Capacitor across resistor?

[BeëlzeM]

Hello,

I am in search for help. I read an instruction somewhere on the internet saying:

Locate the 10K cathode resistor and place a .68uf cap across it

What does this mean exactly? How should I solder it? Somewhat like a bridge from before the resistor over it to after the resistor?

Thanks

 

[Dean Huster]

Yes. If it's a temporary connection, just "tack" solder the cap in place. If it's permanent, be sure to give the leads at least a half-wrap around each end of the resistor and be sure (in either case) to keep the cap leads as short as possible.

Dean

 

[BeëlzeM]

Okay thanks.

So it should look something like this?
I'm not really good yet at electronics.

**broken link removed**

Also, what does it do? What does the cap do to it all?

 

[stevez]

A simple explanation - the resistor by itself conducts DC as well as AC and offers a relatively constant resistance without regard to the frequency of the AC.

A capacitor by itself does not conduct DC and acts like an infinitely high value resistor to the flow of DC current. With AC the resistance to the flow of current decreases as the AC frequency increases.

The resistor and capacitor in parallel offers a 10k resistance at DC and the resistance goes down as AC frequency increases.

 

[BeëlzeM]

So simply said, adding that capacitor to the already existing 10k resistor makes the total resistance for the DC flow 10k as well for the AC flow but the 10k resistance for the DC gets less as the AC frequency gets higher? The AC flow stays 10k all the time?

Or am I totally wrong?

 

[stevez]

No - AC will flow thru the capacitor. The apparent resistance offered by the capacitor to the flow of AC is high at low frequencies and decreases as the AC frequency increases.

So at DC, the apparent resistance for the pair is 10K - like the capacitor isn't there at all. With DC plus low frequency AC the apparent resistance of the pair is like paralleling a 10k resistor with a 10megohm resistor - it's ever so slightly less than 10k but it is less. As you increase the AC frequency the capacitor will behave like a 10meg resistor, then 100k, 10k - you see the progression.

I've used "apparent" resistance to keep from throwing new terms at you. You might have a look at capacitive and inductive reactance to see more of what is involved here. With a little bit of reading you should be able to calculate the "apparent resistance" of the pair at various frequencies.

 

[BeëlzeM]

Thanks for the explaination. I will definately find more about it, and try it out how it works.

 

[Sceadwian]

This is what simulation software is good for. You can sweep an AC signal across the components and look at the circuit responce.

 

[mneary]

Just be sure to be careful and discharge the plate voltages before touching anything. The voltages in tube (valve) amps can kill you.

 

[BeëlzeM]

It is indeed for a simple tube amp modification.
Where can I find instructions to discharge the voltages left in the amp? Or is it just a matter of leaving it unplugged for a while?

 

[Leftyretro]

It is indeed for a simple tube amp modification.
Where can I find instructions to discharge the voltages left in the amp? Or is it just a matter of leaving it unplugged for a while?

Yes just let it alone for awhile after powering off, the resistor across the cap will discharge the cap, the time being related to RC time constant of the R & C valuses.

Lefty

 

[ecerfoglio]

Where can I find instructions to discharge the voltages left in the amp? Or is it just a matter of leaving it unplugged for a while?

If you leave it unplugged for a while the capacitors should discharge across the resistors in the circuit.

But, just to be shure, before puting your hand into the circuit measure the voltages that may sill be there.

Hold one probe of your meter (ussually the black one) to the circuit's chassis (ussually the power supply's negative), select a voltage range that is high enough (at least 500 VCC) and explore the circuit with the oter probe.

Be shure to measure all the high voltage points: Filter capacitors, tube anodes, etc

If you get any reading of 25V or more the circuit needs to be discharged before working on it.

NOTE: This is for a "simple" audio amplifier with voltages up to 250 V or so. For the really high voltage and / or high power circuits (like CRT in TV's, monitors or osciloscopes, microwave ovens, X ray machines, High power radio transmiters, etc) you should take aditional precautions.

If you don't know how to work safely inside one of theese, it's better (and safer ) to keep away

 

[mneary]

And.... never work alone on high voltage (even if discharged).