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Thread: Filter Capacitor Values-- Is it worth tweaking values?

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    Filter Capacitor Values-- Is it worth tweaking values?

    So, I've noticed that on a good amount of Deluxe Reverbs that I work on, 22uf filter caps are installed instead of 16uf identified on a schematic for a vintage blackface at least. I personally prefer the 16uf, the saturation that occurs sounds more natural and I think it does impact the sensitivity of the amp in a big way. Some vintage deluxe's that I've worked on with 22uf sound... I guess overfiltered. It's been really dependent on the individual amp, sometimes I think it sounds ok. One thing I know for sure is it's definitely quieter. What are the risks of overly increasing or decreasing filter cap values? Is it worth experimenting with past what the schematic recommends? It clearly makes a difference, so why? and why not?

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    Lifetime Member Enzo's Avatar
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    There are no risks. Only subtle tonal changes...maybe.

    Keep in mind the 16uf caps from days of old had tolerances or 20% or more, in fact +80/-20% was a common spec. So a 16uf cap can easily measure 20ufs and be in spec. You need to MEASURE the old caps to see what their actual capacitance was.

    50 years ago 16uf might have been common, but it has not been a standard value in many years. 22uf is a standard value. That is why 22uf is what you see in more recent products and in rebuilds.

    Tolerances on all the parts were much looser back then.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Steelwitch View Post
    One thing I know for sure is it's definitely quieter.
    What does that mean? Less hiss? Less hum? Less overall volume? With which filter cap value? Inquiring mind(s) want to know.

    Here I've often used 47 uF main filter in Deluxes. Those playing thru them enjoy less hum, and less gargly sounding modulation of loud single note passages by 120 Hz from the charging power supply. Absolutely NO complaints. So far so good for scores of years now.

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    In a nutshell: "16uF" old caps are basically same as modern "22uF" since thanks to WIDE tolerance typical of Electrolytic capacitors both *actual* values overlap big time.

    The point being that you canīt tell them apart by ear. (yes with Lab equipment)

    Or to say it in another way: in practice a 22uF cap is the same as an old 16uF rated one, Leo himself would be happy to use them.

    So your finding them different can have 2 explanations:

    1) "confirmation bias" ... "anything made in the 50īs 60's is better than anything modern" so you *expect* to hear a difference.

    2) the old cap has dried so it actually has significant lower value, say 8uF , or significant ESR, both of which *do* affect sound.
    Then you are used to old dry capacitor sound and donīt like the modern one.

    Fine with me, but be aware this is not the "original sound" by any means.

    A fresh 1968 16uF cap is actually very close to a fresh 2019 22uF one and swapping one for the other should bring almost inaudible change (if any at all)

    A 50 year old Electrolytic? ... of course not.

    But again, Leo didnīt fit 50 y.o. caps in his amps by any means.
    No way.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by J M Fahey View Post
    In a nutshell: "16uF" old caps are basically same as modern "22uF" since thanks to WIDE tolerance typical of Electrolytic capacitors both *actual* values overlap big time.

    The point being that you canīt tell them apart by ear. (yes with Lab equipment)

    Or to say it in another way: in practice a 22uF cap is the same as an old 16uF rated one, Leo himself would be happy to use them.

    So your finding them different can have 2 explanations:

    1) "confirmation bias" ... "anything made in the 50īs 60's is better than anything modern" so you *expect* to hear a difference.

    2) the old cap has dried so it actually has significant lower value, say 8uF , or significant ESR, both of which *do* affect sound.
    Then you are used to old dry capacitor sound and donīt like the modern one.

    Fine with me, but be aware this is not the "original sound" by any means.

    A fresh 1968 16uF cap is actually very close to a fresh 2019 22uF one and swapping one for the other should bring almost inaudible change (if any at all)

    A 50 year old Electrolytic? ... of course not.

    But again, Leo didnīt fit 50 y.o. caps in his amps by any means.
    No way.
    THIS^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    I want to reiterate what was mentioned here. That is, the older caps rated at 16uf were commonly more than that. The idea at the time being that what you wanted was the most capacitance for your $$$. So loose specs tended to run wide. And then mentioned was the fact that older caps could be lower due to deterioration. And then mentioned was that Leo would never have used old caps that may have suffered deterioration. In fact I'd bet that if he was getting 16uf caps that measured under spec (regardless of the rated tolerance) he would have complained.

    As Enzo mentioned, there are subtle tone changes to be found with experimenting. I would suggest staying with 22uf for the main filter caps since even that value doesn't offer a whole lot of smoothing compared to what could be done alternatively. But for the PI and preamp nodes you could certainly go to 10uf or even 8uf for a different tone. The smaller value won't decouple as much at lower frequencies, allowing more feedback interaction via the HV rail. This can be very significant tonally. For better or worse.

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    What I mean is less hum (120hz) overall, I’ve noticed that the higher the filter value, the more compressed, almost squashed the tone of the amp becomes. Based on some of the responses, it’s seems likely that this might be caused by filtering in the preamp stages more than output stage. So it can be concluded that filtering in the preamp stage 100% will affect the tone of the amp? Although quieter, is it possible that some openness of harmonic response in the amp is being diminished by overfiltering? Or am I totally looking in the wrong place regarding this issue?

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    Quote Originally Posted by J M Fahey View Post
    In a nutshell: "16uF" old caps are basically same as modern "22uF" since thanks to WIDE tolerance typical of Electrolytic capacitors both *actual* values overlap big time.

    The point being that you canīt tell them apart by ear. (yes with Lab equipment)

    Or to say it in another way: in practice a 22uF cap is the same as an old 16uF rated one, Leo himself would be happy to use them.

    So your finding them different can have 2 explanations:

    1) "confirmation bias" ... "anything made in the 50īs 60's is better than anything modern" so you *expect* to hear a difference.

    2) the old cap has dried so it actually has significant lower value, say 8uF , or significant ESR, both of which *do* affect sound.
    Then you are used to old dry capacitor sound and donīt like the modern one.

    Fine with me, but be aware this is not the "original sound" by any means.

    A fresh 1968 16uF cap is actually very close to a fresh 2019 22uF one and swapping one for the other should bring almost inaudible change (if any at all)

    A 50 year old Electrolytic? ... of course not.

    But again, Leo didnīt fit 50 y.o. caps in his amps by any means.
    No way.
    This makes total sense! To be clear I am referring to a comparison between new caps. I often don’t find old filter caps to sound particularly better than new ones, but I can imagine unintended but favorable results from drifting far below spec. That being said, it might be worth matching a lower value in this case. Do you have thoughts on this? Surely lower value filtering will affect fidelity at least!

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    I can think of two ways that filter cap value might affect the elusive and unmeasurable "tone". One is by voltage sag, the other is by allowing higher ripple voltage.

    I can think of a way to separate the two. One might use a second "first filter cap" separated from the actual first filter cap by a resistor to introduce more sag but with less ripple. That would let you have sag without much ripple.

    Alternately, one could A-D some output signal and run a spectrum analysis on it. Actual ripple would stand out as a series of spikes at 120, 180, 240, etc. and you could actually see what was happening.

    I like measurement rather than anecdotal stories of amps sounding better because I rubbed bear fat on the filter caps and such.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

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    Quote Originally Posted by R.G. View Post
    I can think of two ways that filter cap value might affect the elusive and unmeasurable "tone". One is by voltage sag, the other is by allowing higher ripple voltage.

    I can think of a way to separate the two. One might use a second "first filter cap" separated from the actual first filter cap by a resistor to introduce more sag but with less ripple. That would let you have sag without much ripple.

    Alternately, one could A-D some output signal and run a spectrum analysis on it. Actual ripple would stand out as a series of spikes at 120, 180, 240, etc. and you could actually see what was happening.

    I like measurement rather than anecdotal stories of amps sounding better because I rubbed bear fat on the filter caps and such.
    Wow do I wish I could afford a spectrum analyzer

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Steelwitch View Post
    Wow do I wish I could afford a spectrum analyzer
    Although I'm far from expert on modern communication products, I think there are smarty phone apps which will turn your plastic pocket pal into a spectrum analyzer for you. Whether it's looking at what its mic picks up, or how you might input a signal to have it analyzed, I dunno. Smarty phone experts, please help us out here.

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    Supporting Member eschertron's Avatar
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    I've downloaded a bunch of 'smarty phone' apps to play with. Shore look purty! And yes, if you're looking for trends and not the minutia of the data, then a free spectrum analyzer is just the ticket.

    ...and I'm waiting on G1 to weigh in on the bear fat trick

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    Wow! Phones can really do it all! I was just talking in therapy about how I need my spectrum analyzed

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    Quote Originally Posted by Steelwitch View Post
    Although quieter, is it possible that some openness of harmonic response in the amp is being diminished by overfiltering?
    I think you've already decided that is has by using the term "overfiltering"

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    I used to look wistfully at spectrum analyzers, too. Then I got a USB oscilloscope. Can't live without it now.

    Phone apps to record and analyze are even better, if somewhat high end limited. Should be fine for power line and the first several harmonics though.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

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    Oh, forgot.

    Computer microphone inputs can and have been turned into "oscilloscopes" by PC apps. If you can capture it as a wav file, there is at least one free windows app (Spectracizer) that will do spectrum analysis on it for you.

    Be careful not to fry either your phone or computer with connections to a tube amp, though. Those are 3V and 5V logic.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

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    Supporting Member eschertron's Avatar
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    With the limited screen real estate and the sometimes awkward display options, I've used my phone only via the built-in mic. Close enough to accurate for me. Besides, aren't phones supposed to be wireless?

    edit: but seriously, probably good enough to get evidence that low ripple harmonics are present.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Steelwitch View Post
    Wow do I wish I could afford a spectrum analyzer
    Or some bear fat!

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    What I mean is less hum (120hz) overall, I’ve noticed that the higher the filter value, the more compressed, almost squashed the tone of the amp becomes.
    I noticed something similar in vintage PP tube amps, but prefer to describe the effect of higher cap values as less attack responsive. My explanation is the influence of the filter on the time constant of the sag induced screen compression.

    So my answer to the original question is YES.

    To ensure reproducibility of results the actual cap values should be measured with a good LCR meter.

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