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Author Topic: Preamp selection woes...  (Read 14759 times)

darwindeathcat

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Re: Preamp selection woes...
« Reply #15 on: September 16, 2008, 12:51:32 PM »
hmmm... I took a look at the original princton schem (http://www.freeinfosociety.com/electronics/schemview.php?id=802) and the feedback loop does connect to the output of the output tranny. It also connects to the grid (? I think it's the grid, I'm not a tube guy) of the 6y6, which I think is what the ROG guys were copying by having the loop derive from the last FET's drain, although that would go against their logic for the tube to FET switch. Here's what the the ROG guy's say they did:
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"We took the 5F2-A schematic and copied it part for part using MPF102 JFETs in place of each tube stage. Each tube Grid was replaced by a JFETs Gate. The tube Plates were analogous to a JFET Drain. Finally, a tube Cathode was replaced with a JFETs Source. We used a 100k trimmer for the plate resistors on the schematic due the fickle nature of JFETs and the much lower power supply involved."
One interesting thing I see when I look at the original princton schem is that the NF loop is connected to both halves of the 12ax7, and the Pof tweed has it connecting just to the second analagous half (ie, the second FET). Does anyone know a practical reason for changing this? Also, it seems that while the (positive) feedback loop of the Prof Tweed connects to the source of FET #2 (second analogous half of the 12ax7), the original schem connects the NF loop to the plates of the 12ax7 (which are supposed to be the analogical drains of the FET's)... Anyone know why that would be done?
  All that being said, why then does the circuit work and even actually sound good? Not ever having heard a real fender princeton, I cannot say myself that the circuit sounds anything like it is "supposed" to sound, but I've read comments on the stompbox forum that indicate that it does actually sound similar. Is this all a happy accident?
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teemuk

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Re: Preamp selection woes...
« Reply #16 on: September 16, 2008, 02:38:01 PM »
The Princeton 5F2-A (which runoffgroove.com refers to) realizes the negative feedback loop with a 22K resistor connected from the speaker output to the cathode of the last preamp tube, which is, as said, one half of 12AX7 and feeds the power tube. That’s the only feedback loop there. Don’t mistake the power supply for feedback loop….

By the way, to get this over with…
Grid = analogous to transistor’s “base” or FETs “gate”
Cathode = analogous to transistor’s “emitter” or FETs “source”
Plate/anode = analogous to transistor’s “collector” or FETs “drain”

Professor Tweed is theoretically very similar to 5F2-A, except the important lack of output transformer, which can perform phase inversion so that signal appearing at speaker output has an opposite phase in reference to signal appearing at the preamp tube’s cathode where feedback is directed. All you need to do for that is to reverse the leads of the secondary. This makes signal flow inverting, inverting, inverting (resulting to inverting) in oppose to Prof. Tweed’s inverting, inverting (which results to non-inverting).

Anyway, amps/circuits with positive feedback tend not to be very stable and their behaviour can be somewhat unpredictable. I don’t say it’s bad engineering to use positive feedback (as it has several useful applications, such as oscillators to name one) but if you don’t know what you are doing the chances for “happy coincidences” where the circuit works impeccably are slight. I don’t think the designer of Prof. Tweed really knew what he did with that circuit.

I don’t know why it works as well as it does, though. Maybe because the amount of positive feedback is very small and the 22K resistor and 1uF cap in series make a pretty effective shunt to ground through the low 1K5 source resistance of Q2, which effectively attenuates the signal a lot. When you decrease the value of capacitance you decrease this effect, hence more gain. You could also experiment with the value of the series resistor and with a modification where feedback is inserted to the drain of Q2, instead of its source. That would be the conversion to negative feedback.

Jack1962

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Re: Preamp selection woes...
« Reply #17 on: September 16, 2008, 06:10:06 PM »
I hate to disagree with Teemuk , however there are high-voltage FET's that can directly replace certain tubes(not all). However , in Instrument Amplification positive feedback has been used, I would not recommend it for DIY's , but it is used often in production amps . Back to the circuit if your wanting a Fender Champ(tube champ) tone, you will not get it with a solid state circuit, sorry bro , but there 2 different animals, but have fun trying , I have built tons of amps , modified tons more , you can get close , but no cigar. Also the output transformer in a tube amp does 2 things 1st it takes the 2 split phases of the signal and combines them on the primary side (the side of the tubes) 2nd it matchs the impedance on the secondary(the side that hooks up to the speaks) the impedance from the tube is high(around 5k ohms) speaker impeadnce is low(4-16 ohms) transistors, and opamps (generally) have a low output impedance .

                                                Rock On

J M Fahey

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Re: Preamp selection woes...
« Reply #18 on: September 20, 2008, 01:36:28 PM »
Sometimes I can’t but wonder what the heck were the guys at runoffgroove.com thinking. (2)
Dear Teemuk, I couldn't agree more with you.
ROG's ideas are the worst "design" technique I've ever seen, and I've seen quite a few (just as you). Original (tube) circuits component values are NOT "pulled out of the blue" , but carefully calculated to get a given optimal "working point", gain, bias, signal handling and, where appropriate, distortion. None of that is considered by ROG, who probably thinks that "original" resistor values are somewhat magical and shouldn't be messed with. But they mess with everything else!!! Just as an example: a 12AX7 with 820 ohms in its cathode and 100K on its anode has a gain around 50/60, can output around 60 clean RMS volts and stands around 1,5 Vac in its grid (and will stand without dying around 100Vac there, if so supplied by a previous tube). An MPF102 or similar will be grossly underbiased by an 820 ohm resistor (it would need from 2k2 to 10k) and the gain is a meager 3 to 6. Just Proto one and see!! To make matters worse,the so called "100k bias trimmer" is a "max"value 100 k Load Resistor and never used in that value, but typically from 1k5 to 10k at best. The rest of their assumptions are similarly way off the mark. Do they sound good? Well, some do, being minimalistic Fet preamps, sometimes (easily) overdriven. Do they sound like a Princeton/Plexi/Matchless/Hiwatt, etc.? Definitely not!!!! Can Fets be used to get a good guitar sound!! Yes, emphatically, but using datasheets ,measuring them, and using normal good design techniques.

elberto

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Re: Preamp selection woes...
« Reply #19 on: January 23, 2009, 12:49:44 AM »
These are stompbox circuits guys, not tube amp replacements--or even preamps originally in spirit, it's others that have found use in applying slightly higher voltage to drive power amps.  I think the runoffgroove guys would be the first to admit that they're not trying to veritably replace tube amps (and anyway if I'm not mistaken it's Jack Orman and Doug Hammond that get credit for adapting these FET techniques to guitar circuits).

What were they thinking?  They were experimenting and came up with what many feel are great sounding circuits.  what's the big deal here?  like it or not, decades of "bad" design principles, accidents, and cheap shortcuts have shaped present ideas of good guitar tone (who at the dawn of guitar amplification could have predicted extreme distortion characteristics would become desirable, or even a singular focus of amplifier design?).  I'm honestly surprised you guys dont appreciate the spirit behind these articles, or are you "beyond" constructively adding to the discussion?

Anyway I would love to see or hear what you guys think are more well designed FET based circuits.  I read support here for marshall valvestate amps, which I think are kind of a strange choice (and I love early florida death metal!).           

teemuk

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Re: Preamp selection woes...
« Reply #20 on: January 23, 2009, 07:29:00 AM »
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I'm honestly surprised you guys dont appreciate the spirit behind these articles, or are you "beyond" constructively adding to the discussion?

We are not adding to the discussion constructively? Why not? Because we are critisising bad design practices or weird things the ROG circuits feature because their designer didn't seem to understand how the original circuit works? Did you even read the whole thread?

I like the spirit of that site's articles but I just fail to see why every FET gain stage should have a 100K drain trimmer when the world is filled with designs that work greatly without them. Even if someone wants to fine tune the drain resistor value what's wrong in calculating a trimmer value that's approximately in the correct ballbark to start with. They also fail to notice that when you fine tune the drain resistance you are also messing up with the amount of gain as well as with all the RC circuits formed in conjuction with the coupling capacitors. With those trimmers in there you have a hard time in building anything consistent.

If you want to convert tube preamps to FET preamps you actually should do more design work than just replacing all the plate loads with 100K trimmers and all the tubes with J201's.

FET's are tricky and even designers of commercial applications have troubles with them but that's where handpicking or techniques like constant current loading come in.

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I would love to see or hear what you guys think are more well designed FET based circuits. 
Well, how about Roland's Blues Cube series preamps, Gallien Krueger's G.I.V.E. variations, Randall FET-based overdrive circuits, etc.
« Last Edit: January 23, 2009, 07:32:08 AM by teemuk »