Crate / GX-40C+

[SpareRibs]

Hello,
       The reason for the two different designations is because when I was going to contact Loud, it was my understanding they would want the serial number. On the front of the amp GX40-C is silk screened, As I was
looking on the label on the back for the serial number it has GX-40C+. I was going off what it said on the front of the amp. Up until that time, I had no idea there was a discrepancy involving two numbers. Or even two
different models. Sorry if that caused a lot of problems.
        I am searching now for information involving speakers. The two in the Crate are 6" they are listed as 4ohm, but read 5.5 and 5.3. The two 12"s are 5.7 and 7.4 so I am trying to find out if that is close enough.
        I really appreciate your time and effort, Thank you very much.   

[Enzo]

Impedance cannot be measured with a meter.  The resistance of the speaker is what your meter measures.   it is usually close, but not the same.  Most times, the resistance is a little lower than the impedance, like an 8 ohm speaker measures 6-7 ohms.  But that is just a rule of thumb.

[SpareRibs]

Hello,
      That being the case could I replace the 6" ones with the 12"s. I used the meter on the 12"s to come to the conclusion they were 8ohm, they came from an organ. The ones in the amp are stamped on the back 4ohms.                                                                                                                                              I hate to ask so many stupid questions on here. I was able to do some modifications to a Magnavox
record player and convert it to an amp, also a Newcomb PA system converted to an amp. With tube stuff it
is visible on both sides and it seems it is easier to grasp the idea of a path through it all. With solid state it is hard to know where anything goes or what it does, as there is a whole lot of stuff going everywhere all at once.
      I built an Inductance adapter, Capacitance adapter, and a Gauss meter adapter to try to learn about
solid state electronics, however they were kits and it was like a paint by number set.
      There is one amplifier repair shop in this area. While the guy does a remarkable job repairing things, He
told me, He only knows how to diagnose and repair problems, He told me He doesn't know what will happen if you do any modifications to any given circuit.
       I have some electrical books that were used for military training. The library has a limited number of books on things about audio electronics. I learned a great deal about tube amps from a few books by Gerald Weber. 
       So I am trying to learn how to do things but it seems to take forever. Thank You again.

[Enzo]

A solid state amp has a minimum impedance load, you can always use a higher impedance load.   ANy solid state amp will handle an 8 ohm load.  If your amp is rated down to 4 ohms, then 8 ohms will be no problem, the amp will work only half as hard.

[Roly]

As a rule of thumb a speaker will have a DC resistance around 2/3rds of its nominal impedance (and I say "nominal" because it varies widely with frequency).  The impedance of a speaker cannot be lower than its DC resistance (so if you are reading more then 4 ohms on a nominal 4 ohm speaker you have lead or switch resistance in your meter, and should check it by measuring a known low value resistor).

This can be very handy when you are confronted with an unknown box with lebenty-leben screws holding the back on.

Yes, repair and design are fairly different.

it seems to take forever

I first got interested in electronics as a kid, pre-teen, building battery valve radios.  I'm now in my 60's and still learning.  With electronics when you decide to stop following up on stuff you don't know or fully understand, stop being curious, then you will be going backwards.

[SpareRibs]

Hello Roly,
     I have four 12" speakers. I want to put two into the Crate. One into a cabinet I am going to build as a combo for the RMS amp I modified, and I also have a Newcomb PA I converted into an amplifier. On an output plug in the back there are outputs for a 4, 8, or 16 ohm speaker. So I can finally put all of that together. It will be nice to have all of that junk organized into cabinets so at least it can all be stacked instead of laying all over the place.
     I have been really studying that Fender Squier 15 amp schematic. It is starting to make sense. It is a little easier now as the leads coming out of the triangles are part of a rectangular IC and all of the leads represent the numbers of the pins. I also see that there are kind of like loops going out and back into to a line that goes kind of straight through the circuit. The loops mold the sound with pots,caps, diodes, and a lot of resistors. The RC circuits make sense to me because they are in tube amps.
     It makes more sense now, but then I look at the Crate schematic, and that one still blows me
away. Anyway its getting a little better.
      I'll check out that meter. I think it cost about $25.00. I am going to have to buy a better one it
doesn't read high enough to go much further if I continue working on things that have higher voltage.
      Thanks for all your input. 

[J M Fahey]

the leads coming out of the triangles are part of a rectangular IC and all of the leads represent the numbers of the pins.

Ah !! but it's easier than you think 
1) each triangle "is a tube"
2) a tube's gain is defined by 2 resistors, (plate and cathode); an Op Amp's (the triangle) is also defined by 2 resistors.
3) a tube's input impedance is set by the grid resistor; an Op Amp's too, by the positive input resistor, and so on.

If anything, the Op Amp is easier , because it's a "perfect" amp, and actual results are closer to theory.

As in: an Op Amp with 100K/1K feedback resistors will have 100X gain, within a few %, while a tube stage, with 100K plate and 1K cathode will hardly reach 50 , and so on.

[SpareRibs]

Hello,
      I was wondering if in line with a speakers output circuit there is a resistor that the value could be lowered to increase output a bit more? I am also not sure if I should open a different thread to ask this question.

[Enzo]

That's easy:  No, there is not.

[SpareRibs]

Hello Enzo,
      I was working on a small RMS low output amp when I posted that question. I found by making the baffle
more efficient it increased the volume a very great deal. Thanks for your reply.

[SpareRibs]

Hello Everyone,
      OK.Today I called Loud on the phone. I got to talk to a technician, while I was on the phone with him he
searched through the computer and found the GX-40C+ schematic. Its two pages and is crystal clear. It also
has the flexwave circuit temuk made reference to. It involves diodes 3 and 4 in the gain circuit, they both seem to be different values. Would a Schottky diode be of any value anywhere in this diagram ?
       Thanks to everyone who has helped me up to this point. I went to the library and found one book I think
is going to be very helpful it is Electronic Circuits for the Evil Genius second edition, It sounded really stupid at
first, however it shows and explains about fifty components, then how to do projects utilizing them. I think it will help a lot pertaining to more complicated schematics.
        Thanks again.

[J M Fahey]

Leave as-is.
Flexwave is some 1000 times more advanced than any raw diode combination.

[SpareRibs]

Hello,
      Thanks, It looked like the diodes were already offset. I will just have to play with it more as it has 3 knobs
on Lead channel plus a push button called shift,  5 on Rhythm channel, 2 on Reverb, 2 on Chorus.
       I guess I will leave that one as is with the exception of changing to 12" speakers when I run across some
12", 4ohms.
       So overall with every ones help I now have two great sounding amps that were headed to the garbage,
and the knowledge to rescue many more I may come up on that have the same affliction.   

[teemuk]

Would 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.

[SpareRibs]

Hello teemuk,
    While I do not understand everything you said, I totally agree with not tampering with it. Another by product of this search for the schematic, I was able to download a users manual. I know that you are aware it offers a lot of instructions for using the various controls, also patch cables and such. I think with the combinations of Lead, Rhythm, Chorus, and Reverb, it will come as close to sounding like a tube amp as anything is going to get.
     I think I will be much more satisfied once I start playing with it using the manual, I will have an idea about what is supposed to do what. With twelve different knobs it was just a crap shoot fiddling with all of them and not being able to dial it in right.
     I took your suggestion about the tutorial in your post, in addition to some books from the library. I know a great deal more now when I look at a schematic, thanks to you and a lot of other people on this forum. If
you do this because you enjoy helping people, I want you to know in this case it is working very well.

                                                                   :dbtu: