Coupling Cap Value Change

Discussion in 'Amp Tech Center' started by KeepItSimpleTele, May 16, 2020.

  1. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    Got my feet wet working inside an amp solo today without my tech inclined friend. Grounded many points through a resistor and checked several points with a volt meter. Everything was under 1 volt dc so I jumped in....

    I replaced the first coupling cap on one of my 1960’s Traynor YBA-2A amps that had the 0.1 uf value with the more standard .047 uf value. It still works so that’s good. It’s more usable now for guitar without having the bass tone pot rolled almost off.

    My questions is.... would changing the other 0.1 mustard caps down the line to 0.047 as well change the tone significantly more? From my reading it’s mainly the first one that changes HPFiltering?
     

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  2. Commodore 64

    Commodore 64 Friend of Leo's

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    Fire up that soldering iron and replace those blue electrolytics right next to the one you just did.

    I think the .1uF downstream are used to make sure everything that's left gets through. I don't know how much tone-shaping they are doing, but wait for someone else who knows more than me to chime in.
     
  3. johnnylaw

    johnnylaw Tele-Afflicted

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    I did this also on a dark amp. Just the one was all that was needed to dry up and blow away the mud.
     
  4. dougstrum

    dougstrum Tele-Afflicted

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    The first coupling cap has the biggest effect controlling excessive bass. No harm changing other coupling caps to smaller value. I actually like .02 uf.
     
  5. guspac88

    guspac88 Tele-Meister

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    It's just like a gain stage in the sense that the earlier in the signal path you are, the most times the change will be amplified. Yes, changing others will continue the reduced bass effect, but probably not enough to warrant changing them rather than turning the bass knob down. Change whatever caps are old or leaking though!
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    My question is... do you want to learn about this subject or just change some caps?

    There are different philosophies as to which components would be changed to enhance highs or lows. Marshall cut lows in the first stages for a reason. Leo kept lows in his early stages.

    There are two typical ways to make changes. One is by changing RC filters. (You are changing a RC filter in your example). The other way is to change the preamp tube bias capacitor.

    I would suggest you search and read about both. There are handy calculators on the internet to help with knowing what cut off frequency to use.

    If you are not familiar with frequencies expressed as Hertz you should read up on that as well. Perhaps get a ten band Equalizer and become well versed in the function and how the different frequency bands affect the sound of an instrument. (Or a musical composition.)
     
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  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    When an amp goes into overdrive harmonic and intermodulation distortion can add low harmonics that will pass through the amp with those big .1uF coupling caps downstream. If heavy overdrive is too dark or muddy then I'd drop the phase inverter coupling cap down to .001uF and give that a try.
     
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  8. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    Yes...the bigger 40-50 watt amps from Traynor have the 0.022 uF in the first coupling cap. The smaller 15 watt 6v6 ones seem to have 0.047....or the huge 0.1 like mine had. I wonder if this is due to keeping more bass for the smaller powered amps?
     
  9. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    Although I’m mostly simply interested in taking out some of the flubby bass frequencies out of this Traynor 6v6 Bass Mate amp I’m now using for guitar.... I’d still like to pickup some knowledge along the way. I do find it interesting.

    I admit a lot of amp and tone circuit design is over my head right now....guys spend years and years learning this stuff. Still, even though it’s not my work field.. I’ll keep at it :)
     
    Last edited: May 17, 2020
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  10. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    In a nutshell...

    One theory is to reduce the bass frequency early. This is important in over-driven stages. The bass frequencies become too strong otherwise and the overdrive sounds muddy. The lower harmonics of the stronger now over-driven signal are added to the lessened clean bass signal. Rather than over-driven bass, the bass in the signal is relatively clean. The later stages are not restrictive on the bass so the bass can give girth to the over-driven signal without flub.

    Another theory is to reduce the bass in later stages. This works well in clean style amps. (Bass amps as well). The full sonic spectrum is amplified and fine tuned in the tone stack and the bass is reduced in the later stages.

    Of course there is middle ground.

    I have looked at schematics for the Traynor YBA-2A. There are several different versions. I have not found the version you have described. I am a little concerned that some of the advice you have been given in this thread has been offered without knowledge of the circuit you have in front of you.

    Of course trial and error are your friend as you change the bass frequencies to your ear.

    You may want to consider changing the cathode capacitor of the preamp tube to between 0.68uF to 4.7uF. This is a different way to reduce bass frequencies.
     
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  11. Nandrewjackson

    Nandrewjackson TDPRI Member

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    I did a quick search for yba-2a schematic and the most legible one I could find is a yba-2a/b with 6bq5 output tubes. You say yours is 6v6?

    Is this schematic correct?
     

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  12. Ten Over

    Ten Over Tele-Holic

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    I think he has that one that only uses half of the first preamp tube and has a paraphase splitter with cathode biased 6V6's. I've never seen a schematic for that one.
     
  13. Commodore 64

    Commodore 64 Friend of Leo's

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    Paraphase is nice. That's a sweet old amp right there man.
     
  14. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    I actually have both versions :) I’m happy now with the 6v6 version with the lowered coupling caps down to 0.022uF. That seemed to be all that amp needed.

    I am having more of an issue with the EL84 version of the YBA-2. It has an ugly sounding low blocking distortion type sound when the volume is up past noon and I hit a chord hard. I dropped the coupling capacitors and it didn’t cure it. I put in a 12Ay7 and it seemed to help a bit...but I think it just made the amp quieter and thus put off the same issue until further up the volume dial.

    I also changed all the tubes but having same issues
     
  15. Ten Over

    Ten Over Tele-Holic

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    The EL84 version that nandrewjackson posted has a ridiculous amount of gain for EL84's with no NFB. Can you verify that your amp is configured like that?

    I have a schematic for a YBA-2 with 6V6's, but I have never posted it because I am not confident enough about the component values. If you could be so kind, can you note the differences between your amp and my schematic?

    Traynor YBA-2 6V6.png
     
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  16. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    Yeah...the EL84 version really doesn’t sound great when it starts to distort past noon. With pedals for overdrive distortion is sounds great though :) I’d don’t know where to go with this amp. I want to be able to open it up more but it’s just collapses under itself past noon.

    Your schematic for the 6v6 version looks right to me. Some came with stock 0.1uF first coupling caps... and others have the 0.047 value. I tend to change them all to 0.047 or even 0.022 for guitar though. The 6v6 versions sounds great at any volume setting.
     
  17. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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  18. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    You may need some grid stoppers to tame the blocking dist.

    Princeton and 5E3 seem to like 470k to 1M. If you have a 1M pot or trimmer, you can use it as a variable resistor. Attach it as close as possible to V1 pin2 after the 1M resistor.
    That might do it.

    Rob Robinette calls it a sweet spot mod.
     
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  19. KeepItSimpleTele

    KeepItSimpleTele TDPRI Member

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    Yes this is the schematic. It’s the same as the one pasted to the chassis by Traynor :)

    Here is the gut shot....but I have replaced the filter cap with a 50/50 500v and the first two coupling caps with 0.022uF for guitar use.
     

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