General capacitor value (swapping) and function question

[spiral]

From reading these boards i get the impression that swapping cap values isn't a huge deal and that value differences are often fairly minor.

For example i just replaced the power supply caps in my silverface with caps that are supposed to bring it closer to a blackface circuit. my SF had: 3 x 16uF@450v & 2 x 20uF@500v and the new caps were all 16uf@450v. To me that seems like a huge change, but i don't understand electronics, so why the change and why doesn't everything blow up? Well, i haven't tested it yet so it may.

Also, i replaced all of the white mallory caps which were different values, one was 5uF, and the other 5 were 25uF and one was missing as stock. They were replaced with all the same value. That blows my mind: one missing cap, and two different, very specific values seems very deliberate.

I know it is a very broad question but why can you switch caps in and out without the circuit not working? Do filter caps work like audio filters where they are removing electronic frequency? What happens if you just removed all the caps i just replaced ... armageddon? Should i be replacing ALL the caps in this amp? And why or why not?

Thanks!

You probably think i shouldn't be poking around in an amp with these kinds of questions but i just have no theoretical foundation.

[tedmich]

Caps values are somewhat robust, although I would always recommend keeping the highest voltage rating for safety...500V ->450V is courting failure. Also more filtering in the supply is usually better but the old caps may actually have less than the new one because they have degraded.

The Mallory caps if truly in the signal path should be replaced exactly or under the guidance of a seasoned tech, they need 400-600v ratings which may not be noted on the part!

a general rule for cap replacement: keep voltage rating or use higher, keep capacitance value or use higher (PS) or change capacitance value carefully and subtly in the signal path.

[Enzo]

Your five caps were 16uf or 20uf, and you replaced the two 20s with 16s. That 4uf difference is only 20%, which falls within the tolerance of most older caps anyway. Not a huge difference in my mind. In fact, I'd likely go the other way and instead of searching out odd values like 16uf, I'd use the far more common - and a lot less expensive - 20uf or 22uf.

450v is a working voltage rating. I wouldn;t just replace 500v caps with 450v caps, but depending on the circuit voltage and the surge rating of the caps, you might get away with it. A 450v cap will work at any voltage up to 450v all day. You really ought not go over that. If the 500v caps were in circuits that run over 450v then you ought to change them.

Think of a step ladder with 250 pound capacity. A 250 pound man can stand on the rungs safely. If he is carrying 100 pounds of roofing on his shoulder, then the ladder is under rated.


Why doesn;t it blow up? Those caps are smoothing caps, they smooth out the p;otentially bumpy DC into smooth DC. They remove hum essentially. They are sorta like shock absorbers. If you go with smaller caps - 16 instead of 20 - then POTENTIALLY there could be more hum in the amp or it might sag more on peaks. You might not hear much difference... or you might. But that won;t blow it up. Imagine shock absorbers on your car. if you install softer ones, the car might not corner as well or smooth out bumps as well, but the car won;t blow up.

Those are all filter caps, keeping your high voltage smooth. The other caps are in the signal circuits. To over-simplify, the larger the cap is the more bottom end response it allows. If it is the cathode bypass cap in a preamp stage, the difference between 5uf and 25uf might not be all that audible. There is not very much bottom end in a guitar anyway. There is more to the story than that, but it is enough. Back to the car. If I put racing shocks on my car, my mom might not notice much difference driving to the store, but driving hard in a rally it makes a huge difference. Caps are like that - it all depends on what the amp is being asked to do.

To really hear something obvious, offhand I;d say a factor of 10 or more would be an appropriate change. That is why you might see 20uf cathode bypass on one stage but only a 0.68uf bypass cap on another. The difference between 20 and 25 would not usually be audible.

leave them out? That is very different from changing their values. Leave the shock absorbers off your car? Very different from wrong shocks. Those filter caps if removed would leave the amp making loud hum. If you left out those 5uf and 20uf small caps, the amp would lose gain and bottom end. If one was in the signal path directly, leaving it out could break the signal path and then nothing comes out.

[spiral]

Caps values are somewhat robust, although I would always recommend keeping the highest voltage rating for safety...500V ->450V is courting failure. Also more filtering in the supply is usually better but the old caps may actually have less than the new one because they have degraded.

The Mallory caps if truly in the signal path should be replaced exactly or under the guidance of a seasoned tech, they need 400-600v ratings which may not be noted on the part!

a general rule for cap replacement: keep voltage rating or use higher, keep capacitance value or use higher (PS) or change capacitance value carefully and subtly in the signal path.

Thanks for the reply. You are for sure right about the old caps. They were nearly 40 years old and both the larger ones were leaking. I was surprised my amp didn't sound worse than it did.

All of the caps that i just installed are in line with what is specified in the Blackface schematic. The original parts, and the new parts are all 25 and 50v.

Thanks again.

[spiral]

Your five caps were 16uf or 20uf, and you replaced the two 20s with 16s. That 4uf difference is only 20%, which falls within the tolerance of most older caps anyway. Not a huge difference in my mind. In fact, I'd likely go the other way and instead of searching out odd values like 16uf, I'd use the far more common - and a lot less expensive - 20uf or 22uf.

450v is a working voltage rating. I wouldn;t just replace 500v caps with 450v caps, but depending on the circuit voltage and the surge rating of the caps, you might get away with it. A 450v cap will work at any voltage up to 450v all day. You really ought not go over that. If the 500v caps were in circuits that run over 450v then you ought to change them.

Thanks for the great explanations. The 16uf caps were supplied in a kit to bring my amp in line with a blackface circuit so i trust the person i got it from has done the research and worked on more amps than me. I called about it and he assured me that 16uf @450v is the blackface spec and what i should use. I checked the drawings and indeed the blackface deluxe reverb has 5 x 16uf@450v caps. Does going 20uf @ 500v just buy me more insurance? Why wouldn't you use 40uf @ 500v? Cost? Sound?

What you said about the differences between values makes perfect sense and also about leaving all caps out. So basically they just smooth the voltage.

I don't plan to deviate from the kits i got or the original fender schematics but knowing this stuff make me much more comfortable working with it.

Also, i would love to build a car that blew up if it didn't have shock absorbers, or just turned too sharply.

Thanks again.

[txstrat]

Does going 20uf @ 500v just buy me more insurance? Why wouldn't you use 40uf @ 500v? Cost? Sound?

The "over filtering" (not the voltage rating) of the amp can rob it's tonal qualities, due to stiff behavior and muddy tone. The amp will lose sag and be less dynamic. I usually go no more than around one step over the schematic values.

[pdf64]

The voltage rating is the concern.
BF spec caps went in a BF amp with BF transformers putting out BF voltages via BF rectifier (GZ34).
You've got a SF amp, transformers, rectifier (5U4) therefore voltages.
You need to confirm that a 450V rating is sufficient, at switch on/standby, at your maximum line supply voltage (ie not just what it measures today).
Did the guy who did your research look into these variables? How good is his liability insurance?
Stressed caps may explode, short out and destroy your power transformer.
Did you replace the bias supply cap? If so with one rated at least 70V? If not see above, but also include your output transformer in the possible collaterol damage. Peter.

[spiral]

The voltage rating is the concern.
BF spec caps went in a BF amp with BF transformers putting out BF voltages via BF rectifier (GZ34).

Got it. The rectifier is a GZ34. The only question would be the transformer. It is a '71.

You've got a SF amp, transformers, rectifier (5U4) therefore voltages.
You need to confirm that a 450V rating is sufficient, at switch on/standby, at your maximum line supply voltage (ie not just what it measures today).
Did the guy who did your research look into these variables? How good is his liability insurance? Stressed caps may explode, short out and destroy your power transformer.

How can i test that 450v will be ok? The kit is from Torres engineering. I'm not committed to using these values, i was just following instructions. I'm open to and welcome recommendations.

Did you replace the bias supply cap? If so with one rated at least 70V?

I haven't replaced it. Is this the 50pF cap just behind the lamp? If so it is rated at 150v. Should that be replaced as well?

Thanks.

[pdf64]

The fact you are asking such basic questions indicates that you aren't competent to be tackling this job.
Please take it to a tech to check out.
I hate to stiffle your enthusiasm, but a high voltage classic amp is a bad platform for you to learn this stuff. Peter.

[tubeswell]

Thanks for the great explanations. The 16uf caps were supplied in a kit to bring my amp in line with a blackface circuit so i trust the person i got it from has done the research and worked on more amps than me. I called about it and he assured me that 16uf @450v is the blackface spec and what i should use. I checked the drawings and indeed the blackface deluxe reverb has 5 x 16uf@450v caps. Does going 20uf @ 500v just buy me more insurance? Why wouldn't you use 40uf @ 500v? Cost? Sound?

450V should be okay if your power transformer is a blackface deluxe one that puts out 330VAC on each side of the HT winding and you are using a GZ34/5AR4 rectifier. As Enzo said you probably won't notice much difference between 20uF and 16uF.

Also GZ34/5AR4 datasheets indicate they can take up to 60uF reservoir capacitance. You will see that those first two 16uF on the BF schematic are in parallel, making 32uF. You could just as easily have 2 x 20uF there without it changing things too much. It may be slightly less 'squishy' with 40uF instead of 32.

[spiral]

450V should be okay if your power transformer is a blackface deluxe one that puts out 330VAC on each side of the HT winding and you are using a GZ34/5AR4 rectifier. As Enzo said you probably won't notice much difference between 20uF and 16uF.

Also GZ34/5AR4 datasheets indicate they can take up to 60uF reservoir capacitance. You will see that those first two 16uF on the BF schematic are in parallel, making 32uF. You could just as easily have 2 x 20uF there without it changing things too much. It may be slightly less 'squishy' with 40uF instead of 32.

Thanks. It sounds like it would be best to get all 20uF 500v caps for the power filtering which shouldn't affect the sound at all.

The id on my transformer is J025130 which according to the SVVA site means my transformer is identical to the original AA763 blackface transformer: 125P23B.

Also to answer my own question: the bias supply is indeed behind the pilot lamp. I converted the 10k pot to properly bias the tubes a bunch of years ago. I just need to update the bias supply cap as 100v is recommended but it is only 70v right now (original part: 50uf 70v).

[spiral]

I just wanted to thank everyone for the help. I finished recapping (electrolytics only) my amp and put in new tubes. I ended up replacing almost all the caps and it sounds pretty great. Since the power supply caps ended up being the same value as they were originally i only really removed the caps on the power tubes going to ground, and used the BF value for the phase inverter cap.

Now i am just trying to reduce the noise but asked that question as a reply to an existing thread. Thanks again.