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.

'68 SF Twin Reverb/Trem issue

Discussion in 'Amp Tech Center' started by Neener, Jan 17, 2021.

  1. Neener

    Neener Tele-Holic

    Posts:
    745
    Joined:
    Apr 19, 2019
    Location:
    Detroit
    '68 SF Twin Reverb (AC568)

    I'll try my best to describe the issues. Kind of a weird one

    - Footswitch works for both Trem and Reverb

    - Reverb does not work with guitar signal, but works when tank is pulled out and springs manually plucked

    - Reverb can be heard When Reverb is switched "off". It's quieter than when engaged, but with tank pulled out and springs manually plucked, reverb can be heard.

    - There is a loud Trem tick/buzz. The volume of the tick/buzz can be controlled with with Reverb pot
     
  2. BobbyZ

    BobbyZ Doctor of Teleocity

    Posts:
    13,415
    Joined:
    Jan 12, 2011
    Location:
    Snellman MN
    Tried swapping tubes yet? For reverb since the verb channel is apparently working try a different 12at7 (or 12au7) in the V3 spot, reverb driver. If that don't do it try another 12ax7 in the reverb recovery V4, seems less likely but try it anyways. Trem tube is V5 a 12ax7.
    Being a 68 I'd imagine it's had a "cap job"?
    If not that could be it. Sometimes people leave the cathode bypass caps and change the other electrolytic capacitors. Bias and filter caps can kill an amp, so they do those and leave the less critical cathode bypass caps. Well those innocent little (normally) 25v-25uf caps cause all kind of problems, especially with trem and reverb, plus they can be noisy like bad plate resisters.
    Also bad connections on the reverb and footswitch cables can be problematic. Particularly the ground, the outside part of the RCA connectors.
     
    Boreas, Wally and Neener like this.
  3. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Has the amp ever been serviced? Your reverb recovery is working, but the drive side isn't.

    If it hasn't been serviced, no point in trying anything else first.
     
    Neener, Boreas, Wally and 1 other person like this.
  4. Wally

    Wally Telefied Ad Free Member

    Posts:
    38,229
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    If you are hearing a return signal when you shake the amp...or pluck the springs 8n the tank, then the return circuit is working. Your problems would be in the send circuit....V3 as mentioned, the driver transformer, cable, or the tank. Do you have a tech? If not, there are countless sources that explain how to go about things. Go to RobbRobinette’s site and study up on th8ngs. Learn to read schematics. Prior to that, study the vacuum tube, its elements, and the functi9ns of those elements. And....as asked...has this amp been serviced recently? I know you just got it but don’t recall the details.
    now I remember...I suggested that you seek out corliss1 for tech work, didn’t I?
     
    Neener likes this.
  5. strat a various

    strat a various Friend of Leo's

    Posts:
    4,116
    Joined:
    May 9, 2008
    Location:
    Texas
    If you can find a similar amp, you can set them back to back, run your reverb cables to the other amp's tank. That will narrow down whether your tank is good. Maybe at a store or a friend?
     
  6. Neener

    Neener Tele-Holic

    Posts:
    745
    Joined:
    Apr 19, 2019
    Location:
    Detroit
    It's a brand new "MOD" brand tank, newer right angle RCA cables.

    To be honest I don't know the service history, I'll be opening dog house today.

    I have a set of new Filter caps, bias cap as well as bypass electrolytics. I wanted to hold off on the bypass because they are the original cardboard type, which I've heard usually don't fail.

    I'll need to order some tubes, I don't have any in stock. As Wally mentioned, I am a newbie to the amp repair world...

    2.jpg 1.jpg
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    So yeah, that all looks stock and untouched.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,903
    Joined:
    Dec 30, 2019
    Location:
    california
    @Wally I don't find a schematic with the 150 Ohm resistors on pin8 of the power tubes and the terminal strip on the left. The 1k5 grid resistors are only on two of the power tubes?
    I think you have seen quite a few more Fenders than I. Are you familiar with this schematic/layout?
     
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
  10. Wally

    Wally Telefied Ad Free Member

    Posts:
    38,229
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Yeah, Fender only used that circuit from May 1968 until the fall of 1969.

    I think the bias has been changed, has it not? One wonders if they ‘blackfaced’ the dropping resistances in the power rail? I believe I; a full evaluation of newly acquired amps. Good pictures here will let us note the changes, but the voltages and Biasing numbers demand hands on work by an experienced tech, imho....unless an owner is familiar with things such as the circuit and safety measures.
     
  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Those resistors and solder joints look pretty fendery to me :D - always hard to say without seeing it in person.
     
    Wally likes this.
  12. schmee

    schmee Doctor of Teleocity

    Posts:
    13,980
    Joined:
    Jun 2, 2003
    Location:
    northwest
    Also, clean the rca jacks for the trem and reverb. Clean the outside of the jack on the amp and the inside of the "grippers" on the cable. This often can cause reverb or trem issues.

    I had one fender trem that ticked and wouldn't start up but randomly. Eventually I just changed the opti-ocillator and it was fine then. The old one was visibly flashing but didn't work right.
     
  13. Wally

    Wally Telefied Ad Free Member

    Posts:
    38,229
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Right you are, corliss1. The pic in post #6, to which I had not paid much attention, shows the original balancing act as it was when it left the factory.
     
    corliss1 likes this.
  14. Milspec

    Milspec Poster Extraordinaire

    Posts:
    5,565
    Joined:
    Feb 15, 2016
    Location:
    Nebraska
    The 12AT7 tube works pretty hard in those amps, might not be the entire problem here, but do yourself a favor and install a vintage Jan-Phillips military grade 12AT7 in that socket....can make a world of difference on the Twins.
     
  15. Neener

    Neener Tele-Holic

    Posts:
    745
    Joined:
    Apr 19, 2019
    Location:
    Detroit
    Just realized I'll need an external cab to test amp when chassis is out.

    Total noob question but what ohm rating cab will work with the twin?
     
  16. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Why.........not use the speakers in the cab that you're removing the amp from???
     
  17. Neener

    Neener Tele-Holic

    Posts:
    745
    Joined:
    Apr 19, 2019
    Location:
    Detroit
    My work bench is higher than the speaker cable will reach. Or am I missing something completely obvious.
     
  18. jondanger

    jondanger Poster Extraordinaire Silver Supporter

    Posts:
    5,895
    Joined:
    Jan 27, 2011
    Location:
    Charm City, MD
    Making a longer speaker cable would be my approach.
     
    Neener likes this.
  19. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    4,620
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    You'll need a coupler of some sort. Either:

    -solder on a longer cable to the speaker temporarily
    -make some alligator clip leads
    -make a little junction box with 2 1/4" jacks that are wired together. Plug amp into one, plug speakers into another, so you can use as long a cable as you need on the amp side. I use mine all the time for testing. Image below:

    141275748_401299084500459_3515338746970552679_n.png
     
    Neener likes this.
  20. Neener

    Neener Tele-Holic

    Posts:
    745
    Joined:
    Apr 19, 2019
    Location:
    Detroit
    Great options thank you
     
    corliss1 likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.