Amp Books - Grid Bias Excursion Calculator

[RoadShow]

Hi,

Looking at the Grid Bias Excursion Calculator this morning, it's not clear to me where or how the 68k output impedence is derived from in the example.
https://www.ampbooks.com/mobile/amplifier-calculators/bias-excursion/calculator/

From a particular power tube parameter?  Can someone point it out to me?

Thanks...

[AlNewman]

What do you mean when you say the output impedance was derived?
I can plug any number into the 4 variable inputs to the calculator and get different results.

I think you have to come up with your own output impedance, based on your amp design.

[stratomaster]

I think he's asking how to figure that output impedance.  Usually the writeup that proceeds the calculator will have some insight, however in this case there is no mention of it and it just shows up as an input variable.

I'd imagine the PI plate output impedance is a good enough guess as the calculator isn't very responsive to that variable except for the excursion ratio (plug in half of 68k then twice 68k for a quick and dirty sensitivity study). 

[pdf64]

Yes, it'll be the output impedance of the preceding stage, eg LTP.
An LTP with 12AX7 will probably be about 68k, with a 12AT7 it'll be somewhat lower, and lower still with a cathodyne.

[RoadShow]

I think he's asking how to figure that output impedance.  Usually the writeup that proceeds the calculator will have some insight, however in this case there is no mention of it and it just shows up as an input variable.

I'd imagine the PI plate output impedance is a good enough guess as the calculator isn't very responsive to that variable except for the excursion ratio (plug in half of 68k then twice 68k for a quick and dirty sensitivity study).

Correct... where does that output impedence come from...    

Yes, it'll be the output impedance of the preceding stage, eg LTP.
An LTP with 12AX7 will probably be about 68k, with a 12AT7 it'll be somewhat lower, and lower still with a cathodyne.

Thx pdf64, that lines up, I was looking downstream and should have been looking upstream.   

[acheld]

I sent a question to "webmaster" at ampbooks --

In this calculator, output impedance is a variable.   Does this represent the output impedance of the phase inverter (prior stage)?

And got back the following message --

"Yes, driving stage output impedance."

[pdf64]

I sent a question to "webmaster" at ampbooks --

In this calculator, output impedance is a variable.   Does this represent the output impedance of the phase inverter (prior stage)?

And got back the following message --

"Yes, driving stage output impedance."

The poor chap, I was pestering him a couple of days ago about a snag with the LTP balance calculator 

[AlNewman]

Alright, well hopefully someone more intelligent than me can chime in...

But 12ax7@ 250V has a plate resistance of 62.5k.  I'm guessing that's after the load resistor, so realistically we could be looking at 100V at the plates which would be 80k resistance, according to the datasheet.  (or somewhere in between)

From my understanding, the output impedance, in this case, would be the plate load in parallel with the tube resistance, perhaps plus the cathode resistor in series?

So let's just split the difference, let's call it 175V at the plate, which would theoretically be like 70k resistance at the plate under load.

1/220k + 1/70k = 0.018   1/0.018 = 55.5k

That's pretty close.  The grid stopper, cathode resistor, and coupling capacitor may influence the final number, although I personally don't understand how.

There are people who do.

Personally, I would think that the following load on the circuit would also have an effect, for instance a tone stack, a volume pot, etc...but I may be wrong.

[Gregwor]

Buy all of his books. They all cover different things - yes, buy the Basics book as well. They clearly explain how to derive all of the parameters for different circuits. It really is too circuit specific to cover on the forum. The books are affordable when you buy them all at once (paying only 1 hefty shipping fee) and well worth it. I also suggest buying all of Merlin Blencowe's ( https://www.valvewizard.co.uk/ )books as well -- these ones go into more theory but also very very well written. Do it