Tantalum caps for cathode bypass

[RJ Guitars]

I've been looking for a good source for some ~5uF bypass caps to use on an amp build and find that the supply of lower voltage electrolytic bypass caps is not too good. Is there any reason not to use Tantalum caps for the cathode bypass on preamp tubes?

[Structo]

Hey RJ,

I was looking at these the other day.

I asked the seller if they are from Russia and he didn't respond.

But the price sure is good.

http://www.ebay.com/itm/ws/eBayISAPI.dl ... _632wt_825

[Jana]

Watts has Sprague Atoms in 5uf. http://www.turretboards.com/

Messy Booger uses tantalums. I don't care for them myself but it is a personal thing. Electrically, there is no reason why you couldn't use them (as long as proper voltage ratings are observed).

[Structo]

Holy crap! $3 a piece?

[Jana]

Mouser has them--Vishay/Sprague, Cornell Dublier.

[Cliff Schecht]

Tantalum caps are good in applications where you need lower ESR than electrolytics (and in a smaller size per unit capacitance). They also have good performance over temperature changes and tend to be very low leakage.

I only use tantalum caps if they are all I have on hand and they will happen to fit (wrt building amps). They tend to have short leads which make them a pain to use unless you specifically planned on doing so. With SS circuits I only use tantalums when I need the smaller size and lower ESR and can't make the same cap out of ceramic capacitors (especially surface mount stuff).

Also be careful to not reverse bias or overvoltage tantalum caps. The tantalum material inside will get super hot and literally blow all over the place. If this gets on your skin it can cause some nasty burns, if it gets in your eyes it can really do some damage! I've popped enough of these in the past to always want to double check their polarity before applying power. Even the little 5-10V ones will explode!

[rdjones]

I used tantalums in FET preamp designs in the '70s.
Small size, tight high quality specs and not being electrolytics were features that drew me to using them.

I had some reliability problems and the failure mode I saw was a short.
Mostly battery powered and some with 15V supply, so I doubt overvoltage was an issue but since Cliff mentions the reverse thing they might have been marked wrong.

Maybe I just got a bad batch.
Hopefully current production is more reliable.

rd

[Cliff Schecht]

They are higher quality than electrolytic caps and last many times longer than electrolytics because there isn't any gel to dry out (at least with solid tantalum). If I am designing a product that needs to last for over 10 years then I might opt for tantalums over electrolytics, or again if I really need to save on space (especially height).

[katopan]

I've also had reliability problems with tantulum and none with electrolytic. Like rd it was a completely different application (SS preamp stuff) but they were in spec and the right way around. Worked for a while and then failed as a short.

[rdjones]

For the sake of fairness I will add that the type I used were not like the axial 'sealed' type shown.
They were the epoxy dipped radial lead style that are kinda like a baby pale orange drop.
They were used as FET source bypass, with fairly low voltage applied.

There may a few still in operation in "Orange Box" and 801 preamp pedals and Northstar phase shifter clones.

RedDog Steve

[Cliff Schecht]

I've never had them fail randomly. If anything electrolytics have let me down much more than tantalums..

[Nigel Tufnel]

I've been looking for a good source for some ~5uF bypass caps to use on an amp build and find that the supply of lower voltage electrolytic bypass caps is not too good. Is there any reason not to use Tantalum caps for the cathode bypass on preamp tubes?

Hey RJ, I've got some old stock 4.7uf's kickin' around here somewhere. They're the dry bullet type, how many you need for the build? I'll be happy to shoot you a hand full so you can try them out.

edit: I might be able to tell you where I found them too if I can dredge it up from the memory banks

[RJ Guitars]

Thanks folks - many great contributions here,

I want to use these as bypass caps in a Dumblish sorta amp. I finally overcame my prejudice and decided to order some 4.7uF 25V radial caps. I'm not sure how long the leads will be so I also ordered some 4.7uF 50V axial caps just in case. I can find the axial in 25V - even 16V although I'm not sure how much of an issue that is. Merlin's book suggests you stay close to the maximum voltage they will see with the electrolytic's so I was trying.

I do have a bunch of tantalums here and if they don't create a sonic sacrifice I'd like to design around them in the future. They are just a huge unknown and I didn't want to use them or offer them with parts packages to other folks unless they are the right answer.

thanks,

rj

[rp]

RJ why not a film cap? When I need a value from .68 - 5mf and I can find a 100V for the small size I'll usually use a film over a electrolytic.

http://www.mouser.com/ProductDetail/Cor ... wtbYeGs%3d

http://www.mouser.com/ProductDetail/Cor ... lLYNMjA%3d

There are others, try searching 4.7 too. 100V is good, 250V max as they get big.

BTW Greetings from Italy all.

-rp