A simple (I think!) question about impedance (probably)

[nee]

Hi fellows!

Consider the simple 5f1 circuit. The volume pot is a 1Meg between the two halves of the preamp tube. What would be the effect of using a 100K pot instead? Is this affecting the impedance into the next stage?

Thanks!

[drz400]

Hi fellows!

Consider the simple 5f1 circuit. The volume pot is a 1Meg between the two halves of the preamp tube. What would be the effect of using a 100K pot instead? Is this affecting the impedance into the next stage?

Thanks!

Yes but that wont matter much unless you are considering miller capacitance which isnt much in either case. What is more important is the load on the preceding stage
You will loose gain as well as low end from about 6hz to 67hz (-3db point) too unless you increase the coupling cap by an order of 10

[VacuumVoodoo]

100k pot will present a heavier dynamic load to the tube feeding it, it will change gain of the first stage and how it distorts.

[nee]

Ah! I follow! So when the pot is fully clockwise there's only 100K resistance between the signal and earth as opposed to 1M resistance.

What if we keep the 100K volume pot but insert a 1M tone pot before it? Or what about if we have a 100K tone pot first, then a 1M volume pot?

Thanks!

[drz400]

Ah! I follow! So when the pot is fully clockwise there's only 100K resistance between the signal and earth as opposed to 1M resistance.

What if we keep the 100K volume pot but insert a 1M tone pot before it? Or what about if we have a 100K tone pot first, then a 1M volume pot?

Thanks!

It always is a 100K load and could be more depending on whatever else you have going on there. A 12AX7 plate output impedance is at best around 35K. Typical load should be 10x higher unless you have something specific in mind.

They all present a load on the preceding stage, A "tone pot " typically is a low pass filter and is still a load on the preceding stage just at selective frequency.
Unless you are looking to dump level or play with the RC of the coupling cap there isnt a reason to use a 100K pot in a tube circuit as a volume control.

Why are you determined to use 100K pots?

[nee]

Why are you determined to use 100K pots?

I'm just interested in the theory. Most amps seem to use a 1M volume pot - just wondered why.

[fperron_kt88]

I might add this to an already very interesting thread: you would probably like to learn about AC equivalent circuits. Look into books like this one (not related to me...): http://books.google.com/books?id=NmD0SD ... t#PPA86,M1

I know, this might sound scary at first, but AC equiv. is a helpful tool in finding the rationale behind circuits without getting overloaded with the details of the DC operation (like headroom and clipping, that are very important as well, but we want to swallow information in smaller scoops, right?).

The plus side is that you will definitely get to the rationale of the selected circuits very quickly...

In this case, and as already pointed-out above: since the plate load resistor , the plate itself and the traditional volume pot are all paralleled in the AC equiv. circuit, the 10X rule was applied in selecting the volume pot to avoid loading down the gain of the previous stage.

AC equiv. circuits are fun to imagine by inspection. A few simple rules will help you get insight into most guitar preamp circuits (before any clipping happens... yup, this is a bit sad, I know...):

-power supply rails are low impedance equivalents in AC circuits (they act as a ground connection for AC signals)
-a grid input to a tube looks like an open connection (perhaps better: they look like a small value cap to ground)
-the plate of a tube is a resistance in the order of magnitude of the plate resistor (35K seems totally adequate).

You can try redrawing circuits found between two stages by applying these simple rules and figure-out a ton of info. For example: try adding a 10Meg in series with your volume wiper and the grid equivalent of a 10pF to ground... How will that sound, I was thinking low pass...

[drz400]

Why are you determined to use 100K pots?

I'm just interested in the theory. Most amps seem to use a 1M volume pot - just wondered why.

I have seen many that use 500K as well
The output impedance is going to vary depending on components and voltages, 1M is a nice conservative value, much higher and you will introduce noise. Dont over think it, 500K or 1M is fine, listen to each and decide with your ears.