Capacitor polarization help!

[losacco]

Hello, getting ready to start this build and I have newbie question. As far as capacitor polarization, are plus and minus marked on all capacitors? I’ve noticed on one of the capacitors I’m using, one lead is shorter than the other. From the circuit, can you tell the hook up direction? Are resistors hooked up in a certain direction?

Thanks for any help on this.
Patrick

http://www.diyparadise.com/simplepreamp.html

 

[torrence]

Hi Patrick,

Electrotytic (and tantalum) capacitors most definitely have a polarity
and must be installed properly. Electrolytics usually have the negative
terminal indicated on the can either with an arrow or a band. They
often have one lead shorter than the other as well. Confusingly,
Tantalum capacitors that I have seen usually have the positive terminal
indicated.

Other caps (polyester, polypro, metal film, ...) don't have a polarity
even though some expensive decoupling capacitors (like Auricap) have
different colored leads. This is for noise performance, not technically
for voltage polarity, and indicates which lead is attached to the outer
shield.

A nice visual example of identifying the polarity on electolytic caps
is provided by Tubelab:

http://www.tubelab.com/AssemblyManualSimpleSE/Capacitors_SSE.htm

 

[bereanbill]

Just to add to what torrence said, electrlytic caps generally will have the negative lead to ground...i.e. when used as filters, or cathode bypass caps. One exception is when used as filters in a bias circut. Then they will have positive going to ground.

 

[torrence]

The capacitor needs to be connected so that the voltage drop across
the capacitor (from positive to negative terminal) is always positive.
If you are connecting between a grounded cathode and -35V bias on
a grid, then indeed the negative terminal should be connected to
the -35V side. The point is that if the voltage on the "negative"
terminal becomes more positive than the voltage on the "positive"
terminal, the capacitor won't work, and you risk a catastrophic
failure (explosion).

 

[gkenne]

I really hope I'm in the generally correct place for this, I didn't want to post a new thread if I didn't have to. So, I have built a Fender Deluxe 5A3 amp, I started from a Bell Sound Labs integrated amp that I acquired years ago. It has the same exact tubes as one version of the 5A3, one 6SC7, one 6SL7, two 6V6GT's and one 5Y3GT. So that was a perfect fit, the amp actually two 6SC7's and after trying for a while to get the two stages in front of the 6SL7 to work right I decided not to, (at least not yet) try to merge two Fender Schematics. Long story, short. I got it all wired fairly good, I replaced a two of the coupling caps in the Pre-amp stage. Basically the thing is almost exactly like the Fender from 1953 the same year as the Bell 2122-B. It does work after much needed education on my part, trial and error. So, bottom-line is, now it works, I bought a used 12" Fender speaker by Eminence from a Twin, put it in a cabinet I built from a home stereo Yamaha Speaker. I've got the volume, Bass and treble all working very well, the amp has a nice tone and I'm happy with it for now, except after about 5 or 10 minutes it starts this volume fluctuation on it's own. So, I noticed the 6v6 actually fluctuating, as far as the glow also. Up and down. One thing I noticed is that the two coupling caps going to the 6v6's seem to be turn the opposite direction from the other caps. They have the Negative side away from the 6V6's. Is this the normal direction? Would they work at all if they were in the wrong direction? One thing is that this is the same way they were directed in the original amp, despite what the schematic shows. Any help here would be appreciated, I have ordered some JJ 6v6's. That would rule out the problem being the tubes, but could it be any of the other tubes causing a problem like this???? So many questions. Thanks in advance. I'll keep looking around for answers. One thing's for sure, I'm hooked.

 

[DF96]

This would be better in a new thread, probably in Instruments & Amps section, rather than hijack a thread about capacitor polarity.

The issue appears to be motorboating. Which 6V6 glow: heater or anode? The anode should not be glowing at all. If the heater is fluctuating then you have a bad solder joint or a PSU problem. A circuit might be helpful. Coupling caps in valve amps are very rarely polarised, because electrolytics leak too much. Are you sure these are coupling caps, and not cathode decoupling caps?

 

[Kjeldsen]

Just to make sure you know. Building tubeamps will normally include working with high voltage. Just be carefull. I can't open the link, så can't se the operating voltages. Some tube preamps are working at normal low voltage levels, but often it's 300 - 500 volt.

 

[gkenne]

Oh Yes, I'm very careful to discharge the caps very well and then I double check them with a ammeter. I ended up asking the guy over at the Antique Radio store here in San Diego, about the fluctuations. He said it was more than likely the coupling caps, so I replaced them and did a couple of other changes, such as decoupled the cathodes with dual resistors to help them come to their own voltages rather than forcing each other to come to a common voltage, supposed to help with early tube failure. And also acts as a fault protection for the output tubes. Now I just got done playing it for a couple of hours and it kept going very well, I still think I could get a bit more ummph out of it, but it seems to be loud enough. Now I will start searching for how to add the additional 6SC7 as an additional gain stage. Thanks.

 

[gkenne]

Oh sorry, DF96, I forgot to reply to that. The original caps were the old paper sealed in the hard plastic case. I first used electrolytic in the first stage because I had several that I had taken out of an Epiphone Valve Special. So, they worked okay. The other ones are not Electrolytic. Eventually I suppose I will change out the first two, and do some re-wiring to make things a bit more tidy. But, the amp is fairly quiet as far as hum, until I turn on a couple of overdrive pedals. My concern now is getting the other gain stage working, any ideas would be greatly appreciated, maybe I can add tremolo or something else.