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Vintage threads from the first ten years

Re: O'connor books/mistakes & corrections?

7/13/1999 1:49 PM
J Epstein	Re: O'connor books/mistakes & corrections? I have the Morgan Jones book and I find it pretty good : he explains in detail a lot of design tradeoffs and and circuit details in a practical way. It is completely geared to hifi amps and ignores guitar equipment, but I think almost everything in it is applicable to guitar amp building in some way, except the section on RIAA disc equalisation which is pretty fully explained. I recently bought a copy of RDH 3 on Ebay because it's said to be similar to RDH4 but more condensed, and I think it may be useful to have a simpler reference some of the time. I can probably carry RDH3 in my briefcase and take it into the throne room at work - now that's a quality break! Anyway it hasn't come yet so I can't comment from my own experience. I will try to post some comments when I have a chance to road-test it. RG probably has a copy, maybe he can chime in? -j

7/13/1999 1:49 PM

J Epstein

Re: O'connor books/mistakes & corrections?

I have the Morgan Jones book and I find it pretty good : he explains in detail a lot of design tradeoffs and and circuit details in a practical way. It is completely geared to hifi amps and ignores guitar equipment, but I think almost everything in it is applicable to guitar amp building in some way, except the section on RIAA disc equalisation which is pretty fully explained.

I recently bought a copy of RDH *3* on Ebay because it's said to be similar to RDH4 but more condensed, and I think it may be useful to have a simpler reference some of the time. I can probably carry RDH3 in my briefcase and take it into the throne room at work - now that's a quality break!

Anyway it hasn't come yet so I can't comment from my own experience. I will try to post some comments when I have a chance to road-test it. RG probably has a copy, maybe he can chime in?

-j

7/13/1999 8:22 PM
R.G.	...um... I have two copies of RDH3, I bought a second one thinking it was an RDH1 from the description. The RDH3 is an excellent and more condensed reference. It has some ways of looking at things that are a different perspective from RDH4. I like them both. 3 is usually $15 to $25, and a bargain at that. I keep trying to find an RDH1 and an RDH2 for completeness.

7/15/1999 4:51 PM
Mike D	In TUT, the resistance used in the 1/(2piRC)equation is simply the plate resistor. I don't have the Morgan Jones book in front of me, but I believe the correct resistance should be: Rg + (RL \|\| Rp) where Rg = the grid resistor of the next stage RL = the plate resistor Rp = the internal dynamic plate resistance Note that if Rg is sufficiently large compared to RL, you can pretty much ignore RL and Rp altogether. To visualize how the equivalent resistance is derived, you would pretend you shorted the power supplies and stuck an ohm-meter across the cap. This magic ohm-meter can read the dynamic plate resistance as well as static resistances. In a typical Fender stage, RL = 100k, Rg = 1meg, Rp = 62k O'Connor gives the -3db point as 72hz, when it should actually be about 6.96 hz. This is all off the top of my head and i will check for inaccuracies. Regards, MD

7/15/1999 4:51 PM

Mike D

In TUT, the resistance used in the 1/(2*pi*RC)equation is simply the plate resistor.

I don't have the Morgan Jones book in front of me, but I believe the correct resistance should be:

Rg + (RL || Rp)

where

Rg = the grid resistor of the next stage

RL = the plate resistor

Rp = the internal dynamic plate resistance

Note that if Rg is sufficiently large compared to RL, you can pretty much ignore RL and Rp altogether.

To visualize how the equivalent resistance is derived, you would pretend you shorted the power supplies and stuck an ohm-meter across the cap. This magic ohm-meter can read the dynamic plate resistance as well as static resistances.

In a typical Fender stage, RL = 100k, Rg = 1meg, Rp = 62k

O'Connor gives the -3db point as 72hz, when it should actually be about 6.96 hz.

This is all off the top of my head and i will check for inaccuracies.

Regards,

MD

7/15/1999 9:31 PM

ken g

quote:
"In TUT, the resistance used in the 1/(2*pi*RC)equation is simply the plate resistor.

I don't have the Morgan Jones book in front of me, but I believe the correct resistance should be:

Rg + (RL || Rp)"

Actually, all three resistances must be in parallel. And if you want to be anal, include the cathode resistor IN SERIES with the plate impedance.

To add R's in parallel, invert them all, add them, then reinvert the sum.

And you can't ignore Rl and Rp, even if Rg is large. Actually, it's the other way around--R's in parallel end up being less than the SMALLEST R... so as the R goes up, it has less effect on the composite R. IE 100K || 1M = 91K

It's a good thing you do most of your thinking from the MIDDLE of your head

~KG~

7/16/1999 7:55 PM
Mike D.	Ken, If you put all 3 resistances in parallel, ie: 100k \|\| 62k \|\| 1meg you get 36.86k With a 0.22uf cap this gives a 3db down point of 196hz, which is even higher than O'Connors method. It is true that you would parallel all 3 for gain calculations, but I do not believe you do this for the purposes of computing the 3db down point that results from the interstage cap. I reference the following article: http://uts.cc.utexas.edu/~mic/info/amps/resources/Review-The-Ultimate-Tone I modelled the circuit in Ecap. The 3db down point was about 6.9 hz, as predicted. Changing the 100k load resistor to 50k had very little effect, whereas changing the 1meg had the greatest effect. I checked the Morgan Jones book and he is very "anal" about circuit ANALysis (pun intended). He gives the 3db down point of the classic hi-fi RC interstage (0.1uf cap, 1meg grid) as 1.6 hz, totally ignoring the rp and RL of the preceding stage. This is a guy who sweats a 470 ohm impedance mismatch in a phase splitter, adding a "build-out" resistor. He's an absolute wiz when it comes to miller capacitance as well. I'm not trying to make a federal case out of this, just trying to gain a greater understanding. We all agree on the basic principle: bigger coupling cap = more bass, too big = motorboat. That simple formula has been working well for me for 20 years of dorking with amps! Hell, this is rock and roll man! By the way Ken, how's your 600 watt monster doing these days. Regards, Mike D.

7/16/1999 7:55 PM

Mike D.

Ken,

If you put all 3 resistances in parallel, ie:

100k || 62k || 1meg

you get 36.86k

With a 0.22uf cap this gives a 3db down point of 196hz, which is even higher than O'Connors method.

It is true that you would parallel all 3 for gain calculations, but I do not believe you do this for the purposes of computing the 3db down point that results from the interstage cap.

I reference the following article:

http://uts.cc.utexas.edu/~mic/info/amps/resources/Review-The-Ultimate-Tone

I modelled the circuit in Ecap. The 3db down point was about 6.9 hz, as predicted. Changing the 100k load resistor to 50k had very little effect, whereas changing the 1meg had the greatest effect.

I checked the Morgan Jones book and he is very "anal" about circuit ANALysis (pun intended). He gives the 3db down point of the classic hi-fi RC interstage (0.1uf cap, 1meg grid) as 1.6 hz, totally ignoring the rp and RL of the preceding stage. This is a guy who sweats a 470 ohm impedance mismatch in a phase splitter, adding a "build-out" resistor. He's an absolute wiz when it comes to miller capacitance as well.

I'm not trying to make a federal case out of this, just trying to gain a greater understanding.

We all agree on the basic principle:

bigger coupling cap = more bass, too big = motorboat.

That simple formula has been working well for me for 20 years of dorking with amps! Hell, this is rock and roll man!

By the way Ken, how's your 600 watt monster doing these days.

Regards,

Mike D.

7/16/1999 9:38 PM
ken g	The way I look at it, Mike, is I try to visualize all of the resistances which would serve to discharge any AC voltage impressed across the cap. As a result, you've got to take into account all of the paths to AC ground, and that includes impedances to the B+ rail, as well as to signal ground. All of these impedances factor into the time constant of the composite RC characteristic. I could be wrong, though. I've never really emprically verified this. Goes to show you how much you need to characterise tube circuits to get good tones from them! As far as the 600W monster, it still weighs 100 pounds, and it still kicks some serious ass. I haven't heard anything bad come out of it yet. There are a series of tweaks that I'd like to do to it, but I can't seem to get them done. There's two reasons for this: a) it weighs 100 pounds and b) I seriously can't stop playing it long enough to really get in and make big improvements. I can't bear to have it on the bench for longer than 2 days I have never heard a BT sound as good as those KT90's. It is a real shame... ~KG~

7/16/1999 9:38 PM

ken g

The way I look at it, Mike, is I try to visualize all of the resistances which would serve to discharge any AC voltage impressed across the cap. As a result, you've got to take into account all of the paths to AC ground, and that includes impedances to the B+ rail, as well as to signal ground. All of these impedances factor into the time constant of the composite RC characteristic.

I could be wrong, though. I've never really emprically verified this. Goes to show you how much you need to characterise tube circuits to get good tones from them!

As far as the 600W monster, it still weighs 100 pounds, and it still kicks some serious ass. I haven't heard anything bad come out of it yet. There are a series of tweaks that I'd like to do to it, but I can't seem to get them done. There's two reasons for this: a) it weighs 100 pounds and b) I seriously can't stop playing it long enough to really get in and make big improvements. I can't bear to have it on the bench for longer than 2 days ;-)

I have never heard a BT sound as good as those KT90's. It is a real shame...

~KG~

7/17/1999 3:05 AM
moocow	My annoying, arrogant, and rambling comments: 1) 36.86k and .22 uF gives 19.6 Hz. That should be .022 uF. 2) I was at a boring meeting today and derived the equations for bypassed and unbypassed Rk: The equation for the equivalent "R" is indeed: Rg + (RL \|\| rp) as Mike D stated. When Rk is unbypassed: Rg + (RL \|\| (rp + (A+1)Rk)) (A+1)Rk should be added to rp, not simply Rk. The UT guy is wrong. 4) These equations are derived by first drawing the equivalent small-signal circuit model. An ideal AC voltage source is applied to the model and the resultant AC current is calculated. The ratio between AC voltage/AC current is the equivalent resistance. The UT guy states that the equivalent resistance "is the value that an ohmmeter would read...". This is a DC value, not AC. This is why he is wrong. 5) There isn't much of a difference whether the cathode resistor is bypassed (6.97 Hz) or unbypassed response (6.77 Hz). Still, if an equation is posted on this BBS, it should be the correct equation. 6) None of this is accurate if the cathode bypass capacitor has its critical frequency near that of the coupling capacitor. The equations for Cc only work when the critical frequencies are widely seperated. As luck would have it, they two freqencies are very close together in a Fender amp. 7) Because of (6), the best way to analyze these circuits is with PSpice or other circuit simulator. If you really want to know what's happening inside of these amps, you must use a circuit simulator. Those tone control simulators ignore the EQ contribution of the rest of the amp, which is what we're discussing here. 8) The innacurate equation for R = Rg \|\| Rl is popular because it is a synthesis equation, not an analysis equation. That is, the equation is used to choose a coupling capacitor, not determine its cutoff frequency. You use they equation by choosing a cutoff frequency and calculate a capacitance. Then use the next largest value. You never use the value you calculated, so the slight error doesn't matter much. 9) Audio people don't really care where the cutoff frequency is, just as long as it is very low. If they designed for a 2 Hz cutoff and got a 1 Hz cutoff instead, it would be no big deal. Even thoug it is a 1-octave error, it will be inaudible. They can get away with these innacurate rules of thumb, so these rules can still be found in Morgan Jones' book, Audio Cyclopedia, and other otherwise reliable reference texts. Amp gurus then read these books and repeat the equations in their own, without ever having derived them on their own. 9) To find the actual cutoff frequency, we need to use either exact equations or simulations to analyze our favorite guitar amp circuits. These circuits were tuned by ear, so their cutoff frequencies are often in the audible range. In this case, if we designed for a 200 Hz cutoff frequency but wind up with 100 Hz, the difference will be noticable. 10) If someone wants to ignore these equations altogether and tweak their amp by ear, that's fine. In the final analysis, your ear is what really counts.

7/17/1999 3:05 AM

moocow

My annoying, arrogant, and rambling comments:

1) 36.86k and .22 uF gives 19.6 Hz. That should be .022 uF.

2) I was at a boring meeting today and derived the equations for bypassed and unbypassed Rk:

The equation for the equivalent "R" is indeed:

Rg + (RL || rp)

as Mike D stated.

When Rk is unbypassed:

Rg + (RL || (rp + (A+1)Rk))

(A+1)Rk should be added to rp, not simply Rk. The UT guy is wrong.

4) These equations are derived by first drawing the equivalent small-signal circuit model. An ideal AC voltage source is applied to the model and the resultant AC current is calculated. The ratio between AC voltage/AC current is the equivalent resistance.

The UT guy states that the equivalent resistance "is the value that an ohmmeter would read...". This is a DC value, not AC. This is why he is wrong.

5) There isn't much of a difference whether the cathode resistor is bypassed (6.97 Hz) or unbypassed response (6.77 Hz). Still, if an equation is posted on this BBS, it should be the correct equation.

6) None of this is accurate if the cathode bypass capacitor has its critical frequency near that of the coupling capacitor. The equations for Cc only work when the critical frequencies are widely seperated. As luck would have it, they two freqencies are very close together in a Fender amp.

7) Because of (6), the best way to analyze these circuits is with PSpice or other circuit simulator. If you really want to know what's happening inside of these amps, you must use a circuit simulator. Those tone control simulators ignore the EQ contribution of the rest of the amp, which is what we're discussing here.

8) The innacurate equation for R = Rg || Rl is popular because it is a synthesis equation, not an analysis equation. That is, the equation is used to choose a coupling capacitor, not determine its cutoff frequency. You use they equation by choosing a cutoff frequency and calculate a capacitance. Then use the next largest value. You never use the value you calculated, so the slight error doesn't matter much.

9) Audio people don't really care where the cutoff frequency is, just as long as it is very low. If they designed for a 2 Hz cutoff and got a 1 Hz cutoff instead, it would be no big deal. Even thoug it is a 1-octave error, it will be inaudible. They can get away with these innacurate rules of thumb, so these rules can still be found in Morgan Jones' book, Audio Cyclopedia, and other otherwise reliable reference texts. Amp gurus then read these books and repeat the equations in their own, without ever having derived them on their own.

9) To find the actual cutoff frequency, we need to use either exact equations or simulations to analyze our favorite guitar amp circuits. These circuits were tuned by ear, so their cutoff frequencies are often in the audible range. In this case, if we designed for a 200 Hz cutoff frequency but wind up with 100 Hz, the difference will be noticable.

10) If someone wants to ignore these equations altogether and tweak their amp by ear, that's fine. In the final analysis, your ear is what really counts.

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