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What is the function of the 470pF capacitor in this amp schematic clipping? I am 75% sure that it is a treble cut, but there is a part of me that believes that it could instead be a treble boost.

JakeAC5253 wrote:What is the function of the 470pF capacitor in this amp schematic clipping? I am 75% sure that it is a treble cut, but there is a part of me that believes that it could instead be a treble boost.

It's actually a bypass cap, it bypasses high frequencies to ground via the +V supply, this is because the +V supply is at low impedance, and it usually has large value caps going from it to ground...

Soldano do something similar to this in their SLO 50 and SLO 100 Amps, but in this case they use a 1nF cap in parallel with a 100k Anode/Plate resistor....

It might help if you consider that (From the point of view of the AC signal ,,*the sound you hear*) the supply may as well be a second ground plane.
Always remember there are 2 things happening,,, DC and AC.

Amplification is AC floating on a DC potential. The signal cares little about how you derive the DC conditions as long as they will pass AC signal it works.
Phil.

phatt wrote:It might help if you consider that (From the point of view of the AC signal ,,*the sound you hear*) the supply may as well be a second ground plane.
Always remember there are 2 things happening,,, DC and AC.

Amplification is AC floating on a DC potential. The signal cares little about how you derive the DC conditions as long as they will pass AC signal it works.
Phil.

That is true about the supply being a second ground plane, thanks for that clarification Phil....

in other words- yes, it's a treble cut.

I believe in this context it's what's commonly called a Miller cap. http://www.aikenamps.com/MillerCapacitance.html

Thanks gents. I've modified an amp before by using a cap from 12ax7 pins 6 to 7, which raises the floor level of Miller Capacitance, but that would be assumed to be straight anode to grid. In this example the anode bias resistor is between the Miller cap and the anode, and yet this has an identical effect? Or is it an RC filter calculated by using 470pF and 100k?

I'll chalk this up as a learning experience for myself, since I never really fully understood "Miller Capacitance" in the past, until now....

diggum12 wrote:I believe in this context it's what's commonly called a Miller cap. http://www.aikenamps.com/MillerCapacitance.html

No, it's not. In fact, the Miller effect is something completely different and unrelated.

You can call it treble cut or whatever you want. All the capacitor does is reduce the amplification at higher frequencies. Calculate an AC load line for a few frequencies and draw it, and the effect will be clearly visible in the diagram.

The smartest thing anyone can do that really wants to understand whatzup inside these circuits is to come to grips with simulator software.
I learned more in 12 months of *solder free simulation* than I learnt in 15 years of wasting solder only to find I'd created some hazardous waste land fill

A picture says a thousand words and there would be plenty of help from the wealth of knowledgeable folk here I'm sure to help you understand how to use it all.

I use *Circuitmaker free student edition* which has Valves as well as SS bits to play with.
There is only about 3 things that are tricky to setup (so that the sim outputs the right data), The rest can be worked out by anyone with a basic understanding of electronics.
That said they are not perfect but some good reading will cover the missing bits.
For the simple stuff being asked here Sims will work wonders for you.
Have fun, Phil.

I now see there's a difference between Miller capacitance and a Miller capacitor.

DrNomis wrote:Soldano do something similar to this in their SLO 50

No such amp sir!