Fender AA764 / passive equalizer question

[Gerry Rzeppa]

Just wondering... Can I take the circled portion of the equalizer circuit below and insert it in place of the circled portion of the AA764 circuit (also pictured below). Why or why not?

[Leo_Gnardo]

THAT would make a heck of an EQ, sort of 7 band graphic. I think you could successfully drive one or two sections off the plate, as in the AA764 graphic, but 7 sections in parallel adds up to a fairly low impedance to drive. Cathode followers are made for that purpose. Take a look at this pre from Fender's 6G6A white/tan Bassman. Plunk your EQ in there & I think you'll be more satisfied.

[rdjones]

That's an awfully heavy load to drive from the plate of a 12AX7.
There's no DC blocking for R5.

I'd say no, not as a straight transplant it wouldn't work.

At the very least you'd need a cathode follower to drive the stack and appropriate DC blocking.
Then there's the question of insertion loss ...

rd

[Leo_Gnardo]

That's an awfully heavy load to drive from the plate of a 12AX7.
There's no DC blocking for R5.

Let's include C3 "inside the box" then no problem.

Well spotted Mr Jones!

Follow with another triode stage for makeup gain.

[/quote]

[Gerry Rzeppa]

Thank you, Leo_Gnardo and rdjones. I've read up on cathode followers and I see what you're saying. While I'm here, I have two other (unrelated) questions:

1. I notice on the MOD-101 schematic below that they're sharing the resistor and capacitor on the two pre-amp cathodes; what is the effect/advantages/disadvantages of this (aside from lowering the part count)?

2. Do you know of any book or article that explains guitar amplifier circuits from the "electron flow" point of view (as opposed to the "conventional" point of view)?

[Leo_Gnardo]

1. I notice on the MOD-101 schematic below that they're sharing the resistor and capacitor on the two pre-amp cathodes; what is the effect/advantages/disadvantages of this (aside from lowering the part count)?

Glad to help Gerry. I'll take a swing at the first. Separate cathode components gives you another place to "voice" the stage. Combined, yes cost savings. And when the bypass cap goes bad, an unexpected signal path that may cause all sorts of weird noises.

Second question, I just imagine electrons marching down those wires. Little ones can fit thru picofarad caps, and fat ones thru big electrolytics. JUST KIDDING! Sort of reminds me of the Interview With The Bass Player (Bill Murray as it turned out) on a National Lampoon LP. For a more serious answer I'm sure one's about to show up.

[xtian]

Check out Merlin Blencowe's books:

http://www.valvewizard.co.uk

He like to talk about electrons, and he knows his maths!

You can see excerpted articles from his books on his web site.

[Gerry Rzeppa]

Second question, I just imagine electrons marching down those wires. Little ones can fit thru picofarad caps, and fat ones thru big electrolytics. JUST KIDDING!

The reason I asked is because I built a modified Bassman AA864 a while back and had serious hum problems -- until I started thinking about the chassis as a big "pond" of electrons, the wires as "straws" sucking them up, and the electrolytics as "pre-sucked electron holding tanks". NO KIDDING! I found that when I placed the (ground) straws in the pond such that nobody's (unrelated) sucking was close to anyone else's sucking, and that the straws that wanted a lot of "juice" (high voltage potential) were close to the "pre-sucked electron holding tanks", the hum problems went away (hum being pictured as "waves" or "sloshing" or "bubbles" from the other straw's sucking). I was hoping to find a book or article that explained the rest of operation of an amp in similar terms, eg, the transformer as an oscillating electron pump or something, etc.

[Teleguy61]

Second question, I just imagine electrons marching down those wires. Little ones can fit thru picofarad caps, and fat ones thru big electrolytics. JUST KIDDING!

The reason I asked is because I built a modified Bassman AA864 a while back and had serious hum problems -- until I started thinking about the chassis as a big "pond" of electrons, the wires as "straws" sucking them up, and the electrolytics as "pre-sucked electron holding tanks". NO KIDDING! I found that when I placed the (ground) straws in the pond such that nobody's (unrelated) sucking was close to anyone else's sucking, and that the straws that wanted a lot of "juice" (high voltage potential) were close to the "pre-sucked electron holding tanks", the hum problems went away (hum being pictured as "waves" or "sloshing" or "bubbles" from the other straw's sucking). I was hoping to find a book or article that explained the rest of operation of an amp in similar terms, eg, the transformer as an oscillating electron pump or something, etc.

What an organic description of a Star Grounding Scheme!
Loved it Gerry!

[Gerry Rzeppa]

Check out Merlin Blencowe... He likes to talk about electrons...

Thanks for the suggestion, xtian. Unfortunately, Merlin does not appear to be the guy I'm looking for. Quoting from his book:

"This [speaking of current as positive] sometimes causes consternation among students who argue that conventional current ‘goes in the wrong direction’, but this arises due to a misunderstanding of what current actually is. The confusion is understandable because we often describe current as being a flow of electrons, and immediately form a convenient mental picture of what is happening inside conductors...."

Which is exactly the "convenient mental picture" I'm trying to develop.

"But while this explanation is good enough for everyday conversation, it is not the whole truth. Strictly speaking, current does not have any direction in the usual sense because it is not a physical ‘stuff’ that moves around a circuit. Current is more properly defined as the rate of change of electric flux: i = dQ/dt, and is, therefore, an entirely mathematical concept."

Which (I think) is like someone saying, "You can think of this post as words and sentences, but that's not the whole story. It's actually millions of pixels of various colors arranged in a two-dimensional array...". True, but not helpful. (Don't get me wrong -- it's not that I don't believe there is value in mathematics and quantum physics, etc; I have a degree in math myself. But I do think that choosing the correct "level of abstraction" -- and sticking with it -- is critical to understanding anything.)

Now at some level of abstraction I believe it can truly be said that electrons actually move or "flow" from one physical place to another via some kind of conduit or wire or "hose". I'm looking for a book or article by someone who (a) shares this "convenient mental picture", and (b) can explain the whole of a guitar amp in such terms.

[Leo_Gnardo]

Second question, I just imagine electrons marching down those wires. Little ones can fit thru picofarad caps, and fat ones thru big electrolytics. JUST KIDDING!

The reason <snip > oscillating electron pump or something, etc.

What an organic description of a Star Grounding Scheme!
Loved it Gerry!

+1, you betcha! Just as good a way of looking at it as any. If it works for you, no argument from me.

There WAS a 5 page letter to the editor, one of the Audio Amateur mags late 80 - early 90's, that went on and on, mostly being offended by the phrase "flow of current." It's not current that flows, it's charge that moves. Flows, if you wish. The movement of charge is current. On and on it went untill my eyes blurred and crossed. Then I read the writer's name. Turns out to be a git that occupied an apartment I lived in, college days. The guy was impossible to deal with then too. Occasionally right but . . . what a bloviator. The mag must have been short on ads to run, to print that letter. Oh BTW that guy STILL owes a month's rent he skipped out on summer of 1974. For all his perfection, couldn't pay the bill. Hmmm.....

[Gerry Rzeppa]

A bit of internet searching has led me to believe that the "electronic–hydraulic analogy" (derisively referred to as "drain-pipe theory") is what I've been picturing:

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

But with specific application to tube guitar amps, of course. Still looking...

[rdjones]

Thank you, Leo_Gnardo and rdjones. I've read up on cathode followers and I see what you're saying. While I'm here, I have two other (unrelated) questions:

1. I notice on the MOD-101 schematic below that they're sharing the resistor and capacitor on the two pre-amp cathodes; what is the effect/advantages/disadvantages of this (aside from lowering the part count)?

2. Do you know of any book or article that explains guitar amplifier circuits from the "electron flow" point of view (as opposed to the "conventional" point of view)?

1. Pretty sure it's all about parts count, and the cost of caps.

2. http://www.allaboutcircuits.com/vol_1/chpt_1/7.html
The NEETS course also covers electron vs conventional.
In troubleshooting or just going over a circuit conventional flow is easy to follow and probably more intuitive for most people.
But if things worked the way conventional flow says then a tube would not function.
Electron flow helps understand how tubes and other active devices do what they do.

rd

[Gerry Rzeppa]

http://www.allaboutcircuits.com/vol_1/chpt_1/7.html
The NEETS course also covers electron vs conventional.
In troubleshooting or just going over a circuit conventional flow is easy to follow and probably more intuitive for most people.
But if things worked the way conventional flow says then a tube would not function.
Electron flow helps understand how tubes and other active devices do what they do.

Yes, good references. I've seen the "AAC" before, and own and have read the appropriate chapters of the NEETS course.

It's surprising -- I'm referring to some of the comments on the AAC site -- how touchy people can get regarding this topic!

At any rate, it seems to me that not much happens in a guitar amp unless electrons are flowing and so I'd still like to see a complete description of a simple guitar amp circuit entirely in terms of electron flow. I think what I'm going to do is start a thread on the subject over in the "Technical Discussion" forum where I describe my understanding of the thing and let you (more knowledgeable) folks steer me in the proper direction.