Question about Filter Cap substitution

Al Moss · From: Kent,OH,USA

Can a 100mf/100volt filter cap be substituted for a 70mf/100volt filter cap? I'm having the filter caps replaced in my trusty old Fender Concert, Rivera design from about 1983. -suspected that a slight volume loss might be connected to the age on the filter caps. Power tubes and preamp tubes have been checked out as ok and so has the bias, and, I think that my ears are ok too. The snag is that the tech who is doing the work cannot seem to locate a direct replacement for the 70mf/100volt caps that are currently in place. Five of the eight caps have been replaced identically, but, the remaining three are the 70mf ones. He can get a 100mf/100volt but is anxious about putting it in because of the difference in value. I'm out of my depth here. Any advice?
Thanks.

Steve Matlock · From: Bentonville, AR USA

I would say that it depends on what function the cap plays in the circuit design. It could cause some changes in the amp you would not want. The safe play is to stay with the same capacitance rating. You should be safe to go to a higher voltage rating if thats all you can find.

Steve

Mike Wheeler · From: Delaware, Ohio, USA

I agree. I'd stay with the same 70mfd cap. value, if at all possible, but going to a 150v or 200v rating would be fine. Of course, larger volt ratings usually mean a larger cap...but if you've got the space for it, no problem.

You could even go 20% higher, to 80mfd., without any adverse effects. But I wouldn't go higher than that without investigating the design more closely.
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Mike

Mike Fried · From: Nashville, TN, USA

From 20+ years of amp servicing, I would expect no adverse effects from going from 70mF to 100mF for that cap. To keep manufacturing costs down, design engineers often specify the smallest useable (hence least expensive) value for a given circuit component, especially relatively expensive ones such as large electrolytic caps. Power supply ripple filters have a pretty wide range of useable value and it's common for techs to as much as double or even triple (for older amps) the design value in their replacement. That cap is for the power tube bias supply (I just looked it up), and 100mF is very commonly used for that. Go ahead and use 100mF/100v, it'll be fine.
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Robert Leaman · From: Murphy, North Carolina, USA

Filter capacitors have nothing to do with tone quality except to say that if there is insufficient capacitance, tone is degraded by rectifier hum usually 120 Hertz for a full-wave, 60 Hz, single-phase rectifier. You can increase capacitance without adverse effects but do not decrease the volatge rating. If a 400 VDC electrolytic capacitor is operated at 250 VDC, it becomes a 250 VDC capacitor because of the electrlytic forming process. Be sure that the technician bypasses the electrolytics with a 0.1mfd film capacitor rated for the operating voltage. Electrolytic capacitors are very poor at frequencies much above 1kHz since they are relatively inductive. The film capacitor prevents higher frequencies from degrading the rectifier filtering capabilities. This bypass procedure is routine with good audio amplifiers and any amplifier that can reproduce high frequencies.

Greg Cutshaw · From: Corry, PA, USA

At 1 Khz the capacitive reactance of the 100uF cap is 1.59 ohms and the reactance of the 70uF cap is 2.27 ohms. Even at 100 Hertz the figures go up to only 15.9 and 22.7 ohms respectively. Compared to the values of the plate resistors and other circuit values this is a complete don't care. You won't hear the difference in frequency response. Although the series inductance increases with frequency for some caps, only you can decide if you like the effect of that or not.

The 100 uF cap might hold out longer on an extended duration bass note so I might favor it for that. As caps age, they can lose a lot of capacitance, have lower values of leakage resistance and sometimes gain series resistance and these things (aging related) can affect tone a lot more.

greg

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

I think Mike Fried has it - if this is for the power tube bias supply, having extra capacitance sure can't hurt. The objective is to have as stable a bias voltage as possible, in fact I think a bit more capacitance there is a good idea in that direction. Either way, this isn't in the direct signal path, per se.

Ripple or instability in the bias supply circuit can have a major effect on the amp's sound. It's a real good idea to check it out and/or rebuild it in an amp more than 10-20 years old, IMO.

Jim Sliff · From: Lawndale California, USA

At that voltage it has to be a bias cap, and it's S.O.P. in old Fenders to replace the 50 or 70uf bias caps (the 50uf usually also being 50v) with 100uf/100v caps. The only effect it has is increase the safety factor - there is no sonic effect.

You can always increase the voltage with bias and power filter caps, which is a maximum rating; increasing the capacitance has varying effects - in a bias circuit it provides more stability and control over bias settings, a good thing for the power tubes and output transformer; in the power filter section increasing the capacitance will generally tighten up low end, giving you more usable bass frequencies (depending, of course, on whther or not you have a speaker capable of reproducing them).

When doing cap jobs, unless someone wants specific value caps I'll increase the values; 22uf to replace 16 or 20uf, 80uf instead of 70uf (Sprague Atom 100uf's won't fit in the cap can, and I don't use the cheap 100uf caps that fit). At the same time, the carbon resistors go away in favor of wire-wound. FWIW I never replace low voltage bypass caps unless there is a specific problem - in fact, anyone who wants to get rid of blue tubular ones from old Fenders is welcome to mail their "trash" to me! Winking

Overall, a few uf's up or down with electrolytics won't make a noticable change in the sound. Just don't go DOWN in voltage - and absolutely make sure you have the polarity correct or you will get a nasty surprise. Trust me, do NOT solder the second cap wire until you've re-checked the polarity. Just about every tech I know has learned this one the hard way. The smart ones only blow it once....

There are lots of debates about bringing caps up to voltage slowly to "form" them. I used to do it religiously with my Variac. But - nowdays most new caps are pre-formed, and amp manufacturers (except for smaller boutique outfits) NEVER formed their caps - when the parts were installed they turned the things on. Fender wasn't about to slowly bring up voltages over 24 hours on a hundred amps at a time! I stopped forming them at least 5 years ago and have noticed zero difference in sound or durability.

And last (which should have been first) another reminder about safety - discharge all filter caps before working on an amp. Always. No exceptions. Even if it's been stored for months. We've all made this mistake as well - once. And that one time hurts like hell.

Hope that helps....
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No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
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1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional

Robert Leaman · From: Murphy, North Carolina, USA

When filter capacitance is increased in a circuit that uses vacuum tube rectifiers, some thought must be given to the rectifier "hot switching" current. There is an excellent definition of this in the RCA "Radiotron Designer's Handbook" by Langford-Smith.

Jim Sliff · From: Lawndale California, USA

Al Moss · From: Kent,OH,USA

Many thanks to you guys for the responses. -lots of useful info to be relayed to our technician.
Cheers,
Al

Paul Arntson · From: Washington, USA

One more thing to consider is the tolerance of the cap spec. A lot of electrolytics are manufactured with such broad tolerances that a 70 uF and a 100 uF might measure close to the same capacitance on a meter.
I agree it shouldn't be a big deal.

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