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Peavey's steel amps

Started by teemuk, August 30, 2008, 08:48:43 AM

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teemuk

Here's few traced-out schematics for these old gems (Vegas 400 & Nashville 400).  :tu:

J M Fahey

Thanks Teemuk. Always felt curious about "steel amps" , whether they were "special" or just regular guitar amps. JM

Enzo

Without me looking them up and doing a side by side, are these circuits different from the PV published ones?

teemuk

I would know if I had the said published schematics, in which case the first post wouldn't include the ones that are traced out from the PC board prints. The mains side circuit very likely misses stuff like polarity switch and maybe some suppression caps.

Enzo

#4
Here is the factory drawing for the Nashville 400 for comparison for you.


Oh poop, the file is roo large to attach here, so here is a link to it elsewhere.

http://www.ampix.org/albums/userpics/10003/PV_Nashville_400.pdf

teemuk

Looks pretty same to me. The only differences are in the mains side and only when the amp is not an export.

phatt

Hi,
I'm new here but can I ask what U6 and associated components wrapped around the bases of the diff input do?  Yep I see it's a compressor but is U6 a Transconductance unit or similar opamp?

In my limited experience a lot of the comp stuff tends to lack something.
cheers, Phil. [Nambour Australia]

teemuk

#7
Yep, it's the Peavey's Distortion Detection Technique (DDT) compressor circuit:

An OpAmp amplifies the difference between the power amp's differential's inputs. (Theoretically, if the amplifier does not distort/clip there won't be any difference). The OpAmp's output signal then feeds a window comparator (the second OpAmp stage and components within) that triggers if the signal amplitude is high enough. Thus, a sufficient amount of distortion in the power amp will trigger the window comparator. The comparator's output then drives both another comparator/LED driver and the input signal compressing circuit.

The U6 is a basic OpAmp but U7 (or U9 depending on schematic), which I think you actually refer to is indeed an Operational Trasconductance Amplifier (OTA). See:
http://www.nutsvolts.com/media-files/11/April%202003%20Ray%20Marston%20-%20Understanding%20and%20Using%20OTA%20Op-Amps.pdf
and especially (p. 73 (p. 4))
http://www.nutsvolts.com/media-files/11/May%202003%20Ray%20Marston%20-%20Understanding%20And%20Using%20OTA%20OP-Amps.pdf
The OTA is configured as a variable resistor circuit and the output of the window comparator controls the OTA's Ibias. Whenever the comparator triggers the OTA decreases it's resistance, which then (due to voltage dividers involved) causes attenuation of the power amplifier's input signal.

edit: Some corrections to component designations.

phatt

Thankyou Teemu,
                           Alas way past my bed time now so I'll get back when I've done some reading.

I'm interested in the "How" but also looking for comments regarding the colouration [for want of a better word]
Some of these comp circuits seem to come out rather bland and I noticed you mentioned this tonal effect in your book while discussing these things.
catch up soon, Phil.

phatt

Hi again Teemu,
                       Have read the above links, some of which is a bit over my head but yes I do understand the basic principal.
                       
OK here is what I think keeps getting missed with these things.
Most if not all of these Compressor/Limiter circuits work on the basis of turning down the gain at the input "After" the signal has already passed through the Amplifier circuit.

To my mind this is very different to how a power tube stage works when it is at or near full power. "That is my main point."

So far for me at least It has proven to be rather futile trying to get SState power stages to compress in the same way that tube power stages do.
With regard to the "Clean sounds",
The only basic diff between SState powerstage and Tube powerstage  is some bandwidth limiting/colouring [partly valves and AC coupling, partly transformer] 
BUT as they get towards full power the Tube output section develops the magic mojo.
I think it is that part that I'm wishing to recreate/emulate.
[As I'm sure you know, SState power stages just keep on going up untill they run into the rail voltage, sonically not much changes.]

I'm the first to admit that with some modern music I don't know if you even need power tube compression effect as there is so much preamp crunch/fuzz or just plain noise that I doubt if one could hear the difference between tube or SS.
Quite frankly a lot of modern tube amps ALSO have so much preamp gain stages that it becomes almost impossible to attain that magic sweet point of the output tubes compressing and it's just fuzz to my ears.

Vox Valtronic sounded like a good idea until I realised that the gain curve of a triode is vastly different than a pentode. I think that one will ba a passing fad,, but then some rave about it.
Anyway that's my 2 cents worth.   Cheers, Phil Nambour Australia.

teemuk

I think Peavey never devised that compressor circuit to emulate the "soft saturation" behaviour of valves. After all, it was originally conceived for their PA amps. It's simply just a means of clipping prevention and a protection against the effects of severily overdriving the output stage – which e.g. may destroy hi-frequency speaker elements.

I don't think it's futile trying to get SS power amps to compress like tube amps but in whole the topology of the conventional SS circuits does not lend itself very well to the same saturation mechanics that exist in "conventional" tube guitar amps. SS amps have plenty of open loop gain and negative feedback to linearise the circuit. These mandatorily make the transition to clipping very sharp and the clipping thus very harsh one. Of course one can put non-linear elements to the feedback loop (e.g. Trace Elliott) or use an input stage with soft clipping characteristics (e.g. GMT, Yamaha, Trace Elliot) but these are just band aids to turn the inherent behaviour of the topology into something different.

But, there are also examples of SS amplifiers that use less open loop gain and less feedback and these tend to have a softer transition to saturation. If you make the input stage and voltage amplifier soft clipping and then use very little NFB to correct it (almost mandatory if you transformer couple the output like e.g. some Trace Elliot and GMT amps did) the outcome is pretty "tube-like", soft transition into full clipping. However, such circuits are not as efficient (you loose a lot of "clean" output power because the signal swing has a larger non-linear area before total saturation takes place). Not to mention amplifiers such as these are quite unorthodox to design. All in all, they aren't exactly a very popular choice for a power amp.

I think the idea of using linear power amplifiers that amplify the signal from a softly clipping low-current "preamp" stage is a good one. It's likely even a better choice than trying to develop a high power power amp circuit that deliberately performs "poorly". At least, the circuitry is easier to design that way. Coupled to current feedback from the output of the power amp such stages can react very similarly as a power amplifier (with high output Z) driving a complex impedance of a speaker. These circuits aren't very old and refined yet, though. Peavey's T-Dynamics and Vox Valvetronix are pretty much the only examples that come to my mind. I'm pretty sure we'll see more of these in the future, though unless switching and DSP systems take over. Given few more years, I think these things may begin to perform very realistically.

Although I think that Valvetronix is a step into right direction in the various ways to create hybrid circuits I agree about your opinion of it in general. In my experience, at least in the second and third generation Valvetronix amps the tube stage has very little difference in operation to a similar circuit that would have been built with FETs. For example, I can get much softer clipping (that resembles more the clipping of a tube power amp) by overdriving a differential FET stage and summing the differential's drain signals with a differential OpAmp stage. If you even limit the roughest peaks of this differential amp you can create a very smoothly performing circuit.

How "tubey" the output signal from such circuit it is, is somewhat questionable, though, since I haven't seen many tube amplifiers that would have had completely identical clipping characteristics either. The way how they clip has as much to do with the surrounding circuitry as with the types of the tubes. There simply isn't a single way how a tube amp sounds and clips but hundreds, maybe thousands of variations. The clipping of a, say, ultralinear circuit using a lot of negative feedback both locally and globally is not amongst the softest ones. Of course, most modern guitar amps didn't evolve towards this direction, which is a very important point to consider when judging the differences of amps. Many Hifi tube amps did, though (I'm not talking about those low-power SE "high end" amps but about the "true" tube hifi amps). Also, about 99.9% of SS power amps evolved towards extracting maximum power and linearity and this pretty much converts to amps that have a sudden transition to clipping. I think its somewhat unfair to judge how they perform since they perform greatly in what they were devised to do. No-one ever designed them deliberately to perform poorly - and that's what having a large area of non-linear amplification means in most SS designs (e.g. Hifi amps, PA amps etc.) Guitar amps are a small execption and in overall they don't receive a lot of interest. Perhaps we need to see the generation of SS amps designed from a whole another point of view....