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Old 30th August 2003, 11:15 AM   #1
diyAudio Member
Join Date: Aug 2003
Location: Gothemburg
Default Vg=0 distortion

Hi I rather new to this forum and to the art of designing tube amps for guitars.

I drew a schematic for a low watt triod El84 SE-amp with a variable catode-bias for the ecc82 valve in the preamp. In between the two triodes of the ecc82 i put resistors so the second half gets as large a grid peakvoltage as the first.
My intention with the design was to get symetric distortion both against the HT-cutoff and the Vg=0.

The question is: Does the valve cut the voltage swing aginst the Vg=0. One person told me that cupling caps makes the voltage go further than Vg=0 but I didnīt realy understand what he meant. Can someone please help me understand how the valve works when crossing in to +grid voltage and explain how I can get a sharp cut in voltage swing. Does cupling caps have anything to do with it and in that case, what do they do?
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Old 30th August 2003, 02:55 PM   #2
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Join Date: May 2003
Location: Midwest
Default Cutting signal off at 0 volts

Hi Metal. Yes, the signal can go positive, with the input Z going pretty low. Depends on the driver stage how far positive the signal can go. With a 12AX7, it will almost abruptly stop. With a higher current tube, or CF, the signal can go much more positive.

One way you can stop a positive grid situation is simply put a small signal silicone diode (germanium if the PIV will withstand the negative swing) from grid to ground and the signal voltage will only go approx. +0,7 volts (+0.4 volts for germanium) on the grid.
You could supply a slightly negative voltage to the anode of the diode and have the diode stop the signal at the grid to 0 or even a little negative, say -1.0 volts, so the grid will never reach 0. The Z of the negative supply should be low and you can adjust the voltage for the cutoff point.
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Old 30th August 2003, 03:22 PM   #3
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Join Date: Aug 2003
Location: Gothemburg
Thanks a lot. Just what I wanted to know. I was a little worried about the lvl of distortion when the grid aproached Vg=0.

Now I get both sweet, warm compression at 8V-bias and more modern distortion at 4V-bias.
Does anyone have any good advise about diod-distortion. So it sound as good as it can.

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Old 30th August 2003, 11:50 PM   #4
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Join Date: Aug 2003
Location: Gothemburg
Does the cut off respond to changes in resistance in series with the diode.

=> does a large value resistor give a "softer" knee on the cut off compared to no series resistance or a low value resistor.
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