TS calc ?

[shooter]

The TS calculator wants a source Z, I didn't draw a pic, V1a feeds TS, feeds V1b, Isn't  V1a's output impedance determined by Ra in parallel with next stage input R, which is the TS. 

thanks

[PRR]

> I didn't draw a pic

Would be good to draw.

> a source Z, ... Ra in parallel with next stage input R

TSC figures out the interaction between Zsource and the tonestack, not your job.

For Fendery 12AX7 with 100K plate load, you want about 64K||100K which comes to 39K.

For other tubes and other resistances it will be different.

[VMS]

Here's a nifty calculator:

https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/

[shooter]

Would be good to draw.

I posted the clip.  part of my confusion, is the output impedance of the V1A; Ra in || with Rtot from the TS?

TSC figures out the interaction between Zsource and the tonestack, not your job.

I'm happy with that , but, it is a variable i can change,  when I do makes a significant
change in TS loss, or not

[HotBluePlates]

... part of my confusion, is the output impedance of the V1A; Ra in || with Rtot from the TS?

How exact do you need to be? Gnat's-ass perfection? (that question is not intended to be snarky)

1st Approximation:
If you want to do no work, you could take "Zsource" (output Z) to be equal to the plate load resistor/2. A well-designed triode voltage amplifier stage will have a plate load 2x as big as its internal plate resistance.

2nd Approximation:
If you have access to plate curves for the tube, draw a loadline for the known supply voltage & plate load resistance. Draw a second line for the cathode resistance, found by taking an arbitrary grid (bias) voltage and dividing by the cathode resistor value, and plotting a point at that current and that grid voltage. Continue to get at least a point on either side of the plate loadline. The intersection is the operating point, plate voltage & plate current. A tangent to the grid voltage curve at the operating point will be the plate resistance at the operating point.

This will probably be similar or lower than the number you got in the 1st Approximation.

A lot of people just take an assumed fixed value of internal plate resistance off the data sheet, as long as the condition shown doesn't have very much higher/lower plate current than the proposed operating point. You can always refine you estimate later if the design looks promising on the first pass.

3rd Approximation:
From the plate pin, there are 2 paths to ground: Down through the tube's plate resistance and up through the plate load resistor to the filter cap (a.c. ground). So you calculate R_a (plate load resistor) || r_a (internal plate resistance). This number should be lower than the 2nd Approximation but typically not as low as 1/2 the plate load resistor value (when evaluating a triode stage).

4th Approximation:
The coupling cap connects the plate to the following stage's circuit, which usually has a resistor/resistance to ground. This is in parallel with the value found in the 3rd Approximation. However, the plate load resistor is typically around 2x (or more) bigger than the triode's internal plate resistance for good voltage gain, and the following stage's resistance to ground is typically 2x (or more) bigger than the plate load resistor so that it doesn't load the preceding stage significantly. The value you get here should be a little lower than in the 3rd Approximation, but not hugely-lower (or else the stage is designed poorly/voltage gain is severely impacted).

The 4th Approximation is not too difficult if you have a fixed resistor. You're looking at a tone circuit; it is a different impedance to ground depending on the frequency applied and the setting of the controls. Which is why PRR said:

TSC figures out the interaction between Zsource and the tonestack, not your job.

It's a tough mathematical problem, which is why the ToneStack Calculator does the work. So utmost precision only require the 3rd Approximation value (unless you have an unbypassed cathode resistor, in which case there's an added step).

[PRR]

6SN7 is a low-gain low-resistance tube.

It is odd to run it with a 270K plate resistor.

6SN7 datasheet "show-off" conditions are 9mA or 10mA. 270K with reasonable 250V-350V supply means tube current is like 0.5mA. If you really want such a low-current stage, you "could" switch to a skinnier tube, such as 12AX7, and get more voltage gain for your buck. If that is too much gain, you can maybe find a use for some form of loss.

Yes, I know guitar-amp design is often not about strict-best technical design.

I *do* think that the Fenderish tone-stack "should" be driven with a source lower than the slope resistor (150K here), and probably with a plate-resistor <= slope resistor. (Classic Fender, 100K plate, 100K slope.) Different tubes will give different source resistance and different responses, but keeping one resistor in-scale with the other may reduce surprise.

As a low-resistance tube, 6SN7 will never have a "large" impedance like 12AX7.

> as long as the condition shown doesn't have very much higher/lower plate current than the proposed operating point

HBP types faster.

Your values also expose one of several "issues" with Kuehnel's calculator.

1) Case-sensitive. "12Ax7" gives no answer; tube-name must be capitalized as shown. (I reported this to the author.)

2) Excess spaces. "6SN7 " (note trailing space) pops up a complaint.

3) Simple algorithm. If your Rl Rk values give an operating condition "like" the datasheet, the answers are fine. If you go way off from "datasheet" values, it can be significantly far-out. This is very reasonable for a free calculator. Actually estimating the op-point would need a MUCH more complicated calculation (would also need to be told your B+). Since we mostly use 12AX7, and 12AX7 rarely runs far from datasheet, the 'AX7 results are spot-on (within 20%, which is better than real tubes). Running a 10mA tube (6SN7) at 0.5mA (270K Rl), the result is less-right. For 6SN7 at 0.5mA 150V, the G.E. 1954 data gives plate resistance nearer 20K, so stage source Z is near 18K; POIC gives "8K". In this 150K tone-stack case, that is no-difference; in some other systems it might make a difference.

[PRR]

> A well-designed triode voltage amplifier stage will have a plate load 2x as big as its internal plate resistance.

Ratios 2:1 to 5:1 can "make sense".

But the actual plate resistanCE depends on the plate resistOR (because that also affects tube current).

We tend to run 12AX7 at the 2:1 or smaller end. Its internal resistance is already high, and its gain is high, we favor a not-so-high node impedance.

For classic old general-purpose triodes (6SN7), 5:1 does not give a too-huge Zout, and leaves more voltage gain.

I had a rule-of-thumb that Zout would be 1/3 to 1/6 of plate resistOR. However this 6SN7 270K example gives 18K/270K or 1/15(!). The flip-side is that voltage gain is not high, maybe 15 (here again POIC is not allowing for the very-low tube current).

FWIW: I think 12AX7 (6SF5) with 27K plate load will also have Zout near 18K, but twice the voltage gain.

[HotBluePlates]

> A well-designed triode voltage amplifier stage will have a plate load 2x as big as its internal plate resistance.

Ratios 2:1 to 5:1 can "make sense".

First off, I concur with everything PRR wrote. Below is just a bit more in the way of pictures to help you see what was written.

The graph below shows how voltage gain changes when you increase the plate load resistor.

When the plate load equals the tube's internal resistance, voltage gain is 1/2 of the amplification factor, µ, assuming a bypassed cathode resistor. As you increase the plate load resistor to 2x, 3x, 4x and above, you get 2/3*µ, 3/4*µ, 4/5*µ, etc.



The old books say there's little reason to go above 5x the tube's internal plate resistance, as you raise the required grid-to-ground resistance of the following stage (to avoid gain reduction due to heavy loading). That in turn raises the noise in the amp, as at least one form of resistor noise is proportional to its value. And many tubes have limits on how high the grid-to-ground resistance should be for predictable operation.

And as PRR noted, the tube's internal resistance depends on its plate current, which is determined by the supply voltage, plate load resistance, and bias (due to the cathode resistor value). So everything depends on everything else, which is why the empirical rules of thumb often pop up to give a sensible starting point.

[shooter]

It is odd to run it with a 270K plate resistor

PRR, those may have changed, I just grabbed a snipit from "one" of the schematics in the folder.
The amp in question is a working amp I gave my Son, but not *tuned*, His comment; "it's the best, and worst amp you built"  It works great as a keyboard amp, jazz amp, but lacks treble playing guitarin hard rock mode.  So I'm trying to do some "pre-thinking" before I get there to tweak it.

How exact do you need to be? Gnat's-ass perfection

If I hit the side of the barn, I'm happy!
This all started when I *approximated*  Zin, I screwed up and used 270k, the plot line dropped like 20db
hence, the question how to get close

"Zsource" (output Z) to be equal to the plate load resistor/2

That's perfect

I love the TSC, has always come in handy, never changed Zsource, til now.
So the answered info is now read, copied and saved, thanks.

So i'll hi-jack my thread for another question,  If the amp sounds great with a keyboard, that has what, 4 times the Freq range of a guitar, why would a guitar source *lack treble*? (multi guitars used)

Thanks again

[PRR]

> used 270k, the plot line dropped like 20db

IAC, 270K would be unlikely coming off a plate. The plate resistOR is 270K, true; but that is AC parallel to the plate resistanCE, which is almost every practical case is less than the plate resistor. (They could be comparable if plate resistOR were silly-small and the plate DC sat up above 80% of B+.)

The classic Fender circuits work with about 40K "out" of a stage and 220K-270K "in" to a stage. I don't think Leo looked at it that way, it just happened.

There is stray capacitance everywhere. Looking into a 12AX7 we see about 100pFd; purely passive networks have 30pFd or more of stray wire capacitance. 100pFd is 220K ohms at 7,500Hz. So even if we see a 1Meg resistor at a grid, we should remember the input is more like 200K ohms at the top of the guitar band.

When we connect one 12AX7 to another, this 100pFd is driven by the source impedance, typically 39K, and response extends to 42KHz. Yay! Well, we have many-stage amplifiers. -3dB at 42K in one stage is -12dB at 42KHz in four stages, perhaps -4dB at 21KHz, -2dB at 10KHz. So the Fender 39K:220K interface impedance works-out for an amplifier which won't trim the top of the guitar band, yet gives high gain per stage. (But I think he just took the "100K" suggested condition from the tube tables and liked it.)

[shooter]

270K would be unlikely coming off a plate

Agreed, just one of those brain-farts, was *guessing* about 80-100k, then when it came time to key in the value, brain said, 270k.

[HotBluePlates]

...  If the amp sounds great with a keyboard, that has what, 4 times the Freq range of a guitar, why would a guitar source *lack treble*? (multi guitars used) ...

It depends on what the "guitar source" is.

Keyboards are not only full-frequency range (probably wouldn't be 20Hz-20kHz, but perhaps close enough), but all the amplitude you want across that whole range. And depending on how you EQ the keyboard itself, its output can be ruler-flat (or something else if you desire). Guitar pickups are nowhere near full frequency range, or "flat". Low end is gone by 80Hz (perhaps sooner) and highs are significantly down by couple-kHz (or sooner).

The guitar amp is a significant tone-shaper, and has to work well with the guitar's pickup which is almost invariably a relatively high-impedance, which then suffers under stray capacitance and tube Miller Effect. So it takes consideration of high-impedance issues to successfully design a guitar amp.

Since the keyboard is full range, can be flat, and very likely can effectively drive low impedance loads (because of its own internal amplifier circuitry), it is likely to be less sensitive to the amp circuit which follows it. You might say you can more-readily slap something together to amplify a keyboard successfully, but the same design may not work well for guitar.


Some links for reading about pickup response/impedance, and how to go about measuring your pickups:
The Secrets of Electric Guitar Pickups
Measuring Impedance and Frequency Response of Guitar Pickups

You can see the effect of some of that (as well as wiring changes) in real time in the "Guitar Electronics" videos at https://www.youtube.com/user/A2Guitars/videos

[PRR]

> keyboard, that has what, 4 times the Freq range

Bah. Nobody wants to hear 19KHz. Very very fine piano and organ can be done inside 3KHz. Yes, some of Emerson's (RIP) work, also some GD, has synth squiggles far past 10KHz; I miss that now. But I have spent as much time "sweetening" piano by taking OUT an octave or two of the top. Many "music" keyboards are low-passed.

Many electric guitars "lack treble". There is the choice of Hum or hum-buckers (which incidentally don't treble well). Pickup versus guitar cord makes a steep low-pass. String gauge and overwinding makes compromises. Guitar amplifiers generally have rising highs 1KHz-4KHz, in amp or speaker or both, to compensate these droops.

[shooter]

Thanks to both, PRR, HBP, I spent 4 hours last night *tuning* with my Son, and each change got "sweeter", by the time it was all said and done I was completely deaf and my Son was completely satisfied!! I'll post the schematic on the original build thread after I convert the chicken scratch.