Quick question

Discussion in 'Shock Brother's DIY Amps' started by Bitsleftover, Oct 8, 2019.

  1. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Can a bad coupling cap cause a high pitch squeal at higher gain?
    Thanks
    Mick


    Sent from my iPhone using Tapatalk
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

    Posts:
    3,299
    Joined:
    Sep 13, 2008
    Location:
    Lansing, MI
    Yes? No? Maybe? A bad coupling cap would be leaking DC into some area it shouldn't. What that would cause in the particular circuit you're working with is hard to say. Usually the result is static on a pot since there is DC as you turn the control.
     
    King Fan likes this.
  3. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Thank you.


    Sent from my iPhone using Tapatalk
     
  4. muchxs

    muchxs Doctor of Teleocity

    Posts:
    11,561
    Joined:
    Aug 14, 2004
    Location:
    New England
    High pitched squeal is usually a microphonic 12AX7.

    Turn the amp down. Tap the tubes.
     
    King Fan and Wally like this.
  5. Wally

    Wally Telefied Ad Free Member

    Posts:
    32,852
    Joined:
    Mar 17, 2003
    Location:
    Lubbock, TX
    Or microphonic pickups...or being too close to the amp?
     
  6. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Tried swapping all tubes out. Nil fix.
    I chopsticked around and thought I’d solved it. But it came back just as mysteriously as it started.
    I’m working away this week so I’ve had to walk away from it. Which is good, because I was pulling my hair out.
    A fresh set of eyes next week I think.
    Thanks for your help.


    Sent from my iPhone using Tapatalk
     
  7. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Bizarrely, it only happens on the bridge pickups of a few guitars. One Tex Special Strat and a potted humbucker les Paul. Both of which behave through other amps.
    Thank you for the reply


    Sent from my iPhone using Tapatalk
     
  8. muchxs

    muchxs Doctor of Teleocity

    Posts:
    11,561
    Joined:
    Aug 14, 2004
    Location:
    New England
    High pitched squeal is always a possibility in high gain amps. there are certain high gain designs that are never built as combos for that reason. You want to keep the speaker away from the tubes.

    Some combinations are just over the top. They're going to whistle like a litter of hungry piglets. A good recipe for whistle is to stick a pedal in front of a high gain amp. For example a JCM800 with an old Distortion + in front of it will whistle.



    Listen to the guitar part around 2:25. It's trying to whistle. I'll bet they cleaned the guitar track where they could.
     
  9. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Hahaha! Love that one!

    I’ve turned a Marshall class 5 into a 1959slp preamp feeding the class 5 single ended power amp.
    Only having room for one input jack, I hard wired both normal and bright channels in parallel.
    The normal channel is fine from zero to 10.
    The bright channel is sometimes ok from 0-10 but sometimes whistles at around 9.
    With normal on around 7 and bright around the same (unfortunately the start of the sweet spot) the whistling starts and gets worse up to 10.

    I can change the pitch of the whistle by chopsticking around the v2a cathode (.68 bypassed) jumper wire. But I can’t get it to go away. Or certainly not stay to away.
    Because it is a mutilation of a totally different pcb, the layout is less than ideal and there’s not much to be seen nor can be traced from above.
    The PCB is very fragile and I burned a few traces trialing different values caps and resistors so there are a few “compromises” in quality of work which Id rather stab my self in the eyeball with my soldering iron than share with you guys!
    But the main one is that the V2b .68 bypass cap is connected and grounded right next to the wires that go to the tube. The ones that change the pitch of the whistle.
    I think I’ve answered my own question and I think this story will end with me replacing the pcb with a freshly laid out turret board which is what I should have done from the start. At least I’ve done all my tweaking now and have proven the concept.


    Sent from my iPhone using Tapatalk
     
  10. muchxs

    muchxs Doctor of Teleocity

    Posts:
    11,561
    Joined:
    Aug 14, 2004
    Location:
    New England
    Ah, so!

    ThisRemindsMe.png



    Here's what you need to do:

    Change that bypass cap. You may be able to get rid of your oscillation with a 1uf in there. Or 2uf.

    I suppose you're bypassing a 2.7k resistor. As you go up with the cap, go down with the resistor. You can change just the cap if it does the trick. If you want to go belt and suspenders go 1.8k- 2.2k bypassed with a 2uf (or so) cap.

    You can also use a small value cap across the plate resistor for a snubber. A .005uf will almost certainly kill your oscillation. You could start at .005 and go smaller until the piglets come back.



    Yup. That'll do it.
     
    Bitsleftover and Wally like this.
  11. printer2

    printer2 Poster Extraordinaire

    Posts:
    6,314
    Joined:
    May 24, 2010
    Location:
    Canada
    I had an amp squeal at maximum gain, turned out to be capacitance around the input jack, don't remember more than that. You might try temporally shielding parts/wires to try and pin it down. Of course insulate where in danger of contacting live voltages.
     
    Bitsleftover likes this.
  12. muchxs

    muchxs Doctor of Teleocity

    Posts:
    11,561
    Joined:
    Aug 14, 2004
    Location:
    New England
    Bah! Chokes 'n' snubbers' Snubbers 'n' chokes.

    :D :D :D

    I just tore down some sort of old UHF junk. Chokes 'n' snubbers everywhere.

    :cool:
     
    Bitsleftover likes this.
  13. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    Thank you so much for your help! I really appreciate it.
    Something thing just occurred to me when you mentioned a cap across the plate resistor.
    I changed the signal flow through the amp to move the tone stack to after a cathode follower.
    In doing so I’ve changed the flow from
    V1a-V1b-V2a-V2b
    To
    V1a normal\
    V2b-V2A
    V2a bright/

    Which means V1a is picking up its B+ from the right spot. But My bright channel input stage (V2a is picking up its B+ from the wrong side of 10dropping resistor.
    Likewise my next gain stage is picking its B+ up from the same point as my normal channel 1st stage.
    I can rectify that easily when I get home if that would make a difference?
    I’ll edit a schematic later tonight to illustrate invade my rambling isn’t very clear.


    Sent from my iPhone using Tapatalk
     
  14. muchxs

    muchxs Doctor of Teleocity

    Posts:
    11,561
    Joined:
    Aug 14, 2004
    Location:
    New England
    Yeah. Sort that out.

    :cool:
     
  15. Bitsleftover

    Bitsleftover Tele-Meister

    Age:
    46
    Posts:
    219
    Joined:
    Oct 17, 2016
    Location:
    Cheshire UK
    This is the current state of play.[​IMG]


    Sent from my iPad using Tapatalk
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.