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Re: Marshall LED's ,, Diode clipping....plate/cathode resistors..

4/18/1999 3:37 PM
David Manson
Re: Marshall LED's ,, Diode clipping....plate/cathode resistors..
Yeah.. I just replaced the led's with germaniums yesterday... its lost a little crunch... but I can almost get it to feedback at low levels... a good deal more clipping.. (still not enough distortion) hehe,, yeah i'm crazy.. -- i'm looking for resistors leading to the led's along with planning to tinker around with the cathode and plate resistors... Some people have have warned me off of this.. saying it will "imbalance" the preamp... make the bass flabby and all sorts of things... anyone tried this?? probably gonna switch out different caps as well -- anything to give it a "sweeter" tone..  
By the way.. can someone please tell me what a "cathode follower" is and "phase inverter" ??
4/19/1999 10:09 AM
David---> Diode clipping...cathode resistors
OK...This is a very odd and weird thing. So I know many of you will think I've flipped my lid and lost my senses. (where's the bullet proof vest?)  
First let me preface this by say I HATE DIODE CLIPPING ala-JCM900's and Boogies!!! There I said it. Secondly, I don't like it because it's too compressed and buzzy sounding.  
Ok, now on the "flipped-my-lid" part of this. (I feel the tention--just teasing) I tried the diode clipping as a request for a customer. They asked about Diode clipping so I tried it...hated it...ripped it out.  
Late (really late) one night I glanced around and noticed a few different ones sitting there. I removed the .68uf Cap forom the 820ohm cathode resistor on the JCM800 2203. Then I grabbed a couple of test leads and clipped a 1N4004 (silver stripe to ground) and I grabbed my guitar and waited for an UGLY sound to come out.  
The sound was distorted but not really ugly at all (and damn was I surprised!!) It did however add a bass boost to the tone. I shook my head and grabbed another diode. I tied them together (opposite ends) and clipped then on again. This tine there was more gain but the lowend wasn't boost anymore than it was previously.  
Ok...this really got my wagon rolling up hill (well, words to that effect anyway) I grabbed some 1M914's and tried one in the same place. There a gain boost but no bass boost. I tied two together and there was more gain and it boosts the signal the way a 255uF cap kind of does. As odd as this sounds (believe me I know it's sounds odd!!!) It almost boosts the gain but keeps the frequency response flatter than most caps. I know those are bold words so PLEASE, PLEASE try this and see what you think.  
That's not all fellow Ampagers! Sleep was setting in and I decided to place the pair of 1N4004's on the 820ohm cathode resistor (R9) and the pair of 1N914's in place if the .68uF cap (C1)  
It was a very interesting tone. The tone usually associated with diode clipping isn;t the same tone that this gives you. It's almost impossible to describe in words so please try it.  
The 1N914's boost the gain but do not seem to boost the frequencies like a .68uF would do. Again, it's almost a flatter frequency boost. I tried these in both positions and it did not compress the tone like you would think (God knows I thought it would!)  
The 1M4004's boosted the gain and the bass frequencies in a 330uF SORT OF way. With one 1N4004 in both positions it gave the tone the Lowend of a Boogie (I know that sounds odd) with a smoother more "even" distorted tone. With these diodes in both positions it does compress the tone but with the 1N914's it doesn't compress the tone like the 1N4004's do.  
Now these are the only diodes I have tried and there a a mess more out there. So if anyone else has tried please reply so I know I'm not the only guy who's "flipped-his-lid"  
Where was that vest again??  
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4/19/1999 12:32 PM

When you hooked two diodes together, that was back-to-back, cathodes outward? (kind of an AC clipper)  
Pretty neat discovery. You could combine this arrangement with the Ck switching selector for a wide range of tones. Sell kits.  
Better patent this one before you-know-who!  
4/19/1999 2:59 PM
DOC---> Diode clipping...cathode resistors
If it's truly original then how on earth could I patent this??  
As far as the diodes being together there's two ways I did this. Let's just use a JCM800 2203 for the example.  
#1.) I removed C1 (.68uF) and placed one 1N4004 diode in it's place. This is a black diode with a silver stripe. The silver stripe goes to ground. Again, this diode boosts the gain and the lower frequencies.  
#2.) Place another one on R9 (the 820ohm cathode resistor) Again, slap it on the same way as a cap with the silver stripe going to ground.  
This is a pretty "thick" tone and in my opinion you might need to alter a few things to get it closer to what you like, unless you love this tone.  
Here's another way;  
#1.) Remove C1 (.68uF) and place a pair of 1N914 diodes in it's place. This diode boosts the gain abut doesn't seem to attenuate any certain frequencies. It sort of acts like a 22uF (or 25uF) cap in this manner.  
#2.) Place the same diode across R9 (820 cathode resistor) It's a pretty interesting tone.  
Here's the other way to go;  
#1.) Tie two of the 1N4004's together with the silver stripes facing opposite ends of one another and place this where C1 was located.  
#2.) Tie two if the 1N914's together and place them across R9 (820ohm)  
#3.) This is optional but place a single 1N914 on the (R6) 10K cathode resistor. The 1N4004 will work if you use only one here and one in place of C1.  
Doc, try it a see what you think. I know you are pretty darn knowledgeable with this stuff. I have no means of testing these things to see what it actually does on a graph. Let me know what you think!!!  
4/19/1999 4:01 PM
Trace 1n400x diodes have more cap than signal diodes. The diode "clamps" the voltage to a max of about 1v for the 1n400x so the max current that can flow though the tube (omitting the grid) is ~1v/cathode resistor.  
4/19/1999 6:47 PM
David Manson

I'm not messing around with a jcm900 or a boogie.. - i hated the 900-- tinkering with a silver jubilee... which actually sounds pretty damn good for diodes... why does everyone hate diodes... there are quite a few good guitarists that used pedals at one time or another,,, (if it sounds good -- do it!)  
by the way.. when i put in the germaniums it did make it a little flabby.. (prob can fix it with a low value cap) you say you put another diode in series and it got rid of the flabiness?? my amp does sound kinda dull to me though,, but it does that -- even on clean.. pretty sure its the speaker-- hehe,, its a jbl -- not a celestion..  
Anyone got an answer for the "cathode follower" and "phase inverter"?
4/19/1999 7:34 PM

I think most of the amps with Diode clipping sound way too "squashed" or compressed and what most people mean is that that amp has no dymanics.  
The amp you have seems to be the only exception that I'm aware of. I've done sessions with these babies and they are ok to me. It's the wicked high gain JCM900's and the Boogies that bug most people...well me personally speaking that is.  
I don't have a schematic for the amp you have or I'd give you some suggestions. If you have a scanner, please scan it and email it to me!!  
I'd be delighted to help in any way I can.  
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