QuoteClick the link then read Teemuk's first post on that thread... Click his link to download his book.

I don't intent to sound cocky but I do agree that my book is a good source of information. I'm just not convinced that it is a very good start for someone still grasping 101's of electronics.

For that purpose there is plenty of much better reference/learning material.

I assure that if someone is overwhelmed by the basic terminology then my book will only continue to overwhelm them even harder.  :lmao:

A lot of the terminology is explained in just about any book discussing analog electronics. I recommend you get yourself one, if not for learning terminology, for learning electronics in general and for reference.

A decent theory book is always more comprehensive source of information than a simple glossary/dictionary.

For the latter, try for starters:
http://www.hobbyprojects.com/dictionary/a.html

I'm sure few minutes of Google searching will find plenty of other similar sites as well.

The basic operating principle of the "Flexwave" clipping circuit is similar in just about all GX-series amps:



Zener diodes create asymmetric clipping at specific voltage thresholds. A halfwave rectifier / filter circuit gradually modifies DC offset of the circuit, resulting into dynamic variation in clipping symmetry.

Crate amps usually have two of such circuits in series. First one (or was it the second) usually implements a gain control pot. The zeners may be ordinary zeners or zener equivalents built out of transistors and resistor networks. Google search just about any Crate schematic since the invention of the patented "Flexwave" circuit and the basic arrangement is almost the same in nearly all of them.

The GX-40CDSP schematic at Elektrotanya seems to be partial and only shows page 2 of the entire two pages.

The string of two resistors at the output of each regulator, their joining node connecting the "adj" pin of their corresponding regulator, other end connecting the common.

In other words: R54 and R52 for LM317 positive voltage rail regulator, and R55 and R53 for LM337 negative voltage rail regulator.

They are called "dividers" because the voltage at their joining node divides by the ratio of the resistances. This is basically how the voltage adjustment of the regulator works.

I bet you didn't even consult the datasheet like advised. The datasheet would have clearly explained the purpose of these resistors, how they are designed/calculated accordingly and how the regulators ICs work.

www.onsemi.com/pub/Collateral/LM317-D.PDF
www.onsemi.com/pub/Collateral/LM337-D.PDF

You have reference voltages listed in the schematic. Input pin voltages should be -close- to 40VDC, both negative or positive. These seem to be ok, not exact 40 but close enough.

Output pin voltages are obviously incorrect. They should be about 15 VDC, negative and positive. Likewise "adjustment" pin voltages are incorrect: These should be about 1.25V below output voltages. This voltage potential is derived with the resistive divider from the output that serves as a feedback loop. Consult datasheets for further info.

So, given the readings I would check the condition of those resistive dividers as well as the condition of the regulator ICs.

QuoteI was thinking of replacing the filter caps.

It's not just filter caps, it's ALL electrolytic caps that have gone bad.

But I second the opinion that I wouldn't bother unless the amp is obviously showing signs of aged capacitors.

According to schematics, the switch is simple SPST momentary -type switch.

QuoteThe switch has six prongs on it but only two have wires connected to them.

So they had DPDT momentary switches on stock and for SPST function they only needed to use two terminals of them.

The switch itself hooks up to a solid-state flip flop circuit, which further on controls channel switching FETs and indicator LEDs. The momentary switch basically provides a voltage pulse that makes the flip flop circuit change its state, a non-momentary switch would be an incorrect (and probably non-working) replacement for such circuit.

Additionally, have you verified the problem is the switch, not the solid-state switching logic?

Quotedon't give a crap about some scientific transfer function as a sales point!

QuoteAnecdotes don't mean anything with out at least some comparisons to real vacuum tubes...

That's where those "scientific transfer functions" come in. Those are the only objective and universal data about tubes. If transfer curves are identical or similar then the same thing can also be expected about overall performance.

QuoteDo they sound good or not...

Is not a good example of objective and universal data. Everyone can have their own - often highly biased - opinion about that topic.

QuoteSo how the f*uck the peavey is so weaker in comparison?
I suppose it has more modern trannies or something like that...

Indeed something like that. Modern semiconductors can easily feature higher power dissipation and SOA.

QuoteIs there any way to replace the older trannies in the Peavey with more modern and efficient ones?

Yes. You research what replacement devices work, fit them in (with minor circuit modifications if neccessary), adjust, check and monitor bias, and finally monitor for flawless circuit operation (no increased ditortion, hum et cetera., no oscillation, no instability when clipping, matches designed-in output power rating, etc.)

^ As far as I know, that scheme was pioneered by both Raymond G. Anthes, Herbert W. Sullivan and Charles A. Wilkins. Two latter gentleman worked for David Bogen & Co. and the scheme is naturally also employed in some of Bogen's amps.

A little Google searching with those names should find a handful of articles, patents and whitepapers from 1950's.

The idea was using -positive- current feedback to decrease (instead of increasing) output impedance (kinda opposite what someone eventually figured out to do with solid-state amps to fake tube amps). Voltage feedback could have achieved the same goal but due to limitations of tube circuits it was difficult to apply it in ample-enough amounts.

By the way, if you weren't already familiar with it, you might want to check out the power amp design of Seymour Duncan 100W Convertible. ;-)

Current feedback is goal in -some- amps. Having MOSFET or BJT outputs doesn't neccessarily have anything to do with that.

Because current feedback is mostly used to artificially increase output impedance not all designs - like many bass amps, HiFi amps, acoustic amps, etc. - neccessarily benefit from its effects. Whether the designer sees fit to use current feedback also also largely depends on overall design.

For example,

- TubeWorks MOSvalve amps with MOSFET outputs are not configured as source followers (as usual) but common source amps. They also use very little negative feedback. In such setup the output impedance is quite high and there is very little need for additional current feedback.

- Many TDA7xxx series chips (that have MOSFET outputs, now as source followers with plenty of closed loop negative feedback) are often used in current feedback configuration ...at least in guitar amps. ...not so often in bass amps.

- Amp like Ampeg SVT3-PRO again uses MOSFET outputs, open loop. The output impedance is moderately high already.

- Randall's new G3 series power amps use MOSFET outputs, they are open loop, and the amps also fake the characteristic response of high output impedance amp with an additional fixed EQ circuit.

- Traynor DG amps use MOSFETs in class G switching. Other output devices are BJT. They employ current feedback.

- The new Quilter amps have class D outputs and they aqlso employ current feedback (at least accoring to patent).

...I think you get the point. It varies. Anyway, I'm too busy right now to think any more examples.


...So, I can't quite agree that current feedback is "not a goal" as it in many cases in fact is. However, in certain cases the designer(s) have not seen current feedback as important part of the design.

I don't really find that having anything to do with using MOSFETs, though.

Quote...somebody has to call a 100mV clipping level an abomination.

That it is. But it ain't that bad... the famous "Fetzer Valve"  ...supposedly a "clone" of the Fender input stage... already clips close to 20 mV (peak) input while the real Fender input stage can tolerate close to 2 V (peak) input signals before notable clipping. I don't even want to go to issues of frequency response, gain and bias point varying CRAZILY whenever you adjust those stupid 100k drain trimmer pots. They pretty much ensure that each "clone" will sound different from another...and an ungodly amount of field effect transistor -based "clones" are essentially based on a similar circuit.

So I guess I'm kinda saying that I agree with your trife but it's also something that will fall down to deaf ears. These things mimick circuits and for some people it's enough reason to call them "clones". IMO, the whole term is a pretty loose definition to begin with.

QuoteThis preamp is not the Fender Blackface solid-state clone it claims.

Very few clones are. Most have just a couple of similaritites with the things they mimic. For instance, the RedCircuits preamp has similar response from tonestack and distorts if overdriven. Close enough? Maybe.

Likewise, GT-2 isn't a clone of Mesas, Marshalls nor Fenders although it boasts with catch phrases like "California", "British" or "Tweed". Close enough? Many seem to feel that way.

If we start to argue how many stompboxes / preamps manage to precisely nail a specific circuit design and performance of some other unit then we can likely come to figures of about 1% or less. For sure we could exclude most of the DIY populars - a.k.a. "FET versions" - (DR. Boogey, etc.) out of discussion and stick with commercial modeling units that get right down to minuatie details.

If you want to build something moderately simple though, I feel there is no such option and you go with "close enoughs".

If you don't mind building a complete amplifier totally from scratch then yes, it's totally possible.

Practical?

If that's the issue then I'd walk to a store and buy a proper amp from the start.

^ True. But if you want to get such scheme to work effectively (about same output power to each load) then you need to scale up (or down) the power amp's supply rail voltage.

Which is in fact what many manufacturers used as impedance matching scheme for solid-state amps some years ago.

Alternatively one could just fit in a basic impedance matching transformer or autoformer to the output and call it a day. Here's one example of a product formerly made by Peavey but I'm sure that several other manufacturers offer similar products:
http://www.peavey.com/assets/literature/manuals/80370223.pdf