Metalized polyester caps for cathode bypass?

[leadfootdriver]

I can see that it would be good for longevity as they won't dry out like an Electrolytic, but what about the sound? Is there an appreciable difference between a MP and an Electro cap in the bypass position?

[Jazz P Bass]

My opinion is there is not a difference in sound.

Are you talking a low value, like a Marshall .068uf?

[leadfootdriver]

Yes, they are all 1uf in this amp. It's a Jet City JCA5012C.

[Jazz P Bass]

Have at it.

[daz]

I know it will be questioned, but i do hear it. WITH placebo effect in mind as i A/B'd them, i was quite sure. In fact, i once found different electros that sounded quite different. That will likely get me in trouble saying that. But i put some in place of the ones i had before but i had cut the leads too short when i removed them to try something else. I bought some new .68's, and YES i tested them and they read the same as the old ones. They sounded very different in a bad way, and i'm not talking subtle. I ended up soldering lead extensions onto the old ones and putting them back. Why is anyone's guess, but if different brands of the same type can sound different then i think so can different types.

[loudthud]

Small low voltage electrolytics in the 1uF range have very poor ESR. You might actually hear the difference, especially with single coils. Polyester is a better choice if room is available.

[R.G.]

Note that if you can get low-ESR caps like most film types are, then you can "dirty them up" by adding a little bit of resistance in series to make them sound like caps with higher ESR.

[Rhodesplyr]

I have to go with the cork-sniffers on this one. I've done some tests with different cathode bypass caps, and there are subtle differences. Theoretically, the film caps should give you slightly faster attacks and transients, but that may or may not be apparent. Of course, this is just listening to an amp by itself in a room with no other sound. I wouldn't bet that you could tell a difference in a full band situation.

[daz]

I have to go with the cork-sniffers on this one. I've done some tests with different cathode bypass caps, and there are subtle differences. Theoretically, the film caps should give you slightly faster attacks and transients, but that may or may not be apparent. Of course, this is just listening to an amp by itself in a room with no other sound. I wouldn't bet that you could tell a difference in a full band situation.

I think you are correct in your description, tho i think it's not all that subtle. I just popped in to reply on my test i just did and i will describe what i heard/felt. I rigged it up so i could quickly A/B poly caps and electros, both of the first 2 stages having .68uf. And keep in mind that we shouldn't even discuss things like this w/o mentioning what kind of circuit we're talking about, because i doubt it would be anywhere near as obvious in say a cleaner amp like a fender as it is in my cascaded preamp in my marshall JCM style amp ! In any case, i A/B'd them playing each foe a few minutes then changing about 7-10 times. W/O a doubt there was a considerable difference. The electro's caused the tone to get much softer and less clarity like you get if you add a lot of NFB in the output section. That sorta smooth cloudy tone. The polys got clear and articulate. It's been a long while since i went back from poly to electro so i'm glad i saw this thread, as i think i may prefer the polys and will leave them for now. Only fear is they may get too harsh at higher volumes where i need more of that smooth cloudy tone. Anyways, maybe my ears are just overly sensitive but the difference isn't what i'd call subtle, tho then again that word may mean a different degree of difference to some than others.

[leadfootdriver]

Thanks for the insight!

[Alan0354]

I think you are correct in your description, tho i think it's not all that subtle. I just popped in to reply on my test i just did and i will describe what i heard/felt. I rigged it up so i could quickly A/B poly caps and electros, both of the first 2 stages having .68uf. And keep in mind that we shouldn't even discuss things like this w/o mentioning what kind of circuit we're talking about, because i doubt it would be anywhere near as obvious in say a cleaner amp like a fender as it is in my cascaded preamp in my marshall JCM style amp ! In any case, i A/B'd them playing each foe a few minutes then changing about 7-10 times. W/O a doubt there was a considerable difference. The electro's caused the tone to get much softer and less clarity like you get if you add a lot of NFB in the output section. That sorta smooth cloudy tone. The polys got clear and articulate. It's been a long while since i went back from poly to electro so i'm glad i saw this thread, as i think i may prefer the polys and will leave them for now. Only fear is they may get too harsh at higher volumes where i need more of that smooth cloudy tone. Anyways, maybe my ears are just overly sensitive but the difference isn't what i'd call subtle, tho then again that word may mean a different degree of difference to some than others.

You try adding a small resistance in series with the poly to see whether you get back the sound of the electrolytic?

[daz]

You try adding a small resistance in series with the poly to see whether you get back the sound of the electrolytic?

No, I played it for a couple hours last nite and ended up going back to the electros. I have no desire to try and get them to sound like the electros when i can just put the electros back in. I'm usually always up for more clarity, but after i time the poly caps just seemed a bit strident by comparison. This is why i always go back and fourth a few times till i'm sure i like one more than the other. I will likely even try again down the road.

[Merlinb]

Electros tend to have poorer tolerance, higher ESR and worse distortion than plastic caps. A 1uF plastic cap is likely to be within 10% of 1uF, but a 1uF electro could be 20% out. Also, its leakage current will be higher, and will change with duration of use and temperature. Also, capacitor distortion varies with voltage rating and, in electros, with DC bias. Trying to do a fair comparison of plastic versus electro is therefore not easy, because electros vary so much just between themselves!. Exactly which one are you comparing the plastic with?!

[Chuck H]

Going with the popular opinion on this one. Most electro's have poor specs in some critical areas for this circuit. That's part of their charm if you prefer the sound. I don't fuss over the small difference and use plastic for smaller values without flinching. When I first started modifying my own amps I noticed that Mesa used a lot of those solid tantalum caps. Not the little blobs, the bullet shaped ones. I looked up the specs and took to using them for awhile. They were definitely better performers than the elecro's. Not really noteworthy. My main consideration wasn't performance though. I liked that they'll last virtually forever.

[Alan0354]

I use tantalum for all the cathode caps just because I have them around. For 1uF, I use ceramic......because I have them. I make it a point not to use orange drop and the other expensive caps. Unless I have convincing reason, I just not buy into this kind of stuff at all. For the tone caps where I cannot get ceramic caps for hv and high value, I make it a point to buy the cheap caps from China. So far, I have been very happy. I do check them with a meter to make sure the value is ok. So far, the Chinese caps are spot on. To me, only factors about the caps are ESR and accuracy. I don't think ESR comes into play, even for cathode bypass are not that critical. Impedance looking into the cathode of 12A?7 are not very low, nothing like the emitter of a BJT. What is a few ohms in series. Make it 10 ohms!!!!

And I change to either metal film resistor when I order them and use wire wound that are laying around. I avoid carbon resistor all together. I absolutely refuse to use opamp 4558 and TL072 because they supposed to have the "magic". So far it work out very good for me. I buy tubes for the price. So far JJ is pretty good price and have good reviews.