Preamp Understanding;Rp Values

[Tim Hall]

I’m doing my best to learn about guitar amp theory and design...been reading both O’Connor and articles by Aiken. The technical information I’m finding seems to be pretty solid (as far as I know), but I’m having a hard time connecting the dots...I can’t seem to find the little nuggets of insight that explain what it all means in the big picture (with regard to playing guitar).

For example: On setting the bias point for a typical gain stage Aiken says, “the plate resistor is chosen first, typically around twice the value of the internal plate resistance of the tube for most linear operation, although higher values can be used for more gain and output voltage swing.” I looked up the plate resistance of a JJ ECC83, and saw it was 47kΩ, which is indeed about half of the 100k-value Rp one typically sees in a Fender/Marshall type preamp.

So here’s my questions for anyone who might be able to help me connect the dots: What does linearity mean in the big picture? What does it mean with respect to guitar preamp performance? My next question: given Ohm’s law, increasing the Rp value increases the voltage swing given the same conditions...so what might larger voltage swings mean for me as guitar player?

If anyone can point me toward a source(s) of literature that explains amp theory with the big picture in mind, I would really appreciate it.

Thanks,

Tim

PS – I realize there is a lot to learn/know about amps, and it can’t all possibly be published in one place. So this is not intended as a criticism on O’Connor or Aiken...I personally just don’t seem to be learning well from these particular sources.

[bob p]

i was looking at Steve Bench's pages the other day. He had a number of tutorial pages on drawing Loadlines. they would definitely be helpful in learning to connect the dots (pun intended).

[Tim Hall]

Aha!

There are the missing links. Thanks Bob.

I also found this bit of rudimentary theory which helped:
http://www.tubecad.com/articles_2003..._Amplifier.pdf

The problem I was having was Aiken's article seems to assume the reader knows what a load line is, and how to plot one...not likely if you're learning the fundamentals. I also see now that "linearity" simply means the tube is operating within spec...no (or negligible) distortion.

[Amp Kat]

http://en.wikipedia.org/wiki/Linear

Try this

[Tim Hall]

Thanks KB. I'm very familiar with "linearity" in mathematical & geometric terms...electronics is another matter.

[Amp Kat]

If you scroll down there is an explanation of Electronics also. Tubes in general aren't very linear devices but can be plotted on a graph to show the relationship of how a signal acts with voltage vs current and resistance in the form of impedance and frequency. Transistors are more linear which is why the definition used them as an example. It isn't any different in it's terms of math or geology than it is in electronics. As for Aikens sheet the the calculations were for a tubes output impedance depending on the type of tube used in the example of a common cathode amp. Different tubes have different plate impedances so the numbers are going to be different if your using 12AX7,12AT7,12AU7 or any other tube. There are some things they expect you to know so yeah it can be misleading.

[Alex/Tubewonder]

Get Norman Crowhurts classic articles from 1954 on audio amp design.
Go to THIS SITE

Scroll down to Resources and download. Very good stuff.

[Tim Hall]

Very cool link and articles. Thanks Alex!

[Tim Hall]

Ok, I’ve read quite a bit more since my last post on this thread...it’s beginning to be a bit less fuzzy now. I’m still finding that among all the texts I read (O’Connor, Morgan Jones, Crowhurst, etc.), there is some little thing that the author is assuming, which doesn’t quite make everything fall into place for me. I’m hoping someone can verify my understanding of plate load values is correct (or correct me if I’m wrong).

I now understand how load lines are used to find a quiescent operating point (bias point) for triodes within certain performance parameters (negative grid voltage, cut-off, max dissipation). What hasn’t been clear is how these authors go about selecting an Rp value for their examples. One author uses an Rp twice the internal plate resistance, while another uses an Rp three-times the plate resistance. So I decided to split the difference (2.5X) when drawing up my own example...

I’ve been curious about using lower mu triodes in a preamp, so I look at the odd little 6N1P as a candidate with it’s plate resistance of 4.4k. So by my math 2.5 x 4.4k = 12k (nearest value) plate load, right? Immediately I think I must have read the spec’s wrong because I often see a 100k value used for all kinds of triodes in Fender/Marshall preamps. So I cross-reference the 12AX7, and sure enough it has a plate resistance approximately 10x higher than the 6N1P. But then I cross reference the 12AU7, and see that it is much closer to the 6N1P’s spec’s, nevertheless it’s commonly used in preamps with an Rp value around 100k. So what gives here?

It occurred to me that the 12AU7 an 6N1P also have much higher dissipation ratings than the 12AX7. Then it occurred to me that a 12k load might be a reasonable value of output transformer. So this is what I came up with: output stages generally have lower impedances and higher current. Preamp stages have much lower current, higher impedances, and therefore larger voltage swings. The 12AU7 and 6N1P can be used as low-powered output tubes, while the 12AX7 would likely melt down because of it’s higher internal resistance (exceed max dissipation) if the same amount of current was passed through it. So using a 12k plate resistor for a 6N1P in a preamp is probably out of the question.

Am I on the right track here?

[Steve Conner]

There is no "magic" value of plate or cathode resistor that falls out of an equation. The values suggested in the tube datasheet will have been found empirically by the engineers at Mullard (or wherever) while they were designing the tube.

When designing a preamp stage, you need to choose the quiescent plate voltage and plate current. These can be set independently to whatever you want, by choosing different values of plate and cathode resistors, and they both affect the stage gain, maximum signal swing, and distortion performance.

As an example, to optimize signal swing, you may want to make sure the plate ends up sitting at about 2/3 of the B+ voltage, and the idle cathode current is about 1/2 of what you get with Vgk=0 (ie the maximum possible current for that tube)

You probably see 12AU7s in circuits with the 12AX7 component values because someone swapped in a 12AU7 when they were prototyping and liked the tone that it gave.

The 12AX7 would make a poor output tube, because it physically won't pass much current, even with the grid and cathode both at 0V (which is the hottest you can bias it) However, that didn't stop E.A.R. making a push-pull power amp that uses 10 12AX7s per side:

http://www.ear-usa.com/v20reviews.htm

The 12AT7 and 12AU7 will pass much more current and develop more power. The 12AT7 is often used as a reverb driver tube in Fender amps. However, they have lower gain when used as R-C coupled preamp stages like a 12AX7.

[Tim Hall]

Steve,

Thanks for the insight and taking the time to read my long-winded post. The source of information I seem to be learning the most from is Morgan Jones' "Valve Amplifiers." It's unfortunately geared more towards HiFi...he briefly mentions guitar amps in the "Uses and Abuses" section.

Jones talks about optimizing an amp stage for either maximum signal swing or maximum linearity. Since you brought up signal swing, I would really appreciate it if you could expand on that a little(?) In a guitar preamp do we want maximum signal swing or maximum linearity? I know this might get into subjective territory, but I'm just trying to figure out what either of these things possibly means for me as a guitar player. I mean how would this maximum signal swing in the preamp affect the the final outcome? Does more signal swing = more apparent dynamic response when playing?

[Steve Conner]

Hi Tim,

In the case of a 12AX7 the bias points for optimal linearity and optimal signal swing aren't too different. If they were far different, the tube would be a poor choice for R-C coupled audio amplification.

In other words, if you choose either of the datasheet recommended component values for a 12AX7 (100k plate resistor and 1.5k cathode resistor, or 220k/2.7k IIRC) you will get near optimal linearity and swing. The 220k/2.7k setup gives higher gain and the 100k/1.5k can swing more into a given load.

These recommended values are also a good starting point for getting something that overdrives nicely. I once tried an amp circuit where the last preamp tube before the volume control (ie the one that gets overdriven most) was fitted with adjustable fixed bias instead of cathode bias. I found that if I changed the bias current a great deal in either direction, I got an overdrive tone that sounded kind of sputtery and funky as if it might break down any minute.

I also like the tone of a 12AT7 when subbed into the standard 12AX7 circuit (either the 100k or 220k). I'm not too sure what's going on in that case, but I think it biases at a current (or maybe rather a plate voltage) much lower than what would give good linearity, and generates a lot of low-order harmonics for a very warm, fat and dirty sound.

[Tim Hall]

Steve,

Thanks for the reply...that's some good info to chew on.

[Arthur B.]

The 12AX7 would make a poor output tube, because it physically won't pass much current, even with the grid and cathode both at 0V (which is the hottest you can bias it) However, that didn't stop E.A.R. making a push-pull power amp that uses 10 12AX7s per side:

http://www.ear-usa.com/v20reviews.htm

That design pushes the grids of the 12ax7s positive to get its power and bandwidth. They can be biased into the positive grid region, but must be run at low plate voltages.

[Manic]

I also like the tone of a 12AT7 when subbed into the standard 12AX7 circuit (either the 100k or 220k). I'm not too sure what's going on in that case, but I think it biases at a current (or maybe rather a plate voltage) much lower than what would give good linearity, and generates a lot of low-order harmonics for a very warm, fat and dirty sound.

Im a few steps behind Tim in understanding all this great but still a lil confusing info.
It would be cool to run some loadlines to see what is going on with 12AT7 in a 12AX7 setup. Just to see if We can get a 12AX7 to produce those nice 2nd order harmonics ?