Filter cap size question. Fender blackface.

[Lydian]

1968 Pro Reverb. Currently has 2X 100uf 350V and 3X 525V in the doghouse.

AA-165 scheme calls for 2X 70uf instead of the 100's in currently.

Is there a big difference between the two? And should I go back to this size? I am going to replace the electrolytics soon so I wanted to wrap my head around this.

Thanks.
-Dalton

[Enzo]

Does it sound good now? If so, why change it? I mean the value. No, there is no reason to put 70uf in there instead of 100uf. You will find that 70uf is not a standard value, and they will not be easy to find in the parts market. 100uf IS a standard value, and is common. The nearest standard value to 70uf is 68uf, and I can't imagine why I would hunt those up. I wouldn't hesitate using 100uf to replace bad 70uf caps.

[Leo_Gnardo]

You'd have a tough time finding 70 uF now, also when they were replaced the first time. It's not one of the standard values, & may have been specially made for Fender. You're perfectly fine with 100 uF caps, no worries. If anything, just a little better filtering than original. Less hum from the power supply, nothing to complain about there.

I also doubt you'll really be needing 525V. 500V caps are plentiful, easy to find, reasonably priced. Antique (tubesandmore.com) has F&T and JJ brands for instance. (although a long time customer I'm not otherwise affiliated etc... ) If it really worries you, there are a couple sources for affordable 600V axial style electrolytic caps. I'd recommend buying from a dealer, not ebay sources where counterfeit parts proliferate. Although I haven't bought 600V caps from the following sources, others have with good results: JustRadios in Canada, and Studioelectronics in Burbank CA.

Don't forget to replace the bias filter cap(s) while you're doing your refurb.

[Lydian]

I figured I would also change the the preamp electrolytics as well. If stock is 25V will 50V have any improvement here? Or maybe it was 50uf in place of 25uf, I'll have to open it back up.

Thx for the advice guys. I'll throw 100's back in.

[Leo_Gnardo]

I figured I would also change the the preamp electrolytics as well. If stock is 25V will 50V have any improvement here? Or maybe it was 50uf in place of 25uf, I'll have to open it back up.

Preamp cathode bypasses typically 22 or 25 uF. Sometimes I use 20 uF. No way anyone could hear the diff.

25V or 50V, don't matter, whatever you can get your hands on. They only see a volt or two anyway, except for the reverb drive, that one might see 6 to 10V. Rarely do they die of anything but old age 40-50 years or more.

[Lydian]

Preamp cathode bypasses typically 22 or 25 uF. Sometimes I use 20 uF. No way anyone could hear the diff.

25V or 50V, don't matter, whatever you can get your hands on. They only see a volt or two anyway, except for the reverb drive, that one might see 6 to 10V. Rarely do they die of anything but old age 40-50 years or more.

Thanks, I'll leave the preamp cathode caps alone then. I'll change the bias filter once I dig into the doghouse.👌

[Leo_Gnardo]

Thanks, I'll leave the preamp cathode caps alone then.

Have they been replaced sometime in the past? FWIW the white Mallory caps common in some old Fenders as cathode bp's have a bit of a bad rep. Even Crown recommended, get rid of 'em in their SS amps. If you have 'em, replace.

FWIW I typically use 100V rated caps in the bias supply. Haven't had one go bad yet. Check back in another 20-30-40 years.

[Justin Thomas]

Make sure the bias cap is an appropriate voltage rating, too... sometimes they're a 50V cap with 60V on them. They've worked that way for years, sure, but it's cheap insurance to put a 100V cap in. Since it is an absolutely critical circuit, it's not one I like to leave to borderline specs. A failure there could put you in a world of serious hurt.

Justin

Edit: simul-post w. Leo!

[Lydian]

Preamp cathode caps were changed about 10-12 years ago. So, going by the info above, I'd day there fine. I'll make sure to go up towards 100V for safety on the bias filter cap though.

[Mick Bailey]

Many electrolytic caps used to have a tolerance of -20/+80%. That means a 70uf cap could be up to 126uf. Modern caps are much tighter on tolerance and a 100uf will likely measure that. So a modern 100uf cap is within the permissable tolerance of an old 70uf cap.

[keithb7]

Lydian, keep in mind that the first two caps are in series meaning they total up to actually produce 50 uf at 750V protection. This is because when an amp is sitting idle, in standby mode the first 2 caps see higher voltage than
the other caps in the B+ circuit. Probably higher voltage than the 525V that your other 3 caps are rated for. The 3 525V caps are generally rated at 20 uf as seen on the schematic. 22 uf @500 are common and a good replacement here. I also run 100uf at 100V caps in my vintage Fender amp Bias circuits. You can find everything you need in the F&T cap line up.

[Lydian]

I have found some good sites for the doghouse caps. I have three questions.

1. Where are these 25uf 100V axial caps online?
2. Would it be just as fine with 500v rating for bias cap? Overkill, but I'm curious as to weather there is a tipping point on the benefits of upping voltage tolerance.
3. How much happier would sprague a make me, compared to Illinois or F&t?

I've been using tubesandmore.com and amplified parts.com. Out of about 6 sites, these two seem let user friendly to me. Could be cause I'm using an iPhone.

[Justin Thomas]

Re: #3, I doubt Spragues would make you any happier at all. I've been using F&T & Illinois caps for years and not had any problems. And even if one in three WERE bad, it's STILL cheaper than paying for one Sprague!

I like F&T more for looks, to be honest - I use them in amps I build for customers and a couple of my prized Fenders, Illinois in the ones I'm selling off. Repair customers can ask for whatever they want... :P

Justin

[Leo_Gnardo]

For your bias supply 22 uF 100V will work just as well as 25 uF.

No need to increase the voltage rating on these bias filters. If anything, you can increase the capacitance rating, 33 or 47 or even 100 uF. Hardly any better than original values though, slightly less hum as you increase value. A two stage filter with 22 uF caps should be plenty enough.

If you like less money in your pocket, go ahead & buy Spragoo Atoms. Then you have bragging rights: "Hey look I have less money in my pocket!!!!!!!" I'm sure the other cheaper brands will give you a good 30-40 year run before you have to think about replacing them. You could take the money you save, put it in your retirement account. So you can afford to restring your guitar when you're a rockin' grandpa.

tubesandmore.com, amplifiedparts.com one and the same company. No wonder you get along so well!

[Enzo]

25uf is not a standard value, but 22uf is, so look for 22uf 100v caps instead of 25uf, and you will find more. The difference won't matter.

If your bias supply is like -60v, then a 100v cap is fine, there is absolutely no benefit from using higher voltage caps there. If you want to use 500v caps for bias, go ahead, it won't hurt anything, but they will tend to be physically larger, which may or may not cause trouble installing them.