QuoteI've also read that power amps basically do NOT change the sound at all, they just amplify the sound.

This entirely depends on how the power amps in question are designed.

I can tell you for instance that power amps in amps like Vox Valvetronix series or Peavey TransTube & Vypyr series do far more than amplify sound.

Most guitar amps do at least have current feedback, which at least makes the frequency response VERY non-flat. Advanced designs like Peavey's, Pritchard's, Quiolter's, etc. not only do that but may also soft clip, compress, hum and crossover distort like many generic class-AB tube amps.

Saying that power amps only amplify and nothing else is blatantly ignoring several decades of design dedicated to emulating tube amp behaviour with SS.

QuoteI measured the transformer voltage and I got +/-32-35V wich is not that much..

Ignoring effects of loading and voltage sag that voltage range is good for an amp of about 60W - 100W to four ohms.

But that assuming the power supply can sustain that voltage when a lot of current is drawn.

QuoteThe voltage you have is close enough but you will need to check if your transformer can deliver the current needed.

This. If the current rating isn't up for the job and the voltage just drops like no tomorrow as soon as the amp is loaded then the power amp obviously has much less to work with.

Voltage rating is only one part of the equation. Current rating is the second.

http://www.schematicsforfree.com/archive/dir/Audio/Musical/PRODUCTS/ROLAND

Try if you have luck with the contents of the big zip archive titled "Roland JC-120.zip".

Quoteat this stage, because i havent done any real testing, i wont ask for any help other than advice on the whole schematic issue.......are there other ones floating around??

Yes.

If you search persistently enough you will probably find about eight to ten different schematics for different revisions of the JC-120. After circa 1982 Roland made less and less changes to the design and because of that JC-120 amps made post 1982 have a lot in common with each other. Conversely, JC-120 amps made before ca. 1982 can be almost entirely different from one another depending on their circuit revision.

Do note that at least one of the JC-120 schematics most commonly found is a schematic for the JC-120H, which is the head only version. For the combo amps this circuit diagram is entirely useless as no other JC-120 bears even slightest similarity to it. Fortunately the diagram is easy to distinct, not only from the year (1984) but also from overall features it shows the amp having as a combo JC-120 commonly lacks most of them. (e.g. the head is a single-channel circuit and has a "hi-treble" control).

So yes, do make sure you get the right service data. Wrong ones in this case are often totally useless and even misleading.

QuoteI was asked why d1 d2 were stupid. Well they are whats known as hard clipping diodes

Why is everyone overlooking the simple fact that these diodes are practically connected to the inverting input, which practically sits in 0V under normal operating conditions. These diodes do next to nothing when the amp operates normally but I suppose Fender has found out they offer somekind of transient/surge protection either for the switching FET or the opamp itself since they widely seem to be using this scheme in their amps.

But ordinary signal clippers they aren't. More akin to those diode clamps that hook up to supply voltages; operation only when things would go south otherwise.

The most funny part is that I've seen people copy these Fender circuits right down to these protection diodes but sans the FET switching, which effectively means wasting two diodes for nothing.

QuoteWhen I briged send and return loop inputs with cable, problem egzisted only to the point I have removed jack from send input and then amp come alive. From then, I plugged in and unplugged jacks to both inputs several times and amp works as it should.

The issue most likely seems to be of intermittent nature.

Corroded or dirty FX loop contacts are a common problem in that regard and most often easily cured by plugging in and out a coord few times, which scrapes of most of the corrosion and dirt from the contacts.


On the other hand, if the issue is a faulty solder joint it can portray the very same symptons and can be sometimes even cured by the same FX loop method as stress applied to the jacks and even the whole PC board might momentarily revive the intermittent contact. Needless to say, if that was the issue the fault symptoms will usually return after a while and Mr. Murphy ensures it happens right when it's not supposed to.

And of course an obvious question is: has Yamaha been contacted yet?

If they are not willing to share shcematics then it's pencil and paper time.

QuoteThe second stage has a gain of 37X and goes finishes in all out asymmetric relatively soft clipping with 20 volts peak to peak.

You do realize that inclusion of the diodes present in original Randall circuit would reduce that to about 8Vp-p or 1.2Vp-p depending on overdrive setting....

Quote...the power amp chip already clipping with and input 2v peak to peak and even after the tonestack losses theres still way too much.

I wonder why...

Ron Mancini: Op Amps For Everyone - Design Reference (Rev. B)

http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slod006b

QuoteMystery amp, not even Yamaha mentions it in its own page.

Yamaha has plenty of mystery amps in their sleeve.

Evidently they made a whole series of these hybrid amps in Italy.

Quoteif you want the Randall sound, build it original and with 24V .

This.

And the "original" clause also includes those clipping diodes and the proper feedback scheme (both voltage and current) in the power amp section.

Yes, I see plenty of resemblance.

Pro Artiste 30 was introduced in the era when Sound City brand was owned by Dallas Music Industries Ltd.

However, there's very little information of these solid-state amps so it's unknown whether Dallas MI just bought them OEM from some source (that might have manufactured the same design under a dozen different names for several companies), or if Dallas MI manufactured the design itself (and then sold it OEM for several different companies, one of them which might have owned the ELEM brand).

Guys from UK who lived through the 1970's might know if there ever was something like an ELEM music shop. Quite a many music shops had their own "inhouse" brands for their products, though they neccessarily did not manufacture those products on their own.

Should be easily restorable. I don't see anything inside that couldn't be replaced, even with modern components.

Technical information might be impossible to acquire, except by doing it yourself. The electronics are plain and simple, no gooping, no purposedly hidden component values, no microscopic SMT parts or complex multi-layered PC boards, etc. Plain and simple. Therefore sketching a circuit diagram of the problematic parts (or even of the whole thing) should be easy though it might take a while.

This is the first time I see an ELEM amplifier or even hear about such brand. As usual, the plain logo tells very little about background of that unit. Are there any other hints that could tell who actually manufactured that unit?

The size of a choke filter suitable for solid-state amp's power supply would be nearly equal to size of the PT itself. Price would likely reflect this trend.

Capacitance is much cheaper and compact and effectively achieves the same result.

QuoteWould a Schottky diode be of any value anywhere in this diagram ?

Not really. In clipping purposes the greatest advantage of Schottky is the low forward voltage... that is, if you need low forward voltage.

The Crate circuit is tuned for higher clipping voltage thresholds and uses zener diodes. Zener diode differs from a generic diode in that it's breakdown voltage when reverse biased is much lower. In fact, the breakdown voltage is equal to their rated "Zener" voltage. Forward voltage is about the same as with usual diodes.

Crate uses two Zener diode values 3.3V and 6.8V. As you may notice these are placed back-to-back in a manner that clipping threshold needs to always exceed one zenering voltage plus one ordinary forward voltage. The total clipping threshold voltage is different depending on which "side" you look at the circuit but either way the net result is asymmetric clipping due to different Zener values.

This couldn't be achieved with Schottky diodes so at least in that function they would be useless in this circuit.

Diode loading the capacitor providing dynamic DC offset could be schottky. Then again, the circuit works just as well with generic diodes with somewhat higher forward voltages. Schottky diode could be fitted there but the circuit's function still wouldn't change drastically.

So, save the Schottkys to other circuits where they might prove useful. This circuit doesn't need them in any way.