B+ input capacitor question...

I often see in schematics an electrolytic capacitor at the B+ input to an audio circuit after the power supply filtering stages. Assuming that the applied B+ voltage is sufficiently filtered in the power supply section is this capacitor necessary, or is it doing something more than acting as a as a final filtering stage?

etet said:

I often see in schematics an electrolytic capacitor at the B+ input,
is it doing something more than acting as a as a final filtering stage?

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Greetings etet,

If I get what you are referring to,
This cap is possibly an added means to assure a consistent B+ conveyance to the Plate (anode).

But what do I know, I'm just a ShamWow! Loom operator.

Like Zandru I'm not exactly sure what you're asking, could you be a little more descriptive?
(then we/I could answer your question proper)

Usually there are three or four caps. There are a few factors on the "why". The smaller the capacitance the less filtering occurs. With tubes, ac has to be filtered coming off of the rectifier, often times one cap isn't enough to smooth it out. If there isn't enough filtering from the caps you get hum. One may think just sticking a rectifier in a circuit throws pure dc into a circuit, it does not. If you throw a scope on a rectifier before the caps you will see dc but it isn't smooth. Also different tubes require different voltages so this combination of different caps with resistance values allows for different voltage value to arrive off the different caps in the circuit.


At least this is my understanding but I'm no expert.

etet -- You are exactly correct that the lion's share of the filtering action takes place early on in the B+ distribution system. And, as other have pointed out, the caps you are referring to -- such as those likely associated with say the phono and line/tone stage sections of an amplifier -- do help "finish" filtering the B+ from whatever ripple was not removed by the earlier capacitors. But these latter caps you are referring to (associated with the stages mentioned) perform one other very important function called "decoupling". In fact, old timers used to call these caps decoupling caps.

Besides additional filtering, the presence of these caps prevent multiple stages that are powered from a single point from interacting with each other -- not only within a given channel, but from channel to channel in a stereo amplifier. Without them, if two stages of amplification (such as a phono preamp might represent) are powered from a single point, the signal output of the last stage can travel down its plate resistor, back up through the plate resistor of the previous stage, and right back into the grid of the second stage again. In other words, we have an oscillator -- which is exactly why a unit with defective electrolytic caps at this point will often "motorboat" -- so named because of the putt-putt sound the unit makes because the stages are now oscillating. In this example, with a good B+ filter (decoupling) cap at the junction of where these two stages are powered from, a very low impedance (effectively a short to any AC signals) is provided by this cap to ground, that prevents the signal from the second stage from being able to travel back to the first stage.

Knowing this now, since most stereo amplifiers power the equivalent points of each channel in an amplifier from the same B+ point, you can just imagine how much cross talk would occur between the channels without these caps at each B+ distribution point were the caps not there (assuming it would even remain stable without them).

I hope this helps!

Dave

Thanks for all the replies, guess I should have been more precise with the question but dcgillespie must be a mind reader...this is exactly the answer I was looking for and I now fully get it.

Dave, that was an excellent explanation and the examples you cite made it perfectly clear. Thank you.

This type of information may seem quite basic to most of you, but since I didn't pay much attention in tube theory class (when would I ever need to know that old stuff? ) I don't really understand a lot of it despite a lot of reading and building a few very good sounding SE amps.

So...seeing as how there is a lot of in depth knowledge here what would really help me (and others I'm sure) I would really like to see a complete circuit analyses of a simple audio circuit (say an RH84 or something) going through each component with the same type of detailed explanation as given by dcgillespie (hint..hint). While I have tried finding this type of information before it seems it's not all in one place and a lot of it is not as clear as the answer I got about the decoupling caps :thmbsp:

Thanks

etet said:

Dave, that was an excellent explanation and the examples you cite made it perfectly clear. Thank you.

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What a magnificent Idea ET,

Having a simple step by step circuit "Primer" explained by someone like Dave,
would be very helpful for people such as myself and many others alike.

Now Get To work! Gillespie, we're all counting on; and believe in your ability
to easily convey otherwise obfuscating and ambiguously daunting head
scratching while delving into the electronic realm.

Time for another short subject from the lab at Tronola?

http://www.tronola.com/html/dave_s_lab.html

Hey guys...thanks for the links, some good info there.

But, what I'm really after here is a component by component circuit analyses that goes into some detail of exactly what is going on and why from the input to the output of a simple SE vacuum tube amp (to start with) in plain English, kinda of like a Tube Amps for Dummies (like me ).

This would not have to be done in one post...or by one person, there is so much knowledge here that the potential for an excellent series of tutorials is pretty much assured.

Anyone else interested in this?