choosing a treble-boost cap value

[stormbringer]

In my current project amp, I added a 100k resistor in series with my 500k gain pot to limit the maximum signal going to the second gain stage. Now I'd like to try bypassing the series R with a cap to provide some high-frequency emphasis. How do I choose the cap value, and is the cutoff of the filter dependent on the setting of the gain pot?

As an example of this type of bypass circuit, Marshall uses a 470p cap in parallel with a 470k series resistor before a 470k volume pot. I don't have the same value series R, and so I'd like to know how to choose the bypass cap.

Thanks in advance!

[Enzo]

How long is a piece of string?

You cannot take one part out of context and blame it - good or bad - for the tone of an amp. You want to add some high freq emphasis. OK, be we have no way to know what you think of as high freq in this case or how much. Nor can we say without at least seeing the rest of the schematic (and probably not even then) whether this part is where to achieve it. Maybe you'd like a bright cap on the control itself better. Or maybe a smaller cathode bypass cap somewhere might reduce some of the low end emphasis. You'll note in your Marshall there are sometimes cathode bypass caps of 0.68uf instead of 25uf. KNocking off lows is one way to emphasize highs. Or yet something else, or even all these things.

But to answer: pick a few and try them. A couple clip wires or just tack solder some various value caps, and you can tell by ear which are moving in the direction you like. My rule of thumb when trying different parts at random "Just to see," is to use at least a factor of 2 between values. In other words, 100pf, 120pf, 150pf and 180pf is probably not a very revealing selection. Better to use 100pf, 220pf 470pf. If you later find that 430pf really does sound better that 470pf, well go for it, but I'd start with the coarser spacing at first.

[Chuck H]

Enzo is right. Your ears are the acid test and a target frequency doesn't mean much by comparison. Still... Here you go. It's the first one I found with a Google search:

Resistance Frequency Capacitance Calculator

[stormbringer]

Enzo -

I didn't ask for folks to evaluate the entire amp or to determine whether I should swap coupling caps, change cathode bypass caps, or put in a bright cap. I have actually tried several values of caps across this particular series R, and I have developed trust for my ears. But what I like to do is to create excel spreadsheets that I can refer to to quickly get in the right ballpark. So in this regard, I'd like to know what particular formula to use.

[stormbringer]

Thanks for the link.

This calculator computes the cutoff of the series R and bypass cap without regard for the pot that follows. My hunch is that the actual cutoff will vary with the position of the pot. Specifically, the cap bypasses a variable proportion of the series resistance with a variable parallel resistance.

But that is just a hunch.

[Enzo]

For the pot setting to control the freqs, that must mean the variation of the pot changes the resistance of the circuit. SO we would need to know what was connected to the wiper. Without a definition of any other parts, it would seem you would be limited to what Chuck presented.

[tubeswell]

Enzo is right. Your ears are the acid test and a target frequency doesn't mean much by comparison. Still... Here you go. It's the first one I found with a Google search:

Resistance Frequency Capacitance Calculator

And the other part of Chuck's equation

[tubeswell]

Enzo -

I didn't ask for folks to evaluate the entire amp or to determine whether I should swap coupling caps, change cathode bypass caps, or put in a bright cap. I have actually tried several values of caps across this particular series R, and I have developed trust for my ears. But what I like to do is to create excel spreadsheets that I can refer to to quickly get in the right ballpark. So in this regard, I'd like to know what particular formula to use.

Hi-pass shelving filter theory is covered in Merlin Blencowe's 2nd-edition 'Designing Tube Preamps' book - Ch 4 P 95-96.

[pdf64]

I'm not certain that I've understood the arrangement correctly, could you provide a quick sketch, covering at least the pot circuit in question and the stages up and down stream of it?
Pete

[Chuck H]

I'm with Pete. If the series resistor with the cap jumper is before the volume pot, and the volume pot is wired as a typical divider, there is no change to the knee of the circuit due to volume pot setting. If the series resistor with the cap jumper is after the volume pot the cap has virtually no effect because no significant voltage is dropped across the resistor. The only way the pot setting could affect the knee of the circuit is if it were wired as a variable load or if the cap jumpers the pot as well.

EDIT: FWIW it took me a minute to wrap my head around the matter and I do a lot of this. Basically, if wired as I interpret it, the impedance the jumper cap sees never changes.

[stormbringer]

thanks guys.
tubeswell - I like that table; it is from Merlin's book, right? It has been very helpful in the past.

I haven't yet sketched out the preamp, but I'll try to describe the relevant section better in words:
- first gain stage is symmetrically biased with 120k Ra and 2k2 Rk (bypassed with 1uF).
- 2n2 coupling cap leads into 100k series R and then the 500kA pot. The wiper of this pot is connected to a 100k grid stopper for stage 2. There is no bright cap across the input and wiper of the pot.

Let me know if it is still unclear.
Thanks!

[Chuck H]

Let me know if it is still unclear.

A little. You have a 100k resistor on either side of the volume pot and didn't say which one is bypassed with the cap. I'm guessing it's the one before the pot because you called this the series resistor in two posts. So, if your volume pot is wired typically as a divider the impedance the bypass cap sees never changes and therefor the frequency knee of that cap and it's relative dB at that frequency don't change with the volume pot setting.

And it was tubeswell that provided the table from Merlin's book. I provided the link to the on line calculator.

[stormbringer]

A little. You have a 100k resistor on either side of the volume pot and didn't say which one is bypassed with the cap. I'm guessing it's the one before the pot because you called this the series resistor in two posts. So, if your volume pot is wired typically as a divider the impedance the bypass cap sees never changes and therefor the frequency knee of that cap and it's relative dB at that frequency don't change with the volume pot setting.

And it was tubeswell that provided the table from Merlin's book. I provided the link to the on line calculator.

Right. It is the 100k R that precedes the gain pot that has been bypassed.
- I edited my post above, with correct pointer to tubeswell, almost immediately after posting. You are quick!

[Chuck H]

You are quick!

I get that a lot. Mostly from my wife.

[stormbringer]

I get that a lot. Mostly from my wife.

That just leaves you with more time in the shop....