Circuit terminology

[JoeCon]

Guys,

Ok so I'm a little slow with all these circuit terms, but now understand "grid leaks", "stoppers", "peakers", but what is a "Fizz" cap?? A local guy asked me if my Plexi 6V6 has a fizz cap?? I'll bite... where would I look for it?

Thanks...

[martin manning]

That would be a small (50-150pF say), usually ceramic cap across the PI plates, before the coupling caps that go to the power tube grids. See 5F6-A Bassman schematic, e.g.

[dcribbs1412]

Guys,

Ok so I'm a little slow with all these circuit terms, but now understand "grid leaks", "stoppers", "peakers", but what is a "Fizz" cap?? A local guy asked me if my Plexi 6V6 has a fizz cap?? I'll bite... where would I look for it?

Thanks...

Looking at your 6v6 plexi thread
looks like you have a cap(yellow mica) between the 82k and 100k at the PI
I believe this is what Martin is describing
btw nice build...must sound good if your getting asked questions about
components.
congrats

D

[JoeCon]

Thanks guys, I see it now..

DC, yeah it sounds nice to me and really loud too. Those 6V6 ring out!!
And have a bright and normal channel to mix together gives some nice tonal possibilities.

[romberg]

Anyone have a good explaination for (or a link to) what a fizz cap actually does?

I had an amp that produced what I thnk was a bit of an oscillation on the phase inverter output just at the point the power tubes hit cut off. There was a a bit of fuzz on the PI outputs (and this power tube grids). You could see this with a scope and hear it while playing. The fuzz went away if the PI was driven with a stronger signal.

I increased the value of the fizz cap I had between the PI plates from 50pf to 100pf and this oscillation/fuzz went away. But I'm not exactly sure why. Searching the interwebs did not produce any useful results.

Does a fizz cap sorta work like the caps you sometimes see running parallel to a plate resistor on a gain stage? I get how those caps can filter the highs to ground (or B+ in this case). But the fizz cap is not exactly connected to B+ or ground. So, how it does what it does is not clear. It is also not clear how one might calculate a value to use in this spot.

Mike

[Stevem]

The same small value caps can be placed across the plate load resistors in the preamp section.

[vibratoking]

Anyone have a good explaination for (or a link to) what a fizz cap actually does?

You can think of it in two ways. First, a cap is open circuit at DC and a short at infinite frequency. The fizz cap is connected to the out of phase outputs of the PI. As the frequency increases those two nodes are getting closer and closer to being shorted. This action creates frequency dependent cancellation.

The second way is to think of each PI output independently while thinking of B+ as an AC ground. The plate output forms a lowpass RC circuit with the R being the triode plate resistance the C being the fizz cap which is connected to AC ground.

I don't remember for sure, but I think Kuehnel covers this in his bassman analysis if you want all the math.

[JoeCon]

Caps everywhere!!
So I've seen them across the plate load resistor but also connected between the plate and cathode, so what is that??

[martin manning]

So I've seen them across the plate load resistor but also connected between the plate and cathode, so what is that??

Basically the same thing either way- shunting high frequencies to an AC ground for tone shaping or suppression of oscillations.

[vibratoking]

Caps everywhere!!
So I've seen them across the plate load resistor but also connected between the plate and cathode, so what is that??

I am guessing that you need to focus on the concept that the B+ supply, or any supply is equivalent to ground for AC signals. A cap from the plate to B+ is equivalent to a cap from the plate to ground. In a preamp, the cathode is typically isolated from ground by a fairly small resistance. In that case, the cathode is almost ground. In these cases, the cap is shunting some of the AC signal to ground. I hope I am clarifying it and not adding to the confusion. A different way to think about the cap from plate to cathode is in terms of NFB. We can talk about that if you like.

[martin manning]

In a preamp, the cathode is typically isolated from ground by a fairly small resistance.

Which is often bypassed by a fairly large cap...

[vibratoking]

In a preamp, the cathode is typically isolated from ground by a fairly small resistance.

Which is often bypassed by a fairly large cap...

Yes, thank you for the clarification Martin. I forgot to mention that.

[romberg]

Anyone have a good explaination for (or a link to) what a fizz cap actually does?

You can think of it in two ways.
...

Ah! Thank you so much. I've seen some master volume implementations that put a variable resistor across the PI plates. Never occured to me that a cap sorta does the same thing but adding a frequency dependent factor.

I guess that one could achieve the same thing by putting a cap parallel with each PI plate resistor. But then... you would have the price of two caps!

I may have to get a copy of that book. Thanks for the tip.

Mike

[JoeCon]

VK

Yeah seeing the B+ as a ground for AC is a new one for me. But now that you said that it makes more sense.
The cathode is obvious as it's connected directly to ground but that B+ "ground" is not so obvious.

Thanks...

[romberg]

Yeah seeing the B+ as a ground for AC is a new one for me. But now that you said that it makes more sense.

I *think* the theory behind this is that the B+ is connected to ground by BIG filter caps at each power supply node. So... an AC signal is gonna be sent to ground by these caps just like any other noise/ripple. To a AC signal (with a freqency that is not super low) the B+ is a short to ground through the power supply filter caps. Or... I've got it wrong .

Mike