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Re: David---> Diode clipping...cathode resistors


 
4/21/1999 10:17 PM
Steve A.
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Re: David---> Diode clipping...cathode resistors
Trace:  
 
    I wasn't even going to read this thread because I HATE DIODE CLIPPING TOO! But what you've been doing is quite different because they are in the cathode circuit, and not exactly in the audio path (well, if they ARE in the audio path, at least they are buffered from the signal by the tube!)  
 
    Have you tried those diodes on other amps, too? You might want to measure the diodes you are using with your DMM to see if there is any correlation between the measurements and the resulting sound. (1N400X and 1N914 diodes are fairly common rectifiers- right? I wonder what an 1N4148 high-speed switching diode would sound like... Or a zener diode?) Have you tried the diodes on other stages beside the initial gain stage?  
 
Steve Ahola  
 
P.S. Other posts indicate that your discovery is not entirely without precedent, but what the heck- I doubt if many people have been using that trick in their amps these days (and if they are they are certainly keeping it under their hat...) I will definitely have to try it out, maybe on a rotary Ck switch (since 5 or 6 values for Ck is really pushing it).
 
4/22/1999 7:20 AM
Jack Orman
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There is an application note by TEMIC on FET biasing and almost all of the information in it applies generally to vacuum tubes as well. It is extremely useful to have on file.  
 
As I see it, using an LED on a cathode (or source) is constant voltage biasing and using one of the TCR508s is constant current biasing (of course). An FET connected gate to source as a constant current diode could be used on a preamp stage instead of the TCR5xx... a J201 is around 1 - 2 ma. The 2N5457 is slightly less I believe.  
 
http://www.temic.com/  
 
regards, Jack
 
 
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4/23/1999 6:45 PM
Trace
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STEVE A...DIODES.
Steve;  
 
Yes, I've tried these ALOT in a ton of amps and it works in a mess of amps! I've used it on the first stage as well with fantastic results recently.  
 
It's strange how it really does sound good BUT the diodes you use make a big difference in the tone. Again, try a few different ones Steve. I didn't car efor the Zeners but you may like it.  
 
I found the two 1N914's twisted together sounds pretty hip and I like the 1N9004 as well. The 1N9001 doesn't seem to effect the bass as much as the 1N9004 does. These two do add some interesting "beefy'ness" to the tone but it's hard to put into words.  
 
Yes, these are pretty common. Radio Shack carries these and they are mighty cheap indeed!!!  
 
Trace
 
4/24/1999 1:37 PM
Trace
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DIODES ON PLATES
Hey all;  
 
I know someone was asking about trying diodes on the plates of the preamp tubes. So here's another of my silly findings! (ha, ha)  
 
I used several diodes but the 1N9004 and the 1N9001 seemed to work the best for plates. It gives the tone a slight boost that is similar to a cap on a cathode but in a different sort of way. I placed the "positive" side on the tubes pin.  
 
It also gives you a "top-end" boost that is very much useful without ever sounding "harsh" to the ears. It makes a nice mod, if it's wired to a switch, when you switch between a Les Paul and a Strat. It also gives a different "feel" to the amp and makes the tone a bit "bolder" while still adding a nice "top-end." It almost makes the tone a bit "wider" but in a very subtle sort of way.  
 
At lower volume levels it can be heard but it's more effective as the preamp's volume is increased (assuming it's a master volume amp, if it's a non-master then you'll hear more of a difference when simply turning the volume up.)  
 
It didn't make any of the amps I've installed this on unstable in any way and I cranked all the amps wide open.  
 
So, those are my findings! (ha, ha) See what you all think and feel free to comment is it's good or bad!!  
 
:-)  
Trace
 
4/24/1999 5:37 PM
Steve A.
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Trace:  
 
    So you run the diodes in parallel with the plate resistors (with the stripe on the tube pin end)? Have you tried doubling up the resistors as you have done on the cathode?  
 
    Speaking of diodes on the cathodes, I just found the factory Boogie Mark I schematic in my e-mail box last night, and guess what... there are diodes on the cathodes for V1A and V1B! (They were using the optional FETRON on this amp as well as the Mk II so that might have something to do with it- perhaps the diode would set the correct bias for both a glass 12AX7 and a FETRON- as a "constant voltage source"?)  
 
http://www.techaccessinc.com/blueguitar/marki.jpg" TARGET="_top">http://www.techaccessinc.com/blueguitar/marki.jpg  
 
Steve Ahola
 
4/25/1999 12:50 AM
Trace
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Steve;  
 
Actually I placed the stripe on the other side of the resistor. So make sure you don't place the stripe on the tubes pin.  
 
I have doubled up the resistors and it's more gain but it depends on what tone yuo are goign for really. it does work though and doesn't make the amp freak out.  
 
Sooooo...Boogie did use that huh!! (ha, ha) it's not surprising. I figured it had been done before. I think I'll try implementing my kithen blender in there some how. (just teasing...it's 3:00am now)  
 
Steve...again, different diodes will effect the tone differently do try a few and see what you think!!  
 
Trace
 
4/26/1999 7:32 PM
David Manson
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Diode clipping // Marshall Amps...
Well last night I changed out a few resistors and whatnot.. the resistors seem to have little effect on the sound -- very minute changes when you bypass or change their value... (the ones in the tone section and in front of the diode clippers were the ones i was messin with) I did remove some of the clipping diodes and it made a big difference in the sound... it kinda weakened the volume but it made the sound a lot smoother,, not so much crunch.. I figure if i put in a larger value pot in the treble control or maybe some different caps -- i can probably get some crunch back again.. I only have some resistors and diodes to tinker around with,, I'm gonna get some mylar caps tommorrow so I can finally really do some work on it.. otherwise the amp is sounding way better than it used to,,,
 
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