resisor and cap in series on cathode?

[joesatch]

preamp circuit. what is this accomplishing?

[jordan86]

I’ve seen this a few times but never asked. I’d guess that the resistor in series is minimizing the effect of the bypass cap. So you could use a bigger cap for full frequency response, just less of it’s overall gain boost. If that’s not correct, I’d love to know.

[mresistor]

Is it filtering in high frequency components? Was this on a known amplifier?

[joesatch]

Is it filtering in high frequency components? Was this on a known amplifier?

Bogner Ecstasy

[PRR]

It shaves the bottom octave++.

Get a reactance chart and work it out. The cap is "open" at DC and "short" at infinite frequency. The reactance chart tells you where it passes 10k and 2.7k, the corner frequencies.

[shooter]

I wired up a similar circuit, the series R with the cap was a pot IIRC, didn't make it off the breadboard, can't recall any OH, YA moments with my ears though.

[sluckey]

I see asterisks next to those components. What does the note say?

[joesatch]

I see asterisks next to those components. What does the note say?

post 2004 model changes

[T Wilcox]

Change that cap to a 33uF and the 2K7 to a 50K Linear pot and you got the Langner "Fat" knob

[nandrewjackson]

https://m.imgur.com/ZsbvZ7M

[nandrewjackson]

https://el34world.com/Forum/index.php?action=dlattach;topic=19369.0;attach=54190;image




Sorry these are simply links, I don't have enough coffee in me to post images.

[shooter]

Get a reactance chart and work it out.

NO, i'm NOT re-learnin 

How to Use a Smith Chart for Impedance Matching (cadence.com)

[tubeswell]

Its a partial bypass (the limiting resistor limits the gain boost from the bypass cap). By my calculations:


At low frequencies - with an unbypassed cathode resistor, (omitting for now the 470pF plate bypass cap) the unbypassed gain will be:

A = u.Rp/[Rp + rp + Rk(u + 1)] = 100.100k/[100k + 62k + 10k.(101)] = 10M/162k + 1.01M = 10M/1.172M = 8.53


At all other frequencies, the 220nF (0.22uF) cap bypasses the 10k cathode resistor - but the additional 2k7 prevents complete bypassing. As the 2k7 is in parallel with the 10k (at AC), the total effective cathode resistance (at AC) is 2.125k. So the gain will be:

A = 100.100k/[100k + 62k + 2.125k.(101)] = 10M/[162k + 2.125k.(101)] = 10M/0.376625 = 26.6


To AC, both resistors are effectively in series with the 220nF, giving 12.7k, so the 1/2 boost frequency is:

f = 1/(2Pi x 12.7k x 0.22uF) = 1/(6.2857 x 12,700R x 0.00000022F) = 57Hz (As PRR said - the bottom octave)


If you factor in the plate bypass cap, you have a kind of bandpass filter, where the lows and the (very high) highs have been trimmed out.

[PRR]

Smith chart is smut.

Reactance chart is a simple tool.

[shooter]

Smith chart is smut.

ART, is in the eye of the beholder

[joesatch]

so it is limiting the effect on the lower frequencies when the bypass cap is in circuit? A sort of low pass filter?  This will come in very useful  i will experiment. Been looking to tighten up the bass on some of my builds further after the normal methods of lowering the coupling cap values.

[Williamblake]

Cutting away on the low frequencies is a high pass filter by definition. As i am wanting to strum on the thick strings but not wanting to be them as loud as they are in any givven classic circuit i went with a 0.5 to 2 mF capacitor on the first gain stage. Of course this does not mean anything, if you dont know the tube and the plate load used but nevertheless, this is a shelving filter, soprobably a lot of fine tuning you can do with it.

[shooter]

the link in reply 10 is a great "tool" as a start point if you want to save Algebra for a rainy day

[Williamblake]

Who doesnt want a rainy day these times but adjusting by ear is all the fun you never even asked for.

[Williamblake]

To say it all, two jumpered input channels like normal and bright being done by cathode caps and not bright caps is time well spent.

[acheld]

Also,  Kuehnel's calculators are helpful.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/ 

is for cathode bypass cap but it also shows you the effects of all of your variables, and gives a better representation of what you will get from your circuit.

[pdf64]

Partial cathode bypass forms a shelving filter, rather than a high pass.
A regular RC high pass filter carries on rolling off at -6dB / octave (20dB / decade).

[HotBluePlates]

... Get a reactance chart and work it out. The cap is "open" at DC and "short" at infinite frequency. The reactance chart tells you where it passes 10k and 2.7k, the corner frequencies.

NO, i'm NOT re-learnin 

How to Use a Smith Chart for Impedance Matching (cadence.com)

WTF?  "Reactance Chart":

[shooter]

WTF?

Yep, same thought I had when the instructors busted out a similar chart above,  impedance in 3D space, then just for fun He passed out the math.


my next thought, I might like painting ships for the next 6yrs!! 
the course was RADAR Principles I, 60 guys, 26 weeks, in order to "move up" you needed to finish in the top 50%, I was #24.
#60's score, would equate to a C in a typical college.

[dwinstonwood]

So, looking at that reactance chart...

.01uF intersects 100K Ohms at about 150Hz.

What is that telling me? That frequencies below 150Hz are attenuated? I have no idea what I'm looking at.

edit: Oh, I guess it's the resonance frequency. Maybe?

[HotBluePlates]

So, looking at that reactance chart...

.01uF intersects 100K Ohms at about 150Hz.

What is that telling me? ...

Capacitive Reactance:  1 / (2 x π x f x C)
= 1 / (2 x π x 150Hz x 0.00000001 Farads)
= 1 / (π x 0.000003 Farads) = 1 / 0.0000094248 = 106kΩ
     0.01µF = 0.00000001 Farads; the formula for reactance uses units of Farads (or "Henries" when you remove the 1/x term)


Capacitance, Resistance, Frequency.
Pick any 2:  The intersection of those values tells you the value of the 3rd item.
Or you can continue to do the math demonstrated above.

So, looking at that reactance chart...

.01uF intersects 100K Ohms at about 150Hz.

What is that telling me? ...

edit: Oh, I guess it's the resonance frequency. Maybe?

Get a reactance chart and work it out. The cap is "open" at DC and "short" at infinite frequency. The reactance chart tells you where it passes 10k and 2.7k, the corner frequencies.

Corner Frequency (or "Cutoff Frequency" or "-3dB Frequency")

That article doesn't do a great job of just telling you the essential points, but this one does sorta: "corner frequency" is also the "half-power frequency" because the circuit-resistance equals the circuit-reactance (both in Ohms), so half-power is dropped across each element.

PRR told you what to do:
bypass cap is "220nF" which is "0.22µF" ---> Goes a little past the 0.2µF line
One of 2 resistors is 10kΩ ---> find the point where the "0.22µF line" intersects the 10kΩ horizontal line
From the intersection above ----> find the nearest vertical line ---> Corner Frequency

Rinse & repeat for 2.7kΩ.

Takeaway:  The bypass made of series resistor & cap implements a stepped-frequency-response in that gain stage rather than the usual roll-off provided by a first-order filter (bypass cap alone). 

[dwinstonwood]

Thanks HotBluePlates. Some good reading.

[pdf64]

… The bypass made of series resistor & cap implements a stepped-frequency-response in that gain stage rather than the usual roll-off provided by a first-order filter (bypass cap alone).

Rather than a 1st order filter, I’m thinking that partial cathode bypass creates a shelf filter, higher frequencies on a higher gain shelf, lower frequencies on a lower shelf.

Putting a resistor in series with the bypass cap just reduces the gain of the higher frequency section, lowers the height of the upper shelf.

[PRR]

> partial cathode bypass creates a shelf filter,

Yes, and most cathode caps end up working as a shelf. There's a gain you get no-cap, a gain you get fully-capped (for whatever frequencies the cap passes). In tubes, these usually are not very different, but you can add another resistor to make an in-between gain.

[sluckey]

I have a slight variation of that circuit that is used for a gain control in my toaster oven. It doesn't go from zero to max, but it does provide a very noticeable change in gain. Works pretty good. Borrowed that circuit from jojokeo's Little Giant.

     http://sluckeyamps.com/smoky/smoky.pdf