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Vintage threads from the first ten years

Re: rectifier diodes

9/13/1997 8:30 AM
R.G.	Re: rectifier diodes The reason that the green LED's have so notable a set of even harmonics is that the LED's Jack used happened not to be matched. Green LED's are not as easy to make as reds, and vary more widely. Jack just got a pair that weren't identical. I've played with clipping and spectra by using a pulse width modulator and a spectrum analyzer. As you vary the duty cycle, the even and odd harmonics do a dance. Just at 50%, the evens perfectly vanish. Any departure from 50% and they come back up. Dissimilar diodes is an excellent way to get some even harmonic content into a clipped signal. As I noted in the Guitar Effects FAQ, using mis-matched diodes in clippers is a great way to add some second harmonic "sweetening" to a distortion sound. Here's another one for you: as the bias point of an amplifier that is being overdriven shifts, the degree of positive versus negative clipping shifts. The old EH Hot Tubes effect (which I'll have a toner transfer package for if I can ever get the time to put it together) used this by tying the inputs of the inverter clipping sections to ground through a high value resistor. Those same resistors can be taken to a voltage other than ground. In particular, they could be taken to an envelope generator, so the degree of asymetry would change depending on the loudness of the note, with the harmonic spectrum changing depending on how hard you hit notes. So many effects, so little time...

9/13/1997 8:30 AM

R.G.

Re: rectifier diodes

The reason that the green LED's have so notable a set of even harmonics is that the LED's Jack used happened not to be matched. Green LED's are not as easy to make as reds, and vary more widely. Jack just got a pair that weren't identical.

I've played with clipping and spectra by using a pulse width modulator and a spectrum analyzer. As you vary the duty cycle, the even and odd harmonics do a dance. Just at 50%, the evens perfectly vanish. Any departure from 50% and they come back up.

Dissimilar diodes is an excellent way to get some even harmonic content into a clipped signal. As I noted in the Guitar Effects FAQ, using mis-matched diodes in clippers is a great way to add some second harmonic "sweetening" to a distortion sound.

Here's another one for you: as the bias point of an amplifier that is being overdriven shifts, the degree of positive versus negative clipping shifts. The old EH Hot Tubes effect (which I'll have a toner transfer package for if I can ever get the time to put it together) used this by tying the inputs of the inverter clipping sections to ground through a high value resistor. Those same resistors can be taken to a voltage other than ground. In particular, they could be taken to an envelope generator, so the degree of asymetry would change depending on the loudness of the note, with the harmonic spectrum changing depending on how hard you hit notes.

So many effects, so little time...

9/14/1997 8:01 AM
Jack	<<of even harmonics is that the LED's Jack used happened not to be matched. Green LED's are not as easy to make as reds, and vary more widely. Jack just got a pair that weren't identical. >>> I'm sure that probably had something to do with it. I'm thinking about doing a test on different color LEDs to see if there is any difference... anybody got a couple of blue LEDs to donate for testing? I'll probably match some LEDs and see if that makes a difference, as well. regards, Jack

9/14/1997 8:01 AM

Jack

<<of even harmonics is that the LED's Jack used happened
not to be matched. Green LED's are not as easy to make
as reds, and vary more widely. Jack just got a pair
that weren't identical. >>>

I'm sure that probably had something to do with it. I'm thinking about doing a test on different color LEDs to see if there is any difference... anybody got a couple of blue LEDs to donate for testing? I'll probably match some LEDs and see if that makes a difference, as well.

regards, Jack

9/14/1997 5:40 PM
Bruce	Jack, I mentioned this earlier.... has anyone tried using the hexFREDS yet? I'd like to hold them up a bit higher like we talked about once before with an extra pair of Ge's or ???? It seems like an interesting experiment. The ones I looked at were unfortunatly in partial parallel configuration. The cathodes are tied together and short of tieing the anodes together and using another set reverse biased, I couldn't think of an easy way to connect them to the audio circuit. I really enjoy this little spin off on the AMPAGE as I think it will be something we all will benifit from and the results are quite obvious with VERY little cash out!! Any thoughts??? Bruce

9/14/1997 5:40 PM

Bruce

Jack, I mentioned this earlier....
has anyone tried using the hexFREDS yet?
I'd like to hold them up a bit higher like
we talked about once before with an extra
pair of Ge's or ????
It seems like an interesting experiment.
The ones I looked at were unfortunatly in
partial parallel configuration.
The cathodes are tied together and short of
tieing the anodes together and using another set
reverse biased, I couldn't think of an easy
way to connect them to the audio circuit.

I really enjoy this little spin off on the
AMPAGE as I think it will be something we all
will benifit from and the results are quite
obvious with VERY little cash out!!
Any thoughts???
Bruce

9/15/1997 7:08 AM
R.G.	The reason LED's vary so widely is that they are composite semiconductors, not elemental. Silicon and germanium are semiconductors as they sit, all you need to do is dope them. To get light emissions, you have to change the conduction band gap - which changes the forward conduction voltage - up from the 0.2-0.3 of Ge and 0.5-0.7 of Si upwards a lot. Red LED's are usually gallium aluminum phosphide or gallium aluminum arsenic phosphide, which moves the conduction voltage up over 1.4V. The exact conduction voltage varies by the relative amount of each bit of gook in the mix, and so to get different colors, the mix changes. Other ways to get LEDs us gallium indium phosphide, and a stew of other materials. There is a variance within colors of about 10-50% in conduction voltage, and a variance between colors of much more than that, although the ranges overlap a lot as each maker uses a slightly different recipe. Blue LED's are yet another beast. The first ones that existed were silicon carbide, with a forward voltage of 3-4.5V; there is another variety now that makes use of impurity layers in a MOS setup in silicon that is lower voltage, but much more fragile, so there will be two major kinds of blue LED's. In all cases, the degree of overdrive will matter, as you have to have a larger signal to push the LED into conduction, and since the LED forward resistance changes as you drive it over the knee of the conduction curve, you'll get different mixtures of harmonics depending on how hard you drive each one. Yes - there are differences in sound between colors. You could rig up a test fixture to measure the forward voltage of your test diodes at low current, perhaps 100uA, and from the forward voltages predict the degree of asymetry that you will get in any given clipper. The only windage you'll find on that is the degree to which the internal junction capacitance affects the higher harmonic falloff; that will be more difficult to measure and match, althoug for a really fair test you need to match the capacitances as well. I'd be willing to bet money on the predictability of harmonics based on symmetry - or asymetry - of the clippers. The reason is that I spent about six months dealing with the spectrum of harmonics which come out of various duty cycles of rectangular waves as part of my power supply design job. It's as predictable as day and night.

9/15/1997 7:08 AM

R.G.

The reason LED's vary so widely is that they are composite semiconductors, not elemental. Silicon and germanium are semiconductors as they sit, all you need to do is dope them. To get light emissions, you have to change the conduction band gap - which changes the forward conduction voltage - up from the 0.2-0.3 of Ge and 0.5-0.7 of Si upwards a lot. Red LED's are usually gallium aluminum phosphide or gallium aluminum arsenic phosphide, which moves the conduction voltage up
over 1.4V.

The exact conduction voltage varies by the relative amount of each bit of gook in the mix, and so to get different colors, the mix changes. Other ways to get LEDs us gallium indium phosphide, and a stew of other materials.

There is a variance within colors of about 10-50% in conduction voltage, and a variance between colors of much more than that, although the ranges overlap a lot as each maker uses a slightly different recipe. Blue LED's are yet another beast. The first ones that existed were silicon carbide, with a forward voltage of 3-4.5V; there is another variety now that makes use of impurity layers in a MOS setup in silicon that is lower voltage, but much more fragile, so there will be two major kinds of blue LED's.

In all cases, the degree of overdrive will matter, as you have to have a larger signal to push the LED into conduction, and since the LED forward resistance changes as you drive it over the knee of the conduction curve, you'll get different mixtures of harmonics depending on how hard you drive each one.

Yes - there are differences in sound between colors. You could rig up a test fixture to measure the forward voltage of your test diodes at low current, perhaps 100uA, and from the forward voltages predict the degree of asymetry that you will get in any given clipper. The only windage you'll find on that is the degree to which the internal junction capacitance affects the higher harmonic falloff; that will be more difficult to measure and match, althoug for a really fair test you need to match the capacitances as well.

I'd be willing to bet money on the predictability of harmonics based on symmetry - or asymetry - of the clippers. The reason is that I spent about six months dealing with the spectrum of harmonics which come out of various duty cycles of rectangular waves as part of my power supply design job. It's as predictable as day and night.

9/12/1997 8:24 PM
PAUL C	I've seen these caps and resistors on series rect. diodes, but what about a bridge style under normal guitar amp voltages? If so, what values, and better yet - got any math you can share on the matter? Another thing I've seen but haven't got a chance to mess with is something I saw in "Tube Amp Book 4.1" (groove tubes book) There is a block diagram on a Dumble that shows some resistors in the bridge itself. What effect would these have? Here's another! Anybody dinked around with the windings on a Power Trans? I love the sound of EL34's @ 330vdc, so I was thinking of peeling back a winding or two and see if I can get an output around 240vac. I've got a hammond 270hx that I'd like to knock off about 30 volts. Anybody?

9/12/1997 8:24 PM

PAUL C

I've seen these caps and resistors on series rect. diodes, but what about a bridge style under normal guitar amp voltages? If so, what values, and better yet - got any math you can share on the matter? Another thing I've seen but haven't got a chance to mess with is something I saw in "Tube Amp Book 4.1" (groove tubes book) There is a block diagram on a Dumble that shows some resistors in the bridge itself. What effect would these have? Here's another! Anybody dinked around with the windings on a Power Trans? I love the sound of EL34's @ 330vdc, so I was thinking of peeling back a winding or two and see if I can get an output around 240vac. I've got a hammond 270hx that I'd like to knock off about 30 volts. Anybody?

9/13/1997 8:21 AM
R.G.	The old National Semi Linear Data book had a good section on snubbing rectifiers in an appendix. if you can find the book. I'll see if I can dig the copy out of the attic. Dumble: the resistors in the bridge are probably intended to simulate the forward resistance of a tube rectifier. Tranformer voltage: Unless you are already skilled at transformer winding, the chances of getting a dewind to come out correctly in a high voltage power winding are not high. If it's a CT winding, you'll have to dewind both halves an equal amount and the chances ar good that one side is buried in the middle of the windings. An easier way is to use an external bucking transformer to knock some voltage off the HV winding; alternately, 240VAC is a standard mains voltage, why not just put in a filament transformer and a separate 120 to 240 transformer and full wave bridge to get the 330VDC?

9/13/1997 8:21 AM

R.G.

The old National Semi Linear Data book had a good section on snubbing rectifiers in an appendix. if you can find the book. I'll see if I can dig the copy out of the attic.

Dumble: the resistors in the bridge are probably intended to simulate the forward resistance of a tube rectifier.

Tranformer voltage: Unless you are already skilled at transformer winding, the chances of getting a dewind to come out correctly in a high voltage power winding are not high. If it's a CT winding, you'll have to dewind both halves an equal amount and the chances ar good that one side is buried in the middle of the windings.

An easier way is to use an external bucking transformer to knock some voltage off the HV winding; alternately, 240VAC is a standard mains voltage, why not just put in a filament transformer and a separate 120 to 240 transformer and full wave bridge to get the 330VDC?

9/14/1997 8:57 PM
PAUL C	It would be easier to get a diff. trans and mount a heater trans, but what I was wanting to do is this...I've got a '71 Marshall that sounds great on my bench when I run it at 330vdc on the P.tubes. I hook up the heaters to a external supply and readjust the bias to make the amp happy. Now, I've got a trans from a 800 marshall that is the same size, so I was wanting to try to drop the secondary B+ and mount this trans in the amp to get the voltages right but without having to drill into the chassis. I also feel that I'm a little weak when it comes to trans, so I'd like to dig into these chunks of metal a bit more. Know any good books on trans?

9/14/1997 8:57 PM

PAUL C

It would be easier to get a diff. trans and mount a heater trans, but what I was wanting to do is this...I've got a '71 Marshall that sounds great on my bench when I run it at 330vdc on the P.tubes. I hook up the heaters to a external supply and readjust the bias to make the amp happy. Now, I've got a trans from a 800 marshall that is the same size, so I was wanting to try to drop the secondary B+ and mount this trans in the amp to get the voltages right but without having to drill into the chassis. I also feel that I'm a little weak when it comes to trans, so I'd like to dig into these chunks of metal a bit more. Know any good books on trans?

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