50uf filter caps instead of 40uf

[thehoj]

I've got this amp I'm doing a bit of work on, and the schematics call for 4 40uf 450v caps.
Is there any reason I couldn't or shouldn't run 4 50uf 500v caps?

[Robert M. Martinelli]

Hi,
Over-filtering can be a problem only with tube rectifiers, as the bigger initial current demand from the caps starting to load themselves could damage the rectifier tube; the tube rectifiers' datasheets usually indicate the maximum allowed filter capacitor(s) capacitance.

If the amp you're working on has a SS rectifier, you don't need to worry.

( This said, be aware that electrolytic capacitors have rather loose tolerances (say, -10 to +20 % ) so it's also possible that the old ones were already closer to 50 uF than you might think...)

Hope this helps

Best regards

Bob

[PositiveNegativeMan]

+1 I say do it. So long as they are new caps and not oldies. DON'T TRUST OLD ELECTROLYTICS. But if they are fresh and you have them and it saves you the cost of buying newbies... for such a small difference in value it shouldn't cause a problem.

[thehoj]

It is a ss rectifier.
The reason I ask about the 50uf caps is not that I have them, but I can't seem to locate a dual 40uf cap can..
I can easily find tons of dual 50uf cap cans though.

[PositiveNegativeMan]

Go shopping!!

[MWJB]

"It is a ss rectifier." Cool, but note all the commonly use 8 pin rectifiers (5Y3, 5V4, 5U4G, GZ34) won't bat an eye at a 50uf main filter.

[tedmich]

"It is a ss rectifier." Cool, but note all the commonly use 8 pin rectifiers (5Y3, 5V4, 5U4G, GZ34) won't bat an eye at a 50uf main filter.

true..... the blow up at 60uF!


kidding but tube rectifier and "robust" are seldom associated

[pdf64]

My '63 ac30 is still running the brimar gz30 as when I bought it back in '83, and it was old then.

[Hardtailed]

Funny, the OP has a SS rectifed amp and this is turning into a tube rectifier debate

To the OP: note that some electrolytics caps have as much as +30% tolerance. That is, some 40uF cap could be as much as 52uF in reality.

So 50uF? Meh, don't worry

[Enzo]

Not so much anymore, but it used to be on the can caps, that a tolerance rating of +80/-20% was common. Yes, +80%.

[Bruce / Mission Amps]

Not so much anymore, but it used to be on the can caps, that a tolerance rating of +80/-20% was common. Yes, +80%.

Thank you... this is the reason I don't build any of my tweed amps with 16uF@475v caps anymore... the modern 22uF@ 450v to 500v caps are probably much closer tolerance in reality.

And, I don't think I've ever blown a 5Y3GT (rated to 20uF max) with even a 47uF cap.
5R4G are rated to only 4uF cap, but I've run those with 10uF through 16uF caps.
Not sure what the big deal is but if the DC resistance of the secondary is too low, a simple 10 to 47 ohm 2 to 5 watt resistor in series with the B+ rail before the filter cap, seems to take the nasty current surge "tube shock" out of the equation.

[tubeswell]

Funny, the OP has a SS rectifed amp and this is turning into a tube rectifier debate

tubemania

[Robert M. Martinelli]

Hi all,
While we're at it...and just for the sake of completeness...

( please note that, maybe, my opinion is biased by my "worst-case" design mentality, which is part of my everyday job, if so, I beg your pardon ).

It's true that some tube designs operate tubes beyond their ratings ( the AC30 being the most notable example with its EL84s "insanely" biased at some 15W quiescent, which represents the 125% of the published value ), still I wouldn't feel comfortable in recommending this as a "standard" practice.

An example: According to datasheets, the maximum allowed filter capacitance for a 5Y3 is 20 uF, so, even if it is indeed possible that the tube will work with a 50uF filter cap without failing immediately, I think that the extra stress the rectifier tube would have to withstand each and every time the amp is powered on would result in shortening the rectifier tube's life.

I'd say that if we go 10 or 20 % over the published ratings, we could still get away with it, even if we're "stretching the limits", but going as far as some 250% over a published rating is simply asking for a trouble to happen IMHO.

JM2CW

Best regards

Bob

[ThermionicScott]

Thank you... this is the reason I don't build any of my tweed amps with 16uF@475v caps anymore... the modern 22uF@ 450v to 500v caps are probably much closer tolerance in reality.

And, I don't think I've ever blown a 5Y3GT (rated to 20uF max) with even a 47uF cap.
5R4G are rated to only 4uF cap, but I've run those with 10uF through 16uF caps.
Not sure what the big deal is but if the DC resistance of the secondary is too low, a simple 10 to 47 ohm 2 to 5 watt resistor in series with the B+ rail before the filter cap, seems to take the nasty current surge "tube shock" out of the equation.

I've noticed that 4uF rating on the 5R4, too. My (very unqualified) guess is that it has something to do with the super-high voltages it was intended to run at. It never arcs when I flip the standby switch on my 5E7 clone, FWIW.

5Y3s (real ones) are so hardy that they'll last a long time with extra capacitance, too. Note the 5F2A that uses 32uF (160% of max) of input capacitance with one. (Probably helps that it's not nearly drawing the max current of a 5Y3...)

- Scott

[MWJB]

"An example: According to datasheets, the maximum allowed filter capacitance for a 5Y3 is 20 uF, so, even if it is indeed possible that the tube will work with a 50uF filter cap without failing immediately, I think that the extra stress the rectifier tube would have to withstand each and every time the amp is powered on would result in shortening the rectifier tube's life." It won't fail in a typical lifetime at 50uf, plenty of folks run a 5Y3 into a 100uf main filter, it's fine.

"but going as far as some 250% over a published rating is simply asking for a trouble to happen IMHO." data sheet limits are very conservative, they don't know the final application, so err on the side of caution.

If you don't want the rectifier to wear at all, you best not take it out of the box. :-)