Good on you for asking the pertinent question.thetragichero wrote: ↑Sun Dec 15, 2019 2:34 pm is there really any reason (besides needing to replace every couple of decades) behind using something besides a sub-$1 electro cap for cathode bypass purposes?
Probably not. It's really a matter of objectives - what is one (as the amp builder) trying to accomplish?
If you (the amp builder/hacker in general) are after your amp somehow being "original", electros are the way to go. Actually, you want not-very-good electros, the way new ones were back in the 19x0s when whatever amp is being re-born was made.
If you (the amp builder/hacker in general) are after sonic accuracy (?!) using tubes is the wrong way to go about it. Tubes lost to semiconductors in low distortion on all of the various fronts decades ago.
If you (the amp builder/hacker in general) are after the amp sounding good to you personally, you're wasting time on internet forums when your soldering iron could provide even-more-instant gratification by swapping caps.
If you (the amp builder/hacker in general) are only after bragging rights about having the most internet-politically-correct amp, this forum can't help.
If you (the amp builder/hacker in general) are after understanding, you have to learn what the cap is there for in the first place. It's there for self-biasing, in conjunction with the cathode resistors. The cathode resistors cause a DC offset from the idle/bias current, pulling the cathodes higher than the DC-grounded grids. This is right out of the triode playbook for self biasing. It's intended to provide a constant voltage for biasing the tube. Well, "constant" over signal-significant periods of time. It has to average out the lowest frequency signals to insignificance, so its value must be big enough that its impedance compared to the actual cathode resistance must be very small. It has to be small enough that different tubes can eventually make their own bias point.
Imperfections in the cathode cap - ESR, ESL, leakage, nonlinearities - affect the tube operation to the extent that they affect the tube's open loop voltage and current operations. With a many-K reactive plate load, these are going to be small. How small and how much that affects sound gets into a discussion of infinitesimals.
Maybe. Not many people really understand this.i understand not placing electro caps in the signal path
A cathode bypass cap IS in the signal path. It's moving signal >current< to ground. This affects the signal voltage by how much signal current is not bypassed to ground. The power supply filters are actually in the signal path. A shunt-to-ground cap is intended to let a frequency dependent current into ground. If an electro is somehow too "dirty", you can make the effect of the electro's imperfections smaller by making its capacitance larger. At some point, the impedance of the electro cap is just too small for the even smaller imperfections to matter to the audio it leaves behind. Power filter caps are a good case in point. Their value are intended to make them dramatically smaller impedances than the series resistors, inductors, and plate loads that use current from them. When this is true, the remaining effect of the cap is an almost-constant voltage, which is where you were trying to get as a power supply designer.
The whole parallel-the-cap thing comes from RF practice in the early/mid 1900s. RF guys noted that caps have an impedance that is proportional to 1/F, declining as frequency increases... until the cap's impedance dipped below ESR. ESR puts a floor on how low the cap's total impedance can go. Even worse, at some frequency the ESL of the caps' winding and even the parasitic inductance of the leads makes the total impedance rise again. So normal caps have an impedance curve that looks like a bathtub - as frequency increases from near-DC, the impedance goes down as 1/F, hits a floor at ESR, then rises again as F when the ESL impedance gets bigger than ESR. If all of your frequencies are RF, you REALLY want the start of the inductive part to be high in frequency, and you'll happily trade off the low frequency end of the bathtub. If your frequencies are audio, which RF guys think of as DC, you need to be pickier about how low the low end of the bathtub is.but say i paralleled my 22uf electro cap with a 100nf ceramic or film cap (should help with any esr-related issues, no?),
Many caps are made by winding spirals of conductor and insulator until enough surface area has been wound to get the needed capacitance. This necessarily creates some resistance as the signal spreads from the leads' contacts through the cap plates, and some inductance - those spirals! However, modern cap making practice is far better than it was in the bad old days. Cap makers will sometimes make caps with the leads connected to the conductor spirals along the ends of the spiral wind. This means that the incoming current doesn't have to traipse through the whole length of plate to get to the ends, so the ESR and ESL are cut dramatically. Extended foil and low impedance electros can be remarkably low, especially if compared to bad-old-days wound-strip film caps.
Parallel-the-cap schemes can also only help above some frequency where the added/smaller cap's capacitive impedance is less than the ESR/ESL of the cap it's paralleling. Until then, it's just as big an impedance as the ESR/ESL it's trying to shunt. This is all to say - frequency matters. Paralleling caps makes a great deal of sense at high frequencies. But you have to know your caps' ESR and ESL and the frequencies you're trying to pass. Circuit values matter.
And good on you for asking the Right Question. I've laid out the technical background. NOT listening to the hifi-tweako ad-copy mania is a sign that you're a smart, thinking amp builder.besides being able to puff my chest out about spending tens of dollars on mojo-brand film caps? I'm sure we could all come up with good ad copy but actual nuts and bolts technical reasons i am open to hearing