Cap types for large cathode bypass values?

[demonstratedspatulas]

I see a lot of older preamps with something like a 250uF cathode bypass.

As far as I know, you stop hearing an effect somewhere around 1uF? Is that correct?

In addition to this, what sort of cap types do you use here?

I've been using film caps for signal path stuff, and I've heard it's a good idea to avoid electrolytics in the signal path as they don't sound good. (Although I have never had the chance to A/B this. Might be interesting!)

What voltage ratings do cathode bypass caps need? I'm using 630V for my film coupling caps, but certainly this is overkill for a cathode bypass cap.

[pompeiisneaks]

The bassman and original marshall (which was a copy of hte bassman) used that, because it was for bass guitars. Electric guitars do cover a much smaller frequency range and I more commonly see between 25 and 5uF for the cathode caps. The bigger caps can cause some muddiness for sure.

As for voltages on the cathodes, it varies from amp to amp depending on the design, etc. but basically usually I seem to rarely see more than a 25 to 50VDC cap in that position.

Use whatever voltage rating is in the amp, and/or check it if you're replacing one. (see what hte actual value is and at least add 20% more than that if not double it to be safe).

~Phil

[demonstratedspatulas]

Thank you! What sort of cap type is needed here? Or do cap types matter that much?

[roberto]

e-caps are the most used in these cases.
You can see some big polys for the lower range (up to let's say 4.7 uF), but often e-caps starting from 1 uF and above.

[pompeiisneaks]

e-caps are the most used in these cases.
You can see some big polys for the lower range (up to let's say 4.7 uF), but often e-caps starting from 1 uF and above.

Yup exactly. For personal builds it may make sense to pay quite a bit extra for some of the newer film caps that can go that high, but it's significantly cheaper for the electrolytics and big companies nowadays design for the 2-5 year durability if that, so they don't care that the e-lytics are going to wear out in 10-15 years since they hope the amp dies in 3 to 5 so they can get someone to buy another.

~Phil

[roberto]

SCR Solen Fast go up to 47 uF at 630Vdc, so they can be used for power supply too.
Well, if you accept to pay ten times more that cap.

[pompeiisneaks]

SCR Solen Fast go up to 47 uF at 630Vdc, so they can be used for power supply too.
Well, if you accept to pay ten times more that cap.

Yup, I've also seen some other film caps that are some of the other common brands like CDE etc that go up there and also cost a lot more, not sure if it's 10 times more, but it def was a lot more.

Hmm let me peek again, here's a Kemet, which I consider really nice caps with a 40uF 650VDC film cap, for 7.17$ at mouser:.

https://www.mouser.com/ProductDetail/KE ... Ekxg%3D%3D

and the same kind of cap from say antique electronics as an e-lytic with the roughly same rating:
the old sprague 40uF at 500V are $13.13!

That's odd, maybe prices are coming down on them?

Wow this page is a search for anything between about 500 and 1000VDC rating and between 1uF and 100uF ratings. They are all looking MUCH more reasonable lately!

https://www.mouser.com/Passive-Componen ... 1yp7xv3SGT

Of course most have those dual pin outputs so you get more current capacity and that may make it harder for us to use them in boards, we'd have to make PCB's really to pull it off easily, I think using turrets or eyelets for this could be pretty tricky.

~Phil

[roberto]

Thanks Phil!

[demonstratedspatulas]

What about ceramic caps? For example 810-FG18X7R1E105KRT0 on mouser?

[sluckey]

What about ceramic caps? For example 810-FG18X7R1E105KRT0 on mouser?

Nice for pcb work. 1/4" long leads don't work well in turret board or point to point tube wiring.

[demonstratedspatulas]

What about ceramic caps? For example 810-FG18X7R1E105KRT0 on mouser?

Nice for pcb work. 1/4" long leads don't work well in turret board or point to point tube wiring.

Tone-wise they are good though? Thank you!

[sluckey]

Absolutely

[demonstratedspatulas]

Thank you

I have never had a chance to compare ceramic with film. I've just been told to use film for everything and call it day.

In terms of short leads, I don't mind soldering em to some bus wire, if it means saving $3 not buying a film cap.

[thetragichero]

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? i understand not placing electro caps in the signal path but say i paralleled my 22uf electro cap with a 100nf ceramic or film cap (should help with any esr-related issues, no?), 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

[sluckey]

In terms of short leads, I don't mind soldering em to some bus wire, if it means saving $3 not buying a film cap.

If saving money is a primary factor, then buy electrolytics for cathode bypass and power supply filters, and Mallory 150 or Xicon chicklets for couplers. And get long leads so your work won't look crappy.