In this thread I will ask questions about the signal path of a 5f1 Champ

[mort]

I've become a decent "assembler" but there is still very much I don't understand about what actually happens in the circuits so I thought I would start with the simplest of amps and walk my through it, attempting to understand. I will show my ignorance, and please feel free to point out and correct where my understanding is off.. . I'm sure it will be way off in some areas.


http://3.bp.blogspot.com/_BYIkYo9cnn..._5f1_schem.gif



So starting with the input, the circuit accepts the mV signal from the instrument and depending on which input is used, there is a 1M resistor in play and 2 68k resistors. It looks real simple but I'm having a hard time conceptualizing what is actually taking place there. I can see that when plugged into input 2, that the signal goes through a 68k resistor and then into the grid of the 1st tube, and when plugged into input 1, the signal gets referenced to ground with a 1M resistor and then splits and goes through both 68kR's. How does this make for a stronger signal at the grid when plugged into input 1??



next step-- the signal must pass from the grid to the plate of the first triode, and then does that .022 cap block the B+ but allow the signal to pass on to the volume control?? How does the guitar signal get through and not the B+ ??


Thoughts?

[The Dude]

HERE is a thread that discusses the resistor network at the amps input.

On to part two: A capacitor blocks DC and passes AC, so you are correct in your assessment.

[mort]

ok after reading that link the 68k pair make sense now. And the 1meg resistor serves to keep signal from being bled off to ground? Or perhaps there's more to it than that?


And then after the signal passes through the first .02uF cap there's another 1meg resistor(pot) that can be varied in order to bleed signal to ground intentionally, volume control. And then into the screen of the second triode, creating a second gain stage. Then the singal passes through the second plate and into another .02 cap and then into the grid of teh power tube.

And then the 220k tied to ground just before the power tube grid connection. What is it's purpose? Would the power tube stop working if it were removed? Looks to be i a similar scenario to the fixed 1meg and the 1meg pot, but why such a lower value? Is it intended to bleed off some of the signal before hitting the power stage, perhaps to clean it up a little?

[eschertron]

And the 1meg resistor serves to keep signal from being bled off to ground? Or perhaps there's more to it than that?

Yes. More to it. And you've already detected the pattern, as you note below:

And then the 220k tied to ground just before the power tube grid connection. What is it's purpose? Would the power tube stop working if it were removed? Looks to be i a similar scenario to the fixed 1meg and the 1meg pot, but why such a lower value? Is it intended to bleed off some of the signal before hitting the power stage, perhaps to clean it up a little?

The purpose of the 220k grid leak resistor in front of the power tube, and the 1M resistors in front of the preamp triodes (the second stage is disguised as a 1M pot) is to reference the incoming signal at the grid to ground. Note that those sections of the circuit are de-coupled from B+ by capacitors, so they would be electrically floating if not tied to some reference. In order for the cathode bias to work, the grid must be at ground potential (see grounded grid biasing). The values are chosen so as to NOT bleed the signal off, but are essential to the operation of the tube. We use those values (220k and 1M) because the manufacturer's spec sheets recommend them as the maximum allowed values. Can we use lower values? It happens all the time*. Can we use bigger values? According the the designers, we would run the risk of losing the proper bias voltage on the tube, potentially allowing the tube to self-destruct.

That's the theory as I understand it. Seems like you've sussed it out pretty well by yourself.

*disclaimer: please don't ask me to provide examples

[mort]

Ok cool.

Now on to the power tube. I see the same path, signal from grid to plate, but why the second grid with B+ on it? And why the cap on the cathode?

[Jazz P Bass]

Mort.
Do yourself a favor and research the tube types.

The output tube is a 6L6.
Cathode, grid, screen & plate.
The screen is attached to the B+ (higher power amps will use a screen resistor to limit screen current) because that is how that tube type works.

[Audiotexan]

Mort.
Do yourself a favor and research the tube types.

The output tube is a 6L6.
Cathode, grid, screen & plate.
The screen is attached to the B+ (higher power amps will use a screen resistor to limit screen current) because that is how that tube type works.

The output in his 5F1 is a 6v6..
(Yes, same family, but your answer 'outside looking in' is cryptic at best.)

PS: Be kind! He's trying to research theory! So it seems a valid question to me.

[eschertron]

Ok cool.

Now on to the power tube. I see the same path, signal from grid to plate, but why the second grid with B+ on it? And why the cap on the cathode?

Short answer: both replies above are correct.
Long answer: the output tube, 6V6, is a pentode 'type' tube (actually a beam tetrode but not important?) and it has some extra elements in the envelope, the important addition is the screen grid, another grid similar to the triode's control grid. How the screen affects tube operation is best left to a search of this website, or the WWL (world-wide library) in general. But basically it functions by being hooked up to B+ (or a voltage near B+) to keep the cathode from seeing the plate voltage from dropping on each half-cycle of the signal waveform. Almost all power tubes have this structure, so power handling and efficiency must be some of the design goals for this tube.

Cap on the cathode?
Short answer: For the same reason the caps are on the cathodes of the preamp tubes - cathode biased operation.
Long answer: Higher-power amps (most push-pull types, not all) use fixed bias, and do not need a cathode bypass cap. And while a cap is not necessary for cathode bias operation, it improves efficiency. Again, searching with the italicized terms should reveal more accurate and concise info than what I can provide.

[Jazz P Bass]

The output in his 5F1 is a 6v6..
(Yes, same family, but your answer 'outside looking in' is cryptic at best.)

PS: Be kind! He's trying to research theory! So it seems a valid question to me.

I was being kind.

I get upset when people do not want to do a little homework.
Asking a question is not homework.

My gaff on the tube type.

[Audiotexan]

I was being kind.

I get upset when people do not want to do a little homework.
Asking a question is not homework.

My gaff on the tube type.

Fair enough, and TY!

FWIW, I wouldn't have known how to phrase it properly to know what to search for...
So Eschertron's post was very helpful in that regard (and also in the brief synopsis!)

[ArdeliaAhrens]

will you like to take some pics and let's see more details? as this method should works...

[mort]

I was being kind.

I get upset when people do not want to do a little homework.
Asking a question is not homework.

My gaff on the tube type.

You've always been kind and helpful, and thanks for it.


I do attempt my own research but it can be tough trying to find an answer to specific questions concerning electronics. I do admit though that I could be more diligent about looking for answers before asking questions. And it's also true that when looking for answers it's all too easy to find articles that are either more broad or far more specific that the question I'm trying to answer.

[The Dude]

Just my $.02:

I don't see it as a problem. I understand Jazz's point of view, but if someone doesn't want to participate in the discussion, they are free to move on to another thread. There are plenty of threads I read here and decide to just drive on by.