Coupling Caps-Always The Same Value?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by skeeterbuck, May 8, 2015.

  1. skeeterbuck

    skeeterbuck Member

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    I'm working on an amp and thinking of raising the value of the coupling caps from the phase inverter to the output tubes. The current values are .02 uf. If I change them both to .047 and it adds too much bass, can I change one back to .02 and leave the other one at .047?

    Is there a reason that their usually always the same and if so what?

    Thanks!
     
  2. mark norwine

    mark norwine Member

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  3. skeeterbuck

    skeeterbuck Member

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    Thanks for the link Mark. What do you think about keeping both the same value?

    What would happen if I changed just one of them to maybe .01uf? Do you think that I would even be able to tell the difference?

    Thanks!
     
  4. diagrammatiks

    diagrammatiks Member

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    it really depends on the circuit.
    There's no reason to have both output coupling caps be the same value but since push/pull amps have each pair doing half the work, if one half is significantly different then the other half it could sound weird.
     
  5. jay42

    jay42 Member

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    Hmm...read Randall Aiken's article on blocking distortion.
     
  6. pdf64

    pdf64 Member

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    RC coupling doesn't just roll off low end, it also advances the phase around and below the break point, reaching a max of 90 degrees.
    If either side of the balanced pair are having significantly differing phase shifts applied, then when they are re-combined in the OT primary, there may be some cancellation (at low frequencies).
    So the power amp may be working hard driving into itself with little end result.
     
  7. J M Fahey

    J M Fahey Member

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    That's the point.
    Since in PP amps you first split the signal in 2 halves and reconstruct it at the end, in the center tapped output transformer, in principle you want perfect symmetry all along the way, or as close as you can get.
     
  8. skeeterbuck

    skeeterbuck Member

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    So if i'm reading you all correctly, it may make a difference, but it also may not. :huh

    I think to play it safe, if I do change them, I'll change to both to the same value.

    Thanks!
     
  9. donnyjaguar

    donnyjaguar Member

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    You can always increase the values of the resistors after the coupling capacitors to decrease your low-pass frequency. For instance you often see 220k and 270k values here. But you could just as easily use 470kΩ.

    The reality is however, the high-pass function of this circuit chooses values to keep lower frequencies out of the output transformer to begin with. These can sap-power, cause instability and generally not add much to the music.
     
  10. skeeterbuck

    skeeterbuck Member

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    Thanks for your input donny. So a higher value resistor after the cap reduces the bass response similar to increasing the cap value.

    I was curious to see what some other classic amps used. If I'm understanding correctly the Fender and Marshall designs are allowing the most lower frequencies to the output transformers.

    Tweed Bassman 0.1/220K (most all fender amps)
    Ampeg Gemini 0.1/270K
    Ampeg Jet .02/270K
    1954 GA-40 .05/470K
    63 Falcon .02/220K
    JTM 45 0.1/220K
     
  11. pdf64

    pdf64 Member

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    On the face of it, that seems to be wrong; rather as the R term is increased, the RC filter roll off frequency will move down.

    You may find this calculator helpful http://www.ampbooks.com/mobile/amplifier-calculators/pi-pa-bass-response/
    Different power tube types and bias arrangements have differing limiting values of max grid leak resistor; its widely breached by many guitar amps (eg 6V6 and 6L6 in fixed bias it's 100k) but should be kept in consideration.
     
  12. donnyjaguar

    donnyjaguar Member

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    Negatory. The higher value resistors would allow more bass content to hit the output tubes.

    In hi-fi tube amps you sometimes see 470kΩ used with 220n, but keep in mind the design criteria in those is generally 20Hz-20kHz. A guitar open E is 82Hz so no point aiming for 20Hz here. :)


     
  13. skeeterbuck

    skeeterbuck Member

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    Yea donny, I gathered that after looking at the link that pdf64 had in the previous post.

    So that leads me to this... If I want to increase the bass response would I either increase the caps to say from .02 to .047 or .1uf? Increase the resistors from 220k to 470k or do I need to do both?
     
  14. Silent Sound

    Silent Sound Member

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    I would start off with just the coupling caps. When tweaking an amp, it's usually best to just change one thing at a time.

    That being said, if you're trying to increase bass, you should be careful. You might have other bottlenecks in the way that's strangling the bass and simply allowing more bass to pass through one area might result in more muddy and indistinct bass coming out the speaker. So rather than having a fuller, bassier sounding amp, you could wind up with an amp that sounds even worse with the bass than before. For instance, if your OT can't handle it, upping those coupling caps would be a bad idea.

    But for the price of a pair of coupling caps, I'd say give it a shot. And might as well order some of those resistors while your at it, since you'll likely pay more for shipping those resistors than what they actually cost. But I wouldn't change them all out at once.
     
  15. skeeterbuck

    skeeterbuck Member

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    That sounds like sound advise Silent Sound. (pun intended) :rimshot

    I have some .047 and .1 caps on hand so I'll give them a try first. Thanks!
     
  16. skeeterbuck

    skeeterbuck Member

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    Well, I tried out some 0.047uf caps in place of the .02 ones and it added some nice minor boost to the low end. Seemed to balance the tone more evenly. Then I thought I try a pair of 0.033uf I had. Didn't hear much of a change from the original 0.02's. Finally I swapped them out for 0.1's and even though it didn't really seem to add much if any more low end than the 0.047's did, it sounded like it was getting somewhat muddy, so I switch back to the .047's. I think for now, I'll keep them in and leave the 220K resistors as is.
     

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