fender 212r

[Enzo]

Every time I have to explain how some circuit works, I have to first figure out what they are doing. In my work I don;t think about it. This particular amp is so stupidly over complex, I get a headache just thinking about it. I don't know exactly what they are doing there, I just see if the diode is shorted and move on. Looking it over, it seems to me it is there to help turn Q18 OFF faster when the signal drops negative. I could be wrong.

If you get that 2v space and something hauling the whole enchilada up and down, then you understand the right half of the power amp. The left half is the diffy pair and voltage amp. Look at the VERY basic SWR amp here. If you undertsnad that, you know how amps work, and when you encounter something like this Fender, you can look at it with how a simpler amp would have been.

On the SWR, there are your outputs and drivers, Q5,6 are limiters, Q4 is the bias transistor maintaining that space. Q3 is the voltage amp, see how it grabs Q4 by the elbow and shakes the whole thing, space and all, up and down? Q1 drives the Q3, while Q2 watches the output and adjusts Q1 to compensate for any errors. That's it.

Your FM212 doesn;t grab an elbow, they split the bias into two circuits, then they haul it back and forth from the middle. Like shaking it by the neck instead of elbow I suppose. Why? Beats me. If I had to guess, I'd say they are slapping on a lot of extra complexity to try to squeeze more peaks out of fewer output transistors. I could be wrong.

[lowell]

I'll try to absorb that SWR schematic over the next week.

What exactly are Q12 and Q13 doing? Their collectors meet in the middle of the bias circuit. Are they there to make sure the bias transistors emitters are exactly 0v - between the rails?

[Enzo]

I like to look at things intuitively, I tend to visualize and use metphor, whatever it takes to be one with the amp - a zen thing. I talked about having that whole bias separation, and something hauling that up and down to make the outputs follow. And instead of one transistor making 2 volts, we have two transistor circuits joined together doing bias - Q14,15. I mentioned instead of grabbing a corner of the bias tree, this particular amp grabs the middle betwee those two biases. That detail doesn't matter, they could have designed it to grab from the top if they wanted. Well on this amp, the handle for output-yanking is the point between the two bias spacers.

Q12, Q13 are a push pull stage latched onto that handle. They are what pulls the output circuits up and down to make music. It's balanced, Q9,10 each drive one of them. Note the circuits driving each are the same, though mirror images. See the diodes and their relationship? You even have Q11 there to flip it over for the V- side.

[Enzo]

Here is an even more basic circuit, a rare old early Peavey. Every part in it is a classic, and I am sure the circuit was a straight lift from a transistor manual. Much like early fender amps were straight lifts from the RCA tube manuals.

[lowell]

Cool thanks for referring me to these classic schematics. I'm gonna print 'em out and carry 'em with me so on down time I can absorb how they work.

Ok, so I kind of get Q12 and Q13... but why not just have transistors Q14 and Q15 connect at the output bus? Wouldn't then the up and down push/pull motion of the output bus do the same thing?

[J M Fahey]

Want SIMPLE and time tested?
I have built over 10000 (yes, not a typo, over ten thousand) amplifiers in over 40 years.
The first 7000 or 8000 were like this:

The last 2000 or so, were simpler, thanks to TIP142/147:

The board (including PSU) is:

This batch was upgraded to a Vce multiplier bias instead of the three diodes.
How can I produce so many?
I make them in batches of 40 or more, 100 in good times:

I make lots of speakers too; hese are the frame stampings, already zinc plated and with the iron disks soldered.(Batches of 50 to 100):

These are the 12" speakers almost ready. They are hand made in smaller batches, some 8 or 10 each.

Well, that's a glimpse of the "Little Factory in the Pampas".
Lost too much time here, now I'll go milk the cows and feed the chickens.

[Enzo]

And yet simple or basic, they all do the same thing. A couple diodes works just fine as a bias spacer. Of course the step between one diode or two is a whole half a volt. Fancier amps will use a transistor so the bias can be more finely tuned. But still the whole point is to maintain that operating space.

[lowell]

JM that is amazing simplified. Thanks for sharing the pictures, looks like a nice operation you got goin' there. Wish I could build 40 amps at once. It's so much easier when you can get the assembly line goin'. As of now my building is slow and sluggish, though I am almost done with my first 5 tube heads and am excited.

Anywho, it's nice to see a simplified version.. or should I say not unnecessarily overcomplex... and to see that after building 10,000 amps that THAT is your choice of style and approach.

Back to my last question... why not tie the center of the bias transistors to the output bus instead of using Q12/13's collectors?

[Enzo]

Then what would pull the driver bases around to drive them? The bias is there to keep a separation between the driver bases, not between the bases and output. The bases already have a relatiion to the output, they are a couple of junctions away from it.

[lowell]

Ok. Here's my confusion. I've been picturing it this way so far: The AC signal coming off of Q8 and Q11 collectors was modulating the driver bases up and down through D28/29. Is that not the case? What I think you're saying is that Q12/13 are modulating the connection between the bias transistors up and down thus moving the driver bases up and down while also maintaining 2v between driver bases. My description? Your description? Or maybe both?

[Enzo]

Q14,15 are one big happy bias family. Q12,13 are a push-pull arrangement that drives them around. D28,29 look to me like clamping diodes of a sort. They are there to ensure certain aspects of the amp cycling.

This is not really my expertise, but here is how I read it. Your output bus sits at zero, then either Q20 or Q21 turns on to allow positive or negative current to flow through the load. You can't have both conducting at once. If you do, it doesn;t help the load, it just makes extra current flow betwen the V+ and V- rails. SO while it is important for the signal to turn on those transistors as the waveform rises their way, it is equally important they turn OFF when the waveform swings the other way. I see D28,29 as part of that. Now I could be way off base, and those diodes are the driving force and the Q12,13 pair are doing the housework, but I don;t think so. Any latency in transistor turn off would result in current spikes and god knows what else. And in my opinion, that is why there is so much extra junk hanging on this circuit - they want to get everything they can from it without adding more heftiness, so they slather it with all manner of self-protection.

I am certainly interested in alternative analysis.


Here is another bare bones power amp circuit to ponder.

[lowell]

I read in another thread that the power amp has both voltage and current feedback, but the current feedback is AC coupled.

Is the voltage feedback via the output bus and the current feedback via C59 on the black speaker terminal? I understand that negative feedback linearizes a stage or section of an amp, but don't get what the difference between current and voltage feedback are.

[R.G.]

Voltage feedback tells the input what the output voltage is doing. Current feedback tells the input what the output current is doing.

Output voltage and current are only linearly related with a perfect resistor as a load. Speakers are far from a perfect resistor.

Voltage feedback and current feedback may both be either positive or negative in nature.

In the typical guitar amp setup, the current feedback is usually set up to increase the output voltage a bit on lower current loads - that is, positive in nature, but just enough smaller not to make the amp go unstable. This has the effect of making the power amp look like it has a lower damping factor, literally not as tight a grip on the speaker voice coil. This mimics the nature of a tube amp to deliver more power into moderately higher load impedances, and emphasize any speaker response related frequency funnies. It sounds more like a tube amp is driving the speaker, possibly, maybe, could be. That's the objective anyway.

I think... 8-)

[J M Fahey]

You can easily test the difference, which is quite audible, by building both networks and switching one or the other.
As an example, in a generic SS poweramp, 100W into 4 ohms, and considering it a "big Op Amp" (which it is), the feedback can be:
1) 10K to the "-" input, 100uF to ground (I chose a big one so it's essentially a short at any guitar frequency) through a 470 ohms resistor.
The voltage gain will be around 20 (10000/470), and absolutely independent of the speaker impedance.
The amplifier will be a perfect voltage source and the damping will be very high.
That's 99% of SS amps.
Bass will be dry and tight, highs relatively "soft".
2) 10K to the "-" input, speaker to ground through a 0.22 ohm resistor, 100uF to ground through that resistor.
The gain is again around 20 (4/0.22) so "we should be the same as before".
We are ... if the load is a resistor .... but those 4 ohms written on the speaker label are nominal, really vary a lot with frequency.
The higher impedance means higher gain.
An 8 ohm speaker can have 30 to 60 ohms at resonance (around 100Hz) and around 16 ohms at 5KHz, so you have a boost of 6dB in highs and around 15dB at resonance.
The sound will be very different: thumpier and edgier.
You can do the same with a graphic EQ, of course, but this one is free.
The amplifier behaves as a perfect current source, and the damping is very low.
This pure current feedback is very extreme, only Randall and maybe a couple more use it (you know the buzzy thumpy famous Randall sound), most others use a mixed feedback, not as bland as an old Kustom or Acoustic, not as hot as a Randall.
Some amplifiers come with that switch , a Polytone one ("Guitar"/"PA") and the Valvestate 2x40 or 2x80W rack poweramp , don't remember the label.

[lowell]

In the other thread regarding fixing this amp someone mentioned that the opamp U6 was the DC servo. Isn't a DC servo also called a Vbe multiplier? Is this not what Q14/15 are as well?

JM I'm trying to decipher your post regarding feedback. I would think that a perfect current source would provide higher damping. I definitely don't understand ss feedback loops and how that reacts to the speaker. I'll keep your post in my head for a bit and see if I can't make sense of it. Your example #1 above is DC feedback correct? Q: Your typical tube amp global negative feedback around the power amp is AC voltage feedback right?... no DC feedback because it's tapped from post-OT. ??