Tube Amps / Music Electronics
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|4/24/1999 2:42 AM|
||Re: DOC---> Diode clipping...got the schematic..|
I've got the schem for the 2555 -- probably email it to ya tommorrow Trace..
|4/24/1999 12:35 PM|
Great! Please do and I'll do my very best to help you! I know there is a few other who also would be more than willing to help you out.
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The Ultimate Tone, Volume III by Kevin O'Connor
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|4/19/1999 10:56 PM|
||Re: DOC---> Diode clipping...cathode resistors|
Set up in this way, Trace, you are essentially providing the tube with a fixed bias source of about 0.7V, the forward Vf of the 1N4004 diode. At anything less than 0.7 Vk, the diode "dissapears" to the tube. At anything more than 0.7 Vk, the diode is a VERY low impedance conductor, which would explain the gain increase. The BASS increase can be explained by the fact that there is no frequency dependant impedance as there is with a cap bypass--i.e. no RC "break" frequency.
By wiring the diodes thusly you are simply limiting the voltage (for BOTH polarities) that can appear across the pair. The 4004's will have a higher Vf than the 914's.
Pretty interesting findings, though. I plan to impliment a diode bounding scheme on the crunch channel of the preamp I'm twisting together. You simply cannot beat the diode bounds for low noise, effective crunch!
IMO, of course.
|4/20/1999 12:44 AM|
Couldent this idea make a low noise high gain preamp,If the diode turns on at a set point than way not connect the band end to a cap.
so guitar volume down no caps no noise, volume up caps on full gain??????
|4/19/1999 10:14 PM|
||Re: David---> Diode clipping...cathode resistors|
Doc, do you think this is similar in function to Tom Mitchell's constant current sourcing method in the preamp sections of that elaborate FX loop we talked about last year?
Also, I've seen this type of thing discussed in great length by many folks including RG in FX DIY forum and over in Jack's DMZ.
Remember all the chatter about different "turn on" voltage/currents for different Si and Ge diodes, IR diodes and colored LEDs? Not to mention unbalancing them with odd numbers per side.
We use to use Ge diodes or HP Shotky diodes in product detectors... why not here too?
IR diodes were used to bias dual gate mosfets.
And let's not forget our Canadian friend(s)!
I think Kevin dedicated quite a bit of print to it too, especially the use of back to back zener diodes swamped with caps or held up with resistors to round them over a bit.
Still, lots of configurations to goof with and cheap!
But fairly old ideas, I think.
|4/20/1999 5:35 AM|
There is a tube book in the public library here in town that describes using a zener diode in the cathode circuit for bias purposes and it describes the effect on gain and also the low dynamic impedance of the diode. I can't remember the title but I'm sure I could find it again since I just looked at it this weekend. Also, I have previously described the use of an LED on the source of an FET for constant voltage bias, which has been used in Aron's original Shaka Braddah circuit. http://surf.to/stompbox/
Not saying that it's not patentable, just pointing out a couple of references.
best regards, Jack
|4/20/1999 2:59 PM|
||Re: David---> Diode clipping...2555 schematic..|
Does anyone have a scanned copy of the schematics for a marshall 2555 series -- 50 watter... (silver jubilee) -- I have a friend that has the marshall book.. It may take awhile if i go and get it from him...
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