Results 1 to 7 of 7

Thread: Univox U45b Tremolo

  1. #1
    Member
    Join Date
    Jun 2012
    Posts
    162
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    8

    Univox U45b Tremolo

    Hi folks, I have a U45b on the bench, sounding pretty good after replacing a handful of drifted resistors and leaky caps. Ditched the grid leak bias on V1 for a more familiar cathode bypass, and I'm happy with the results.
    I also slowed down the tremolo, it's nice and deep but imparts some very harsh distortion. Voltages look ok, I've tried swapping tubes, adding a grid stopper to the PI, etc. Can anyone suggest a remedy?
    I don't have a scope to play with, unfortunately.
    Had good luck with the Paul C Mod on another bias-vary-trem amp with a similar problem, but I am unsure how the 6BM8's would react to the higher cathode voltage?
    Click image for larger version. 

Name:	3324d1225222064-u45_orig.jpg 
Views:	490 
Size:	65.4 KB 
ID:	26228

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 534/3
    Given: 289/0
    Rep Power
    26
    Is the distortion apparent at lower volumes?
    The reason I ask is the fact that the Tremelo, at higher volumes, can drive the output into distortion.
    If you are playing clean at a loud volume and then engage the Trem, the Trem can put the signal 'over the edge'.

    0 Not allowed! Not allowed!

  3. #3
    Member
    Join Date
    Jun 2012
    Posts
    162
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    8
    Yeah it's only at higher volumes. Is there a way to tame the distortion while retaining the trem depth?

    0 Not allowed! Not allowed!

  4. #4
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 534/3
    Given: 289/0
    Rep Power
    26
    The Trem is driving the amp into clipping.
    That is a consequence of the design.
    You are most probably playing right on the edge of distortion & the Trem is taking it over the top.
    If only the bass notes are driving the amp into clipping when the Trem is engaged, you could try a lower value cap in C15.

    0 Not allowed! Not allowed!

  5. #5
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,160
    Thumbs Up/Down
    Received: 1,890/7
    Given: 0/0
    Rep Power
    55
    Or maybe reduce R21?

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  6. #6
    Member
    Join Date
    Jun 2012
    Posts
    162
    Thumbs Up/Down
    Received: 1/0
    Given: 0/0
    Rep Power
    8
    Good suggestions. After swapping a few values, I'm realizing it's a fine balance between taming the clipping, and losing the lower speed trem entirely.
    Too bad, the amp is really impressive otherwise!

    0 Not allowed! Not allowed!

  7. #7
    Bent Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    15,448
    Thumbs Up/Down
    Received: 2,426/4
    Given: 2,807/0
    Rep Power
    30
    Is this a garbled, phasey sort of distortion or just clipping? I ask because the trem takes the bias in both directions. Not only can it cause the power tubes to clip, but it can also cause them to cut off. In fact it SHOULD do both when a high intensity is used. Check the actual bias of the power tubes. Experiment within a safe bias range and the trem intensity and hopefully find a sweet spot.

    0 Not allowed! Not allowed!
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "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

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: 05-08-2013, 03:56 PM
  2. Univox u45b no tremolo bad output trans
    By davohilts in forum Vintage Amps
    Replies: 0
    Last Post: 05-07-2013, 07:43 PM
  3. Replies: 3
    Last Post: 07-07-2012, 09:08 PM
  4. Univox u45b video!
    By tribaltattoos in forum Vintage Amps
    Replies: 5
    Last Post: 04-02-2010, 06:03 AM
  5. Univox U45B adding a line out
    By darrellcheng in forum Mods & Tweaks
    Replies: 3
    Last Post: 03-27-2008, 06:04 PM

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
  •