Any danger with increasing capacitance of main caps in old gear?

[J!m]

So, lately I seem to be collecting receivers. And they're old and generally neglected and otherwise screed up (that's how I can afford them!)

But, it seems reasonable sooner than later, they will need more significant TLC than an oil change and new spark plugs. Like rings and gaskets (AKA recap).

So, large caps in the 1970's tended to be quite a bit larger than their modern counterparts with the same ratings. Going up on voltage is a given (and possibly required, to get close to the old physical size), but often the modern ones are both shorter and smaller diameter as compared to the old ones with the same ratings.

Bottom line: for the big main PS caps that would need to fit a physical space and original clamps, is there any harm/danger in going up with the capacitance as compared to the originals? Were the original values "good enough" since they had to design to a price point as well? Would a "bypass cap" be a good idea, regardless of what the final value of the beer cans are?

(trying to lean while traffic here is low)

Thanks!

 

[markthefixer]

older caps tended to be specified as -20% +80% of value.
Thus they could be almost double in value and still be in specification.
One CAN go higher in capacitance, just mind the rectifiers...

Thus one can safely double the capacitance.
We / I DO encourage new (replacement) diodes (1 amp or 3 amp) if this is done, thus old tired diodes don't mess things up.
(1 amp 1n4004 -1n4007, 3 amp 1n5404 -1n5407)

To make physical size, increasing the voltage rating is a very effective (and within limits, harmless) way to find the physically larger caps.

 

[8991XJ]

A couple things.

Depending on who makes the new caps they may be closer to -10% than spot on or higher than the value stated on the cap. Nichicon confirmed they supply large caps below the rated value but well within the ±20%. Building them to 10% low will still have a range of values but still within the ±20% spec and that 10% material savings can really add up. So if I get Nichs for the large PS caps, I go big or go home. Got to to get the designed value installed.

Going much over +20% might be looking for issues with things like power switches, fuses, rectifiers and such as the unit was designed for a particular inrush current using the parts specified in the original build. Many manual or the original caps might have info as to the designed limits for the cap. Mark mentions -20 to +50% and some were but others were ±20% limits.

Just be mindful of what you are doing and what parts are in the unit that will be stressed a bit more and adjust as needed.

 

[George S.]

Jim, something Joe taught me when I was upgrading the PL preamps was this. No ceramic caps allowed. Use silver mica caps wherever economically feasible, usually low values as they're expensive. Replace electrolytics with film caps, except where a film cap would be too large to fit, so use a electrolytic.
Granted this isn't going to work in a tuner section that is populated full of ceramics and designed so. But there are many electrolytic coupling caps between the various sections that can be upgraded to film and perhaps silver mica in the phono section.
If I was recapping a receiver, I'd probably just go with what it came with. Separates are easier to modify.

 

[J!m]

I wasn’t thinking about the switches. Would largerbcapacitance increase amperage across the switch contacts?

The rectifier diodes probably should be changed when recapping anyway; but going to higher wattage makes sense and I see no downside there. Even at higher wattage they’ll probably still be physically smaller.

Resistors are not the sort of part that “wears out” as far as I know, unless cooked much beyond medium-rare. Dark areas of a board are always prime targets for further investigation.

 

[J!m]

I like film caps too. The phono pre (and headphone amp) are loaded with them. You can get pretty tight tolerances on them too.

 

[George S.]

My understanding is larger storage caps create a larger inrush of current when the unit is turned on. This creates a potential to burn out a switch. The design engineers have this all figured out when they spec the parts. I've read that some builders get around this by installing a soft start relay.

 

[George S.]

Resistors, the old PL preamps have carbon resistors. I upgraded mine to 1% metal film. It's debatable if it makes a difference. Doing so on a receiver would be a massive job.

 

[wattsabundant]

older caps tended to be specified as -20% +80% of value.
Thus they could be almost double in value and still be in specification.
One CAN go higher in capacitance, just mind the rectifiers...

Thus one can safely double the capacitance.
We / I DO encourage new (replacement) diodes (1 amp or 3 amp) if this is done, thus old tired diodes don't mess things up.
(1 amp 1n4004 -1n4007, 3 amp 1n5404 -1n5407)

To make physical size, increasing the voltage rating is a very effective (and within limits, harmless) way to find the physically larger caps.

At my day job, we used 7700uf caps by the train load. They tended to run close to 10,000uf. Then 15,400 became available in the same can. We started doing capacitor change outs 1 x15,400 for 2 x 7,700 and found out the 15,400's were on the low side of the spec and we didn't have enough overall capacitance. We switched to 2 x15,400 for 3 x 7700 and that works out. When you're changing out 30 or 40 caps the savings in labor offsets the material cost. So today the manufacturers have better quality control and you don't get free microfarads. You get what you pay for.

If you look at the specs on rectifier diodes you find out they can withstand huge (10x) amounts of inrush current. the 1N4004 diode is rated for 1amp and has a surge curreent of 30 amps. Worst case inrush current I've been able to record on a 700B was about 150 amps. The stock 25 amp rectifier has a surge current of 300 amps. I don't believe there is a need to change out the bridge rectifier when doing rebuilds because you can't hurt it. The line fuse will go before the rectifier fails. The impedance of the transformer limits the current. Short the bridge out and blow fuses all day long and you won't hurt the bridge. The on/off switch is a different story.

As for the switches, .01uf ceramic was the standard cap across a switch. I see no reason for going larger. The pupose of the cap was to avoid a loud pop in the speakers from the arcing of the contacts opening. Phase Linear recommended the .01's on the rectifier for instances of RF interferance.

 

[wattsabundant]

here is a link to a great paper on snubbers and quencharcs. Paktron quencharcs

 

[J!m]

Pioneer uses a similar (possibly same value) cap on their switch.

What about a “bypass cap” on each main cap? Seems to be all the rage now.

 

[J!m]

Interesting they use metallized film in the quencharc, rather than the typical ceramic (for that purpose).

 

[8991XJ]

The use of a bypass cap on the bigguns is fine. Most of the better gear I have has them.

 

[J!m]

It seems like a simple “mod” with no real downside.

 

[Gepetto]

Interesting they use metallized film in the quencharc, rather than the typical ceramic (for that purpose).

They are self healing is generally why...

 

[Gepetto]

It seems like a simple “mod” with no real downside.

No upside either, those bypasses on bulk are too far away from where the need for low ESR decoupling is. Put your bypasses right at the sensitive load spots in the circuit design. That way you get the benefit of the wiring inductance leading to the load.

 

[J!m]

That seems to be exactly what Wyn did with his designs, although it’s not unusual to have those caps at the chips and op amps these days.

On the other hand, if you “improve” a vintage circuit, at what point does it no longer represent the original? You (perhaps) lower noise and distortion, increase slew rate and eliminate power supply sag… and then what? It no longer behaves like it originally did, so it no longer sounds like it originally did.

Like “resto-modding” a car. Old car with new car performance. But why? The charm of the old car is the rattles and squeaks; leaks and damn choke that never operates properly. Make it perfect and it’s just another new car with an old body on it.

I’m all for modding, but I personally try to preserve as much of the “old n busted” as I can. I restored my ‘71 land-rover totally stock because I wanted to experience driving a new 1971 land-rover. Soon I made changes; quite a few preparing for Africa. But that original experience was important to me.