help!

[mr.jetski]

hi all,

if you have mixed ac and dc signal, say plate voltage. to block the dc one would use a cap inline. if you theoretically wanted to block the ac signal and let the dc pass. how would you do that?

a rectifier cuts half the ac signal so thats out.

[Bruce / Mission Amps]

hi all,

if you have mixed ac and dc signal, say plate voltage. to block the dc one would use a cap inline. if you theoretically wanted to block the ac signal and let the dc pass. how would you do that?

a rectifier cuts half the ac signal so thats out.

Choke/inductor who's reactance is very high at AC and very low or a dead short at DC.

[mr.jetski]

hi bruce,

thanks, can you elaborate a bit more. how do you find the right one mathematically?
i want to use both just separated

[Enzo]

Can you give us a specific example of an application of this? How would you use it?

And don't write off diodes just yet. When you rectify AC centered around zero, you do remove the one polarity, but if your AC waveform is elevated it remains all DC of one polarity, it just changes voltage.

I am just guessing now, but if you wanted to follow some level and have the amp react - some sort of servo circuit - or you wanted to create some voltage that related to the signal level, you might look up AGC/AVC in a text like the RDH. Automatic gain control/automatic volume control. Used mainly in radio receivers, the AGC senses signal level then makes a DC voltage that corresponds to level then uses that to control stage gain. It tries to keep your radio reception at a steady level as the signal comes and goes. Of course this might have nothing to do with your project.

[mr.jetski]

thanx enzo,

a left turn for a second. what does a cathode cap do? function?

[mooreamps]

thanx enzo,

a left turn for a second. what does a cathode cap do? function?

The cathode cap bypasses the ac signal around the bias resistor. Different values will give you different types of voiceing from that gain stage. If, given a typical value of 1.5 K ohms of a bias resistor, typical values of a cathode cap I have seen in the range from 0.68 to 22 uF. But if you end up building a new pre-amp with my para-amplifier mods, those pre-amp gain stages will be fixed bias, and you will not be using either bias resistors or by-pass caps.

-g

[mr.jetski]

thanx g,

thats what i thought. i have an idea....

[tubeswell]

Cathode bypass caps also function to hold the cathode voltage steady when the Plate voltage is going up and down (when a signal is being amplified through the tube). The cathode would otherwise not be at a flat voltage because a cathode biased tube has a voltage across the cathode resistor, and because this is in the current path of the electrons flowing to the plate and the cathode resistance is fixed, the voltage through the resistor invariably follows (albiet with less severity) the voltage swing on the plate. When you add a cathode bypass cap, the cap functions to store charge on the upswinging voltage (through the cathode resistor) and discharge on the downswinging voltage (through the cathode resistor), helping to hold the cathode voltage flat. If you didn't have the cathode bypass cap, you wouldn't get as much gain out of the tube because the voltage between the plate and the cathode is reduced (because the cathode voltage would be swinging in synch with the plate voltage, so there is relatively less voltage difference between the plate and the cathode, and hence less gain). With the cathode voltage held flat by the bypass cap, you get more gain out of the tube