Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on August 24, 2022, 01:36:56 pm

Title: resisor and cap in series on cathode?
Post by: joesatch on August 24, 2022, 01:36:56 pm: preamp circuit. what is this accomplishing?
Title: Re: resisor and cap in series on cathode?
Post by: jordan86 on August 24, 2022, 02:17:09 pm: 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.
Title: Re: resisor and cap in series on cathode?
Post by: mresistor on August 24, 2022, 02:18:44 pm: Is it filtering in high frequency components? Was this on a known amplifier?
Title: Re: resisor and cap in series on cathode?
Post by: joesatch on August 24, 2022, 02:26:43 pm: Quote from: mresistor on August 24, 2022, 02:18:44 pm
Is it filtering in high frequency components? Was this on a known amplifier?

Bogner Ecstasy
Title: Re: resisor and cap in series on cathode?
Post by: PRR on August 24, 2022, 04:49:06 pm: 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.
Title: Re: resisor and cap in series on cathode?
Post by: shooter on August 25, 2022, 03:56:06 am: 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.
Title: Re: resisor and cap in series on cathode?
Post by: sluckey on August 25, 2022, 05:32:39 am: I see asterisks next to those components. What does the note say?
Title: Re: resisor and cap in series on cathode?
Post by: joesatch on August 25, 2022, 06:34:23 am: Quote from: sluckey on August 25, 2022, 05:32:39 am
I see asterisks next to those components. What does the note say?

post 2004 model changes
Title: Re: resisor and cap in series on cathode?
Post by: T Wilcox on August 25, 2022, 02:42:36 pm: Change that cap to a 33uF and the 2K7 to a 50K Linear pot and you got the Langner "Fat" knob
Title: Re: resisor and cap in series on cathode?
Post by: nandrewjackson on August 25, 2022, 03:34:10 pm: https://m.imgur.com/ZsbvZ7M
Title: Re: resisor and cap in series on cathode?
Post by: nandrewjackson on August 25, 2022, 03:36:51 pm: 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.
Title: Re: resisor and cap in series on cathode?
Post by: shooter on August 25, 2022, 04:58:57 pm: Quote
Get a reactance chart and work it out.

NO, i'm NOT re-learnin :icon_biggrin:

How to Use a Smith Chart for Impedance Matching (cadence.com) (https://resources.system-analysis.cadence.com/blog/how-to-use-a-smith-chart-for-impedance-matching)
Title: Re: resisor and cap in series on cathode?
Post by: tubeswell on August 25, 2022, 06:26:52 pm: 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.
Title: Re: resisor and cap in series on cathode?
Post by: PRR on August 25, 2022, 09:34:08 pm: Smith chart is smut.

Reactance chart is a simple tool.
Title: Re: resisor and cap in series on cathode?
Post by: shooter on August 26, 2022, 04:14:39 am: Quote
Smith chart is smut.
:laugh:
ART, is in the eye of the beholder
Title: Re: resisor and cap in series on cathode?
Post by: joesatch on August 26, 2022, 09:11:01 am: 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.
Title: Re: resisor and cap in series on cathode?
Post by: Williamblake on August 26, 2022, 09:45:53 am: 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.
Title: Re: resisor and cap in series on cathode?
Post by: shooter on August 26, 2022, 09:48:13 am: the link in reply 10 is a great "tool" as a start point if you want to save Algebra for a rainy day
Title: Re: resisor and cap in series on cathode?
Post by: Williamblake on August 26, 2022, 10:02:32 am: Who doesnt want a rainy day these times but adjusting by ear is all the fun you never even asked for.
Title: Re: resisor and cap in series on cathode?
Post by: Williamblake on August 26, 2022, 10:06:44 am: 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.
Title: Re: resisor and cap in series on cathode?
Post by: acheld on August 26, 2022, 10:16:51 am: Also, Kuehnel's calculators are helpful.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/ (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.
Title: Re: resisor and cap in series on cathode?
Post by: pdf64 on August 26, 2022, 12:38:44 pm: 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).
Title: Re: resisor and cap in series on cathode?
Post by: HotBluePlates on August 27, 2022, 07:09:21 pm: Quote from: PRR on August 24, 2022, 04:49:06 pm
... 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.
Quote from: shooter on August 25, 2022, 04:58:57 pm
NO, i'm NOT re-learnin :icon_biggrin:

How to Use a Smith Chart for Impedance Matching (cadence.com) (https://resources.system-analysis.cadence.com/blog/how-to-use-a-smith-chart-for-impedance-matching)

WTF? "Reactance Chart":

(https://www.rfcafe.com/references/electronics-world/images3/reactance-chart-electronics-world-may-1959.jpg)
Title: Re: resisor and cap in series on cathode?
Post by: shooter on August 27, 2022, 07:26:14 pm: Quote
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!! :icon_biggrin:
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.
Title: Re: resisor and cap in series on cathode?
Post by: dwinstonwood on August 28, 2022, 08:57:02 am: 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. :dontknow:

edit: Oh, I guess it's the resonance frequency. Maybe?
Title: Re: resisor and cap in series on cathode?
Post by: HotBluePlates on August 29, 2022, 03:29:27 pm: Quote from: dwinstonwood on August 28, 2022, 08:57:02 am
So, looking at that reactance chart...

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

What is that telling me? ...

Capacitive Reactance (https://en.wikipedia.org/wiki/Electrical_reactance#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.

Quote from: dwinstonwood on August 28, 2022, 08:57:02 am
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?
Quote from: PRR on August 24, 2022, 04:49:06 pm
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") (https://en.wikipedia.org/wiki/Cutoff_frequency)

That article doesn't do a great job of just telling you the essential points, but this one does sorta (https://www.electrical4u.com/cutoff-frequency/): "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).
Title: Re: resisor and cap in series on cathode?
Post by: dwinstonwood on August 30, 2022, 07:03:38 am: Thanks HotBluePlates. Some good reading.
Title: Re: resisor and cap in series on cathode?
Post by: pdf64 on August 30, 2022, 07:14:37 am: Quote from: HotBluePlates on August 29, 2022, 03:29:27 pm
… 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.
Title: Re: resisor and cap in series on cathode?
Post by: PRR on August 30, 2022, 12:28:41 pm: > 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.
Title: Re: resisor and cap in series on cathode?
Post by: sluckey on August 30, 2022, 12:37:02 pm: 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