    Thank you!
    Matt

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member eschertron's Avatar
    Join Date
    Aug 2012
    Location
    Great Black Swamp
    Posts
    2,164
    Thumbs Up/Down
    Received: 357/0
    Given: 914/1
    Rep Power
    9
    If the early clipping follows the tube, then the tube is at fault. I do not expect this behavior to be considered normal.
    What audio power do you get when adjusted right at the point it begins to clip?

    1 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
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


  3. #3
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    14,864
    Thumbs Up/Down
    Received: 1,911/4
    Given: 2,148/0
    Rep Power
    29
    To expand on this...

    You absolutely can expect asymmetry in most tube amps when clipping, but typically only when clipping hard. Hard as in pretty much a square wave. I mention it just so tubes won't be your first suspect when you see it under this circumstance. In this case, particularly since the asymmetry follows the tube, I'd agree with eschertron and call out the tubes. Is it normal? Nope. That's way too much mismatch. Even with "unmatched" tubes you'll never see that much difference. You've got a very weak tube there.

    0 Not allowed! Not allowed!
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

  4. #4
    Member
    Join Date
    Nov 2017
    Posts
    23
    Thumbs Up/Down
    Received: 7/0
    Given: 1/0
    Rep Power
    0
    Thanks folks. Threw in a pair of new 6l6s I had waiting for another project and it looks nice and powerful and symmetrical. Much appreciated!

    3 Not allowed! Not allowed!

  5. #5
    Member
    Join Date
    Nov 2013
    Posts
    262
    Thumbs Up/Down
    Received: 89/1
    Given: 24/0
    Rep Power
    6
    Quote Originally Posted by Drummer4gc View Post
    Hi all,

    I just finished working on a Fender Blues Deluxe Reissue (new filter caps, rebiasing to factory spec). I'm sending a 1khz sine wave into the power amp input, and this is what I get on the scope:

    Click image for larger version. 

Name:	IMG_0434-1.jpg 
Views:	44 
Size:	927.7 KB 
ID:	54230

    One side of the waveform is clipping much sooner than the other. If I flip the power tubes, the top of the wave clips early instead of the bottom.

    Is this a case of mismatched tubes? Is this expected in tube amps? Is this considered normal operation, or is there something else I should be looking at?

    Thank you!
    Matt
    That is the waveform of a single tube,the other is not conducting at all

    5 Not allowed! Not allowed!

  6. #6
    Senior Member SoulFetish's Avatar
    Join Date
    Jan 2016
    Location
    Massachusetts
    Posts
    1,104
    Thumbs Up/Down
    Received: 325/1
    Given: 284/0
    Rep Power
    6
    Quote Originally Posted by alexradium View Post
    That is the waveform of a single tube,the other is not conducting at all
    ^^^^^ exactly right. The audible symptoms are usually very low output volume and heavily distorted. Swap them out with some pulls and see if it fixes it. If so, then one of your tubes is shite.
    If it still looks like this, then dollars to donuts that one of the plate resistors in your phase inverter is open.

    2 Not allowed! Not allowed!
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  7. #7
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,405
    Thumbs Up/Down
    Received: 1,351/21
    Given: 3,337/8
    Rep Power
    22
    I think he wrapped it up in post #4.
    Good info anyway though.

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Replies: 5
    Last Post: 11-01-2017, 02:09 PM
  2. mismatched speaker impedance?
    By dmartn149 in forum Conversion Projects
    Replies: 9
    Last Post: 10-21-2016, 06:54 PM
  3. Mismatched coil
    By Marco78 in forum Pickup Makers
    Replies: 43
    Last Post: 11-30-2013, 06:32 AM
  4. Mismatched coils
    By David Schwab in forum Pickup Makers
    Replies: 14
    Last Post: 11-15-2013, 01:48 AM
  5. One power tube socket is not sending power to tubes in my Mesa Express 5:50
    By Freckles in forum Maintenance, Troubleshooting & Repair
    Replies: 17
    Last Post: 02-11-2012, 02:02 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
  •