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Thread: Yet another post about Cathode Caps!!

  1. #1
    Member PositiveNegativeMan's Avatar
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    Yet another post about Cathode Caps!!

    Sorry to do this, but I'm still new to building.

    So far I've been very conservative in my approach, so in my preamps I have been using 100K anode loads, 1.5K cathode resistors and 22uF cathode caps. 12AX7s and HT values in the region of 325 to 360V. Nothing ground breaking but it works, and so far everything has been usable and sounds passable.

    Now I know that changing the value of those caps will change the response of the stage, but I was curious as to how much of a cap value change would be needed for a noticable difference?

    What I mean is suppose I have a 22uF cap on a stage and I decide I want it more bassy. I could put a 47uF cap in parralel to give 69uF, but how much difference is that in the real world?

    Is it something detectable, but only by men in white coats in labs with specialist measuring equipment? Or by an experienced musician with a trained ear? Or would the difference be so massive anybody would notice instantly?

    Likewise if I swap the 22uF with a 4.7uF, I know the stage emphasises more treble, but would I need ro be looking at 0.47uF for it to be noticable? Or would 10uF be enough? Or would only bats notice the difference?

    I know the best way is to experiment and listen, and believe me I will be doing so, but before I do, I would like a clue as to what I should be experimenting with to get a difference that can be "heard", not just "measured".

    I did try looking up some bass amp schematics on the web to see if they could give me a clue,but the only ones I could find to my surprise all used 22uF caps on preamp cathodes, so I am now totally confused as to the effectiveness of changing the cathode cap value to alter frequency response.

    Sorry again for a post that may seem like repetition to some!!

  2. #2
    Senior Member Enzo's Avatar
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    The simple answer is "try it." With a low value in the circuit, you can tack larger ones in parallel to experiment. Hard to parallel a 22uf down to anything... Up is easy.

    In general, I think at least a factor of 2 if you want to hear anything. Off the top of my head, I am not so sure you will get any more bottom from your guitar above 22uf. From 22uf I'd try 4.7, sure or even 1uf. Plenty of amps out there with a .68uf bypass on a stage.

    Same thing with plate load resistors, 100k to 120k won;t make much difference. The step up to 220k from 100k is a natural change to try. Of course any change needs to be considered in the context of the whole circuit.
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    Quote Originally Posted by PositiveNegativeMan View Post
    Sorry to do this, but I'm still new to building.

    So far I've been very conservative in my approach, so in my preamps I have been using 100K anode loads, 1.5K cathode resistors and 22uF cathode caps. 12AX7s and HT values in the region of 325 to 360V. Nothing ground breaking but it works, and so far everything has been usable and sounds passable.

    Now I know that changing the value of those caps will change the response of the stage, but I was curious as to how much of a cap value change would be needed for a noticable difference?

    What I mean is suppose I have a 22uF cap on a stage and I decide I want it more bassy. I could put a 47uF cap in parralel to give 69uF, but how much difference is that in the real world?
    No cathode cap increase on earth is going to make a difference on the LF side, because 22 uF is already more than enough for the cathode to be completely bypassed, so the lower cutoff frequency of your stage is currently in the infrasonic region ( I'd say around 5 Hz ). You could make it sound LESS bassy by DIMINISHING the bypass cap value, but, to get an AUDIBLE difference, you'd have to move to values around 2 uF or less, as, with some 4,7 uF the lower cutoff freq is still below the audible range, while with a 3,3 uF cap it moves in the 20 Hz region ( so it still doesn't make any difference unless your guitar is tuned VEEEERY low ).

    Hope this helps

    Best regards

    Bob
    Hoc unum scio: me nihil scire.

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    I'm just a bystander... but I mix a lot of audio.

    I'm curious if anyone thinks there is any merit to the idea that raising the cut off point might relieve the amp of the necessity of wasting energy on infrasonic content.

    Is it possible that raising the cutoff point will have the perceived effect of increasing the output of the lower frequencies?

    That's a common notion when mixing multiple tracks together.

    best regards,
    mike

  5. #5
    Supporting Member Chuck H's Avatar
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    You will hear the difference with values up to 10uf. But the audible difference between a 3.3uf and a 22uf are quite small. In fact, I never use the values between 3.3uf and 22uf because all I get for it is wolfy notes and flat spots on the low E string. As in, using a 6.8uf seems to seems to give me all my low end overtones on the A string but the E sounds a little weak. That's not useful for me so the values I typically use are the ones that seem to make a USEFUL audible difference and they are: NONE, 1uf, 2.2uf, 3.3 uf and 22uf.

    The difference between the 2.2uf and 3.3uf is subtle but very useful when voicing the midrange character. By the time you get to 4.7uf you get into that dicey area between the A and E string notes and it's best to just fully bypass with a 22uf IMHE.

    Here's a chart that shows the effected frequencies for some bypass cap values. But the audible effect to my ears is a little different.

    Chuck
    Attached Thumbnails Attached Thumbnails bypasscap.pdf  

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    Here is an alternative graph showing how different values affect the frequency response. Notice that halving the capacitance shifts the response up by exactly one octave.

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    Quote Originally Posted by Merlinb View Post
    Here is an alternative graph showing how different values affect the frequency response. Notice that halving the capacitance shifts the response up by exactly one octave.
    Hi Merlin,
    very useful, there's only a small typo in it, the value of the cap "before" the 10 uF curve is 47 uF, should be 4,7 uF IMHO.

    Cheers

    Bob
    Hoc unum scio: me nihil scire.

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    Quote Originally Posted by Robert M. Martinelli View Post
    Hi Merlin,
    very useful, there's only a small typo in it, the value of the cap "before" the 10 uF curve is 47 uF, should be 4,7 uF
    Oops! You're right, I have corrected it.

  9. #9
    Supporting Member tubeswell's Avatar
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    Mostly what the others have said. I find that with a guitar amp, 2.2uF on the first stage will cut out real deep bass that 22uF clean fender sound otherwise allows, but 2.2uF still allows a decent bottom E string chug on a combo with a single 12" speaker. ( I must qualify this by saying I build my combos on the slightly larger side of normal, so they are naturally more bassy anyhow). I can't hear any difference between 10uF and 20uF (or even between 4.7uF and 20uF) on V1, let alone 60 or 70uF. (But its different when you're doing a reverb recovery stage on a fender stand alone reverb for some reason, where the stock value is something like 250uF)

    Why not wire up your V1 cathode bypass stage with 2.2uF and then add a switchable (say) 22uF in parallel and experiment for yourself (put the switch on the ground side of the parallel cap)
    Building a better world (one tube amp at a time)

  10. #10
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    Quote Originally Posted by mike_mccue View Post
    ...I'm curious if anyone thinks there is any merit to the idea that raising the cut off point might relieve the amp of the necessity of wasting energy on infrasonic content.

    best regards,
    mike
    Hi Mike,
    I'm not sure I'm getting what you meant with the above statement, maybe due to my rather poor grasp of the English language, if so, bear with me....

    For a guitar amp, amplifying ( or trying to amplify ) anything in the infrasonic range is useless and, sometimes, even "harmful", as the amp often loses definition and character. ( The speakers' resonant frequency has to be kept into account as well ), so, yes, I actually think amplifying infrasonic frequencies is, to say the least, a "waste of energy" in a tube guitar amp.

    Back to the thread's main topic,
    he asked if it was possible to add "more bass" by increasing the bypass cap's value, so what I tried to explain him in my previous post is that, being the cathode fully bypassed already, there was no chance to add "more bass" by further bypassing that cathode with a larger cap.

    I also tried to emphasize the fact that a ( gain ) stage's "voicing" can be shaped by using a smaller cap, to shift the cutoff frequency to a higher point ( in the audible range ).

    As I stated, and as others can confirm, a bypass cap larger than, say, 4,7 uF in a tube gain stage is practically useless IMHO, with a negligible effect on the frequencies within the "useful range" for a guitar, while smaller bypass caps are very useful to shape the sound.

    Cheers

    Bob
    Hoc unum scio: me nihil scire.

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    I did something similar to what you describe. I made a 6 position switch act as a "bass" control on the cathode of the preamp tube. My experience is that beside the change of volume, the effect is certainly noticeable especially at higher volume. (I used 22uF-10-4.7-1-.1-.01) I was surprised when the guy I built it for said he used around the lowest setting-- he was thereby able to give a lot more gain.

  12. #12
    Senior Member Old Tele man's Avatar
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    ...to paraphrase:

    A) Too much of a good thing, isn't a good thing.

    B) "More is better," ain't always so.

    C) from the old Bryllcream commercial: "...A LITTLE dab, will DO..."
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    Hi Robert,
    I agree with everything you have expalined.

    I was just speculating that tuning the circuit so that it, technically speaking, may have less bass response MAY cause the listener to hear a more focused and clearly defined bass causing them to have the impression that the bass has actually become louder.

    It's a useful trick when mixing in a summing bus. Clean up all the sub frequencies to allow the energy at 60-80hz to pop out. The amplitude of the 60-80 hz region may not increase but the enhanced clarity can give the impression that it has.

    It seems you have just described the same idea when you mentioned the potential to lose definition and character.

    So, I was just trying to provide support to your idea that you can not "add" bass in the circumstance described.

    very best regards,
    mike

  14. #14
    Old Timer tedmich's Avatar
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    here's a little unit I made up to help me fine tune the cathode bypass of my preamp tubes


    its a nice Greyhill DIP switch which puts 4 tant caps in parallel for values from 0.1/1.0/4.2/9.9 uf, so really 0.1-16uf. There is also a series resistor trim pot on top which lets me vary the boost with 0-20kohms. Its all held together with epoxy putty and can be used in a breadboard or temp soldered in. Next one will have a cap bypass to make it easier to take resistance reading that sounds best for final build; 4 of these allow for some complex tweak experiments!

  15. #15
    Senior Member hasserl's Avatar
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    I hear a difference between 4.7uf and 22uf, and Merlin's chart above shows why. I like 4.7uf with a Fender style 1k5 cathode resistor, it gives a good strong bottom end, but reduces the tendency of the amp to get woofy when pushed.

  16. #16
    Supporting Member Chuck H's Avatar
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    See... It's all subjective idn't it?!? I don't like 4.7uf with a 1.5k cathode R because it makes my open A louder than my open E with my guitar. So it starts to become a matter of personal taste AND the individual amp. I like tedmich's decade dip doodad. Good idea.

    Remember I mentioned that the charts don't seem (to me) to represent what I hear... Well I gave that some thought and realized that the -3db point for different value caps in this application represents half the db increase. So I guess it becomes relative. I hear the effect of different preamp bypass caps closer to their peak frequency or maybe at -1db or so. Pop a 1uf across a cathode R and I dont hear +3db at 150hz. But I sure do hear +5.5db at 1khz. Just sayin'

    Chuck

  17. #17
    Senior Member Enzo's Avatar
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    Context matters to everything.

    Just because the bottom end response of one triode stage rolls off at 5 Hz instead of 20Hz instead of 50Hz doesn;t mean that any signal in that freq range was getting to the stage in the first place. Shark repellent doesn;t do you much good in Denver, no matter how effective the stuff is. it also doesn't mean that any bottom end stuff will make it out of the stage either. That 20Hz will have a tough time making it through a 500pf cap into the PI stage of a Fender.
    Education is what you're left with after you have forgotten what you have learned.

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