newbie capacitor question.

[Capacitordeath]

As you can guess by my name, I have a fascination with these things, but know very little about them. I'm going to be tackling an organ conversion very soon, and with little (more like no) electronics knowledge, I'm hoping to learn as I go along by diligently reading and asking on this forum, and reading several ebooks I've found on the subject.

Anyway...so here I am, planning to build a 5e5 or 5e7 out of an old Wurlitzer speaker power amp. I know that I have to either rewire the power tubes to 6l6 power, which is apparently easy, or stick with the 7025a tubes I have. I understand they're very clean...seems simple enough.

I have to test the transformers (or have a dude do it for me) to see if they're still running. Again, not too bad.

I'll also need to decide on keeping the 7199 for the preamp, or buying 2 12ax7s instead of 1. Also...need another socket.

NOW comes the tough part for me, and the part I don't understand. I see that the 5e5/5e7 have what...2 dozen capacitors of varying...uhm...resistance? no...whatever "farads" are. Well, anyway, not many of the caps I have in my organ amp match the spec'd caps in the Fender build. I understand the voltages can be a little disparate, as long as the rating is higher than necessary, but I think I'll buy all new just the same. Now the thing is that I have a pile of chubby cardboard firecrackers, apparently "electrolytic" caps. The chassis layout on the 5e5 looks like electrolytic, and the Bandmaster circuit at Ceriatone looks like it has a couple of electrolytics. BUT, I go look up what a capacitor is, and apparently they come in all sorta flavors, from epoxy, to electrolytic, to tantalum (oooh). Does it matter what style of capacitor I use in this build, as long as they match the specs listed on the schematic? The thing is, I priced the difference between SPrague "orange drop" 750? capacitors, and a repop Sprague electrolytic capacitor, and it seemed a huge jump, like 10x the price. I didn't practice due diligence and check for a .22mf/600v little cap and the same size electro cap, but it looked pretty equally unbalanced down the list.

So..my question is, what if I use little caps on my board? I'm not looking for period-correctness (I'm going to be building a wood combo cab for it, no tweed involved anyway), but I am looking for more mojo in the tone, and I am looking to spend as little as possible, as I'm doing a trial build for my friend who has my amp's sister. Seems to me that, if there is very little difference in tone from switching to a more solid-state capacitor, then it'd leave me to spend a little more on the speakers or something.

Thanks for putting up with my rambling, and thanks in advance for any answers ya'll have.

[tedmich]

Keep the cap values (usually in micro-farads, or millionths of a farad) and the same or slightly higher voltages. The size will be a ton smaller, I just replaced a cap in a 1970's SS amp and it was ~1/3 the size! Unless the amp was fired up recently you probably don't need to "bleed" the caps. Caps store energy (see the Wiki) and can release it all at once into YOU unless you take care; do a google of "rail gun capacitors" to see some real deadly ones.

FYI bleeding just means shorting the leads carefully with a 1-3watt 220k resistor to lose the stored energy

[Enzo]

Capacitors have "capacitance," measured in farads or fractions of farads, such as microfarads - one millionth of a farad.

[Steve Conner]

In tube amp circuits, small capacitors are metallized plastic film, and larger ones, 10 microfarads ("uF") and above, are electrolytic.

This selection of caps covers the needs of most tube circuits:

0.01uF, 0.022uF, 0.047uF, 0.1uF, all 600V plastic film

22uF, 25V electrolytic

10uF, 22uF, 47uF, 100uF, all 450V electrolytic

[tubeswell]

How a cap works
A capacitor is a device with two metal sheets (one lead is connected to each one) separated by a medium which prevents flow of electrons between the plates. However when you hook a current up to one of the plates it charges up with static electricity, and the other plate develops a corresponding charge. The cap will hold the charge while it remains hooked up to the electric current but it will gradually release the charge when you remove the current (sort of like a temporary but short-lived rechargeable battery). Direct current cannot pass between the two plates, but alternating current can, because of the way the indirectly charged plate corresponds to the directly charged plate when the directly charged plate is charged.

Capacitance values
The amount of charge that a capacitor stores is measured in farads. This is directly related to the rate at which a capacitor releases the charge. Caps with a smaller farad value (a micro farad - uF - one-millionth of a farrad) store charge for a shorter time than caps with a bigger farad value (say 1 farad). Caps with an even smaller farad value (a nano farad nF - one thousand millionth of a farad) store charge for even shorter time. Caps with even smaller farad value (one pico fara pF - one million-millionths of a farad) store charge for the shortest time. This is useful is designing circuits, because it can be used to manipulate the frequency of a signal that passes from one side of a cap to the other side. Higher frequency signals have shorter wavelengths and the high frequencies, with corresponding high rate of charges and discharges, will pass across a cap with a small value, because they are not capable of holding bigger charges with a lower frequency release rate (which would otherwise require a cap with a larger farad value). In this way you can get a frequency filter which filters out the low frequencies and only lets the high frequencies get amplified, and so on. Whereas a cap with a bigger capacitance value will let a wider band of (both high and low) frequencies pass from one side of the cap to the other.

Different cap types
As you note, you can get different caps made from different materials. But the higher the cap value, the physically bigger the cap is. Polypropylene or polystyrene or metal film caps in the microfarad range are physically a lot bigger than the equivalent value in an electrolytic cap. So we typically see electrolytic caps where caps values of several microfarads are called for, in order to save physical space. It so happens that electrolytic caps are relatively short-lived compared to other types of caps, so they need replacing more often. Electrolytic caps also have a polarity (whereas other types of caps don't), and polarised electrolytic caps need to be put in the right way around, otherwise they will explode. The '+' sign on the schematic shows the orientation of electrolytic caps w.r.t. to the rest of the circuit.

Sorry about the ramble - hope that helps

[Capacitordeath]

thank you for the responses. Electronics articles tend to start getting deep into jargon territory, using lots of numbers and graphs that I care little about, though I suppose I will start caring soon...

I have read in many places to watch out for capacitors. this machine has been unplugged for months, but I still plan on shorting the caps anyway when I get to unsoldering.

So, they only make the big caps in electrolytic style, eh? oh well. I was hoping to pinch a penny there. On the 5e5/7 chassis layout, there's a row of 16uf/400 volt caps with 25/25 caps bookending them. I was hoping the electronics would fit in a bit smaller of an area. I guess if I want that, I need to build a champ. heh

Anyway, thanks again for the replies. Keep on going, if you like.

[mooreamps]

Ah, the world of capacitance. Depends on how deep you want to go. In this pond, one can ponder the workings of a very good low ESR Tant filter cap, as used in a power supply, or study the various inter-element capacitance of different vacuum tubes, and calculate the miller-capacitance of each as both the function of gain and frequency ; or in the RF world reading the complex impedance of a transmission line ; R + Jo or R - Jo on the Smith Chart. { -Jo being the capacitive component}.

However, in the world of your project, perhaps to point out, that those different types of caps are used for different jobs. For example, those orange drop caps you mention. Most designers use those for signal coupling between stages. The bigger electrolytic caps for power supply filtering. The Tant's are a good choice for low voltage power supplies.

Fortunately, in the grand scheme of things, tube amps in this respect can be simplistic in nature. Just use the right cap , for the right job, be very mindful about the working voltages in your amp, and the voltage ratings on your caps, and be very mindful to make sure you keep the polarity "correct" on the filter caps, you'll be OK.






-g

[Enzo]

A word of advice. 16uf is an odd value. 50 years ago it might not have been, but it is now. SO if you go looking for 16uf caps, they will be expensive. 20uf or 22uf is a LOT more common a value. Those will cost far less, and will work just fine. You will find modern caps will be a whole lot smaller physically than they were in the old days.