Fender Squier 15

[SpareRibs]

Hello,
      OK, I have been doing research. I went back and pulled the amp apart. The diodes in the amp are as follows (D1-.617 - D2-.642.) I am thinking they are both the same value as the readings are so close.
      The diodes I have go from (.742 to .485) that is the range between 10 of them. After taking the amp apart I eliminated the larger ones as the ones in the amp seem to be 1/2watt, or at least I think. I am going to use one of the ones closest in size to the originals.
       What I would like to know now, is it your suggestion to remove one of the existing diodes and replace it with a different one ? If so I am wondering if the .742 would exceed a limit. The .742 and a .735 are the same as the ones in the amp as they are glass. The others are black material with silver markings. So would it be best to go up or down to start ?
        I have an idea that having two different value diodes will cause somewhat of a ripple in the flow
of the current and slightly distort the sound coming from the speaker. Hopefully in a good way.
        One more idea I would like get your opinion on, how about using two wires and alligator clips run to the outside of the amp to clip in different diodes without taking everything apart every time?
         Anyway Thank You in advance for any information, you may provide, and for all you have given
me to date. I can't thank you enough. 

[Enzo]

Try it this way, pull the amp chassis out of the cabinet, then run a couple clip leads back to the speaker from the existing speaker wires.  Now you can poke inside the amp all you want while it is running.

If you wonder what type each diode might be, look in the schematic.  CR3,4,5,6 say 1N4003 by them, and that is the type they are.  They don't have a value, they have a type number.  CR1 and CR2 are used as clipping diodes and are type 1N4148, says right next to them.  CR7, CR8 in the power supply are 12v zener diodes, type 1N4742.


PLEASE don't mess with any of the diodes other than CR1 and CR2.   CR7,8 set the voltage for your ICs to run on, don;t change them.  CR3-6 are simply rectifiers in the power supply.  They need to be what they are.

CR1,2 act to clip the signal when it exceeds their junction voltage.  Junction voltage is what your meter is measuring.   How large they look is not the issue.   1N4148 is a small glass diode.   There is no "limit."  You could remove them entirely, the amp would try to be louder, but wouldn;t clip until the signal outgrew the IC they serve.   I have no idea which diodes you were measuring, but the 1N4148 should have a junction drop of roughly half to three quarters of a volt.  Germanium diodes have a junction drop about half of the silicon ones, 4148s are silicon.  If you installed germanium diodes, the amp would clip even harder.  CLip at lower levels.   You can add an extra 1N4148 in series with each existing one, that will double the clip voltage, so the signal can get twice as large before clipping.   SOme amp makers use LEDs for this.  Depending upon color, LEDs have something in the 1v ro 3v range of junction drop.  Remember, and LED is mostly used for its light, but it is nonetheless a diode.


If you stuck different pairs of diodes into CR1,2 places, you might get different amounts of clipping.   If you mix types, say a 1N4148 facing one way, but an LED facing the other, then the clipping would be uneven.  You might like that or you might not, up to your ear.

[Roly]

Diode forward voltage drops, (cut-in voltage or Vgamma) depends on a number of factors, the material; the type, signal or power; the current; and the temperature, among others, but we normally generalise that a silicon diode has a forward drop of 0.7V (or 0.65V depending on who you are reading).  Schottky diodes are similar to germanium in that they have a forward drop around 0.1V.

I did a small model of this diode clipper in LTSpice, then took a spectrum of the output for a signal level of 0.1V, i.e. below the clipping level, for 1 volt, then 1 volt with a Schottky diode added in series with one of the original diodes.



At 100mV and the diodes below conduction the output consists of just the 1kHz input frequency.  There is a tiny content at 3kHz but it's 120dB down which would be totally inaudible.


Lifting the signal level to 1 volt causes both diodes to conduct on peaks, and the result is new content of odd harmonics, 3, 5, 7 ... kHz.  Now the 3kHz component is only about -12dB on the fundamental and should be clearly audible.


Finally I added a Schottky diode in series with one of the existing diodes producing a composite diode with a cut-in voltage of around 0.8V against the other with 0.7V, giving asymmetric clipping.  The comb of odd harmonics hasn't changed much, but even this slight imbalance now produces a strong comb of even harmonics, 2, 4, 6 ...kHz as well.  While they are not quite as large as the odd harmonics they are still fairly large and their contribution will again be audible.

[SpareRibs]

Try it this way, pull the amp chassis out of the cabinet, then run a couple clip leads back to the speaker from the existing speaker wires.  Now you can poke inside the amp all you want while it is running.                                                                                                                               If you wonder what type each diode might be, look in the schematic.  CR3,4,5,6 say 1N4003 by them, and that is the type they are.  They don't have a value, they have a type number.  CR1 and CR2 are used as clipping diodes and are type 1N4148, says right next to them.  CR7, CR8 in the power supply are 12v zener diodes, type 1N4742.
PLEASE don't mess with any of the diodes other than CR1 and CR2.   CR7,8 set the voltage for your ICs to run on, don;t change them.  CR3-6 are simply rectifiers in the power supply.  They need to be what they are.
CR1,2 act to clip the signal when it exceeds their junction voltage.  Junction voltage is what your meter is measuring.   How large they look is not the issue.   1N4148 is a small glass diode.   There is no "limit."  You could remove them entirely, the amp would try to be louder, but wouldn;t clip until the signal outgrew the IC they serve.   I have no idea which diodes you were measuring, but the 1N4148 should have a junction drop of roughly half to three quarters of a volt.  Germanium diodes have a junction drop about half of the silicon ones, 4148s are silicon.  If you installed germanium diodes, the amp would clip even harder.  CLip at lower levels.   You can add an extra 1N4148 in series with each existing one, that will double the clip voltage, so the signal can get twice as large before clipping.   SOme amp makers use LEDs for this.  Depending upon color, LEDs have something in the 1v ro 3v range of junction drop.  Remember, and LED is mostly used for its light, but it is nonetheless a diode.
If you stuck different pairs of diodes into CR1,2 places, you might get different amounts of clipping.   If you mix types, say a 1N4148 facing one way, but an LED facing the other, then the clipping would be uneven.  You might like that or you might not, up to your ear.

Hello Enzo,
All of your points are well taken. I will try a few things in the next few days. All within the parameters you have laid out. I guess the amp is older than the schematics posted online as I bought it I guess about 1985, the diode numbers are not on the board. The legend on the schematic calls diodes 1-4 (DS442). That may well be a substitute for 1N4148 but it does not make reference to it. I think I will try one in series to start. The graphs in Rolys post look promising and its an easy mod to start with. I will have to experiment with what I have for the time being.
Radio Shack here only sells flashlight batteries, cell phones, and coax cables. Everything else is special order. They will not be open until Monday anyway. The speaker change encouraged me to try and refine the amp, now that I know it has a little more potential than just encouraging people to junk it and buy a bigger one.
                Thank you for your input and all of the valuable information you provide for not only me but
everyone in general. Without your guidance the landfills would be overflowing with good equipment that
would probably only need a 10 cent fix.

[SpareRibs]

Hello Roly,
        I am going to add the Schotty diode in series with D2. That should be a good starting point. Since I
now know that is the area that will have the most effect on the sound the speaker conveys that is where I will focus my attention. Enzo also had some interesting ideas about changes in the same area. So armed with all of the information I can now experiment without the fear of of billowing clouds of smoke. With a
soldering iron, DMM, and an assortment of hand tools, I feel there is nothing I cannot tear up.
        Thank you again. I will probably be posting some results later today. Good or bad !!!!!!
        This will probably be me later today    :grr   :grr   :grr

[SpareRibs]

Hello Roly,
      I put a schottky diode in series with D2 as I thought that was more towards the end of the circuit. It is
exactly the sound I was hoping for. I can only try to explain it by saying, it took away the solid state brittle
sound and made it sound fuller and more resonate if that makes any sense. I cannot believe the difference
the 12" speaker and one diode made. Now I am going to do something very strange, I am going to call it complete and not make any further changes to it.
       Thank you for all your input and assistance. I only knew what I wanted I had no idea how to make it happen. This is one of the better forums on the internet for beginners in electronic modding. Thanks again.
                                                                 
                                                                     :dbtu:

[SpareRibs]

Hello,

      Thank you Enzo for your input and advice concerning my amplifier modification. It worked out great.

                                                                    :dbtu:

[SpareRibs]

Hello,

  Thank you J M Fahey for your input and advice concerning my amplifier modification. It worked out great.

                                                                    :dbtu:

[Roly]

It won't make any difference which diode you add the new diode to, either will have the same effect.

It is exactly the sound I was hoping for.

Amazing the difference a bit of 2nd harmonic distortion can make, eh?

That's what we like to hear - enjoy.   :dbtu:

[SpareRibs]

Hello,
      Yes, The second harmonic has the effect of the technique musicians sometimes use recording several takes of the same song and blending them to come up with a harmony using more than one of them. I am
in awe how simple it was.
       Now on to the next project. I have another small amp RMS-200, I was trying to find a schematic to  no avail ! TO MY SURPRISE right in the middle of the board was the same little circuit, two IN4148 diodes and a 2.7K resistor. That is very encouraging because I just happen to have four more 12" speakers and a few of the Schottky diodes. It seems to get better and better. Now it seems some my hoarded up junk is not so useless after all.
        So I am on my way to adventure, in the words of the Terminator (ALL BE BACH) !!!!!!!!
                                                               8|

   

[SpareRibs]

Hello Roly,
      Last night I added a diode to the RMS-200. I put it into the cabinet with the 12" speaker, same results.
Instant full rich sound. It is unbelievable.
      I have a Crate GX40-C, it contains two amps and I think I see the same type circuit in both, coming off of the IC's. It is comprised of two diodes a resistor and a capacitor. I am wondering if I can achieve the same effect with that one ? Anyway I will have to remove it from the cabinet to see if it has the same configuration. The schematic from Loud Technologies is not very good. The lettering is not plain when it is magnified. I'll have to search around and see if I can come up with a better one.
       Any suggestions would be greatly appreciated.Thank you in advance.
                                                                   
       

[Roly]

Be careful, amps normally have a few diodes that can do significantly other things you don't want to mess with.

The dead giveaway of a clipper in the signal path is two diodes connected together in opposite directions (and generally not too far away in the circuit from the main volume control).

Just remember, all amps are different and YMMV; some amps respond well to this sort of mod, others not so much.

[SpareRibs]

Hello,
      I think you are right about this. I have a lame schematic for the GX40-C. I cannot read any of the values, and it doesn't seem to have the preamp on it so I guess I will hold off on that until I can find a better schematic. Anyway two out of three ain't bad.

[SpareRibs]

Hello,
    Well all of the small amps are done. I think I will check with the amp tech in town and see if he has acquired any hopeless case amps. There is a Guitar Center in town so there should be a boat load of Squier 15's around. They come with the starter packs, I can imagine its not long before the bigger amp bug bites, most especially since they sound so junky new.
     I think that could be an untapped resource. The only problem would be acquiring larger speakers but I can struggle along. There is also a electrical recycling center close. That may provide some likely prospects for experimentation. Anyway on to the next hopeless looking project.
     I would like to thank each and everyone of you for all of your assistance, I couldn't have done any of these things without all of your input.

                                                                :dbtu: