Cathode resistor math for dummies? - preamp tubes

[jordane]

I would like to know how to calculate values for a cathode resistor on a given preamp tube.

I understand that raising the resistor value can bias the tube cold, and lowering has the opposite effect. I have seen 820ohm (warm), 1.5k (neutral), and 2.7k (cold) explained on various websites and schematics for a 12AX7.  But, how do we "arrive" at a 1.5k resistor being neutral for a 12AX7? 

For example... how do I change the cathode resistor to neutrally bias a 12AX7, vs. a 12AT7, 12AU7, etc.?

Another way of asking this: let's say I want to design a preamp using a 12AU7 tube in V1A/V1B instead of a 12AX7 (AB763 schematic attached for reference).  How does one calculate an appropriate cathode bias resistor?

I'm aware I might be asking the wrong questions, or irrelevant questions.

[mresistor]

This link has all the info on the first gain stage of a tube amplifier, including the biasing. http://www.valvewizard.co.uk/Common_Gain_Stage.pdf

[Lectroid]

Anyone here could probably explain this better than I can so I won't try.  But it is not something that can be explained very clearly in just a paragraph or two.

A good place to start would be with a book written by Merlin Blencowe, (aka Valve Wizard.)  "Designing Valve Preamps for Guitar and Bass."  Luckily he gives away the first chapter on his website.

Follow this link:    http://www.valvewizard.co.uk/Common_Gain_Stage.pdf.   Start reading at Section 1.7 on page 9.  He gives a high-level look at what you need to know.

Honestly, buy the whole book asap; most or many people here will recommend it.  I refer to it constantly.  But that free first chapter alone has a wealth of information explained as well as any others I've read.

[SILVERGUN]

As a bit of a dummy myself I found this kind of informative
https://vimeo.com/33244955

[Lectroid]

Nice video. Basic but great visualization! 

[jordane]

This forum always delivers.  Thanks for the references!  Links bookmarked in my (ever expanding) diy amp folder.

[acheld]

Also, Richard Kuehnel's Ampbook pages have first rate calculators for a bunch of stuff, including for cathode bias resistors.   I use these frequently.

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

[shooter]

i've had this image, in one form or another since the 70's
still comes in handy

[jordane]

Also, Richard Kuehnel's Ampbook pages have first rate calculators for a bunch of stuff, including for cathode bias resistors.   I use these frequently.

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

Thanks, I had seen that.  I wasn't sure I was getting enough information about whether the tubes are biased appropriately... maybe I'm misreading the graphs that pop up.

[PRR]

I would like to know how to calculate values for a cathode resistor on a given preamp tube.....
I'm aware I might be asking the wrong questions, or irrelevant questions.

CALCULATE??

The G.E. or RCA suggested R-C Amplifier tables were good enough for Leo. And, obviously, 99% of his copy-cats.

[shooter]

You'd be amazed the time saving when you stand on the shoulders of giants, any field that has design engineers, has lots of data.
once it's up and running, decide if it's off to the mountains or just one more tuning pass.


 

[jordane]

I would like to know how to calculate values for a cathode resistor on a given preamp tube.....
I'm aware I might be asking the wrong questions, or irrelevant questions.

CALCULATE??

The G.E. or RCA suggested R-C Amplifier tables were good enough for Leo. And, obviously, 99% of his copy-cats.

I did not know these existed.  Or what they are.  Thanks for the input.  I will look...

[HotBluePlates]

... it is not something that can be explained very clearly in just a paragraph or two. ...

It can be done if we focus on "What" and set aside "How/Why" for a bit.

I would like to know how to calculate values for a cathode resistor on a given preamp tube.
... how do I change the cathode resistor to neutrally bias a 12AX7 ...

PRR is right about the R-C tables.

But if you want to calculate, you need to start by specifying your supply voltage.

Next you need to specify your plate load resistor value.  This is gonna be recursive (unless you use those R-C tables), because you oughta use a plate load resistor that's around 2-5x the tube's internal plate resistance.  But we determine the internal plate resistance at the operating point, which we won't know until we know our plate load & cathode resistors.  HINT:  Real-World, we guess close to "typical internal plate resistance" then do the calcs, then adjust for a 2nd pass if desired.

Draw a loadline for the plate load resistance used at the chosen supply voltage.  Throw a dart halfway between the "0v gridline" and the really-bent stuff near plate current cutoff.

Estimate the grid-volts implied by this spot on the loadline (it will certainly be between 2 existing grid-volt curves).

Make a note of the plate current at this spot on the loadline.

Volts / Current = Ohms of the cathode resistor



Any place where you say "Huh?" or don't know how to complete a step are knowledge-gaps you should identify & fill.  Ask how to do them, or where to look to find out.

[jordane]

Estimate the grid-volts implied by this spot on the loadline (it will certainly be between 2 existing grid-volt curves).

Make a note of the plate current at this spot on the loadline.

Volts / Current = Ohms of the cathode resistor

Thanks, I will work on this!

[thetragichero]

in addition to the ampbooks calculators i find this useful: https://hosenlander.nl/triodecalculator2/index.html

[tubeswell]

12AX7 triode load line examples


HT = 280V,
Plate Resistor = 100k,
Bias point  = -1.5V
Plate (or cathode) current = 1mA


And same thing with 220k plate resistor, bias voltage = -1V, plate current = 0.65mA

[jordane]

I found this while searching, might just be the thing someone needs reading this in the future: the RCA R-C tables--

http://www.tubebooks.org/tubedata/hb-3/Receiving_Tubes_Part_1/Resistance-Coupled_Amplifiers.PDF

Thanks to all who chimed in.