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13th December 2006, 04:40 PM  #1 
diyAudio Member
Join Date: May 2004
Location: Toronto, ON

Q on voltage swing
I have come to realize that I think I was mistaken on the capability of a tube being able to provide voltage swing as being a product of its mu. It really is defined by how spread out the curves are in the standard tube curves correct? And how far you can go before hitting the 0V and other constraints (to put it simple)?
So even though the 12AX7 has a high mu it can't swing a lot of voltage because its curves are bunched together, is that right? What has higher voltage swing potential, the 5687 or the 6SN7? Thanks, Josh 
13th December 2006, 05:15 PM  #2 
diyAudio Member
Join Date: Oct 2002
Location: Brooklyn, NY

The curve spacing can be changed just by changing the scale of the graph.
Relative to its bias and plate voltages in its linear range, the 12AX7 does have a good output swing! It's not fair to compare it a 6SN7. Very different tubes. 
13th December 2006, 05:25 PM  #3  
diyAudio Member
Join Date: May 2004
Location: Toronto, ON

Quote:
I was assuming fixed scale. And I am not comparing the 12ax7 to the 6SN7, rather the 5687 v 6sn7. I was using the 12ax7 as an example to try to illustrate my point, which I am still not so sure of. 

13th December 2006, 06:57 PM  #4 
diyAudio Member
Join Date: Jun 2002
Location: Macedon NY

Most any tube can swing the voltage from supply voltage (nearly off) to nearly ground (saturated)...but... it may not be very linear doing so... Look at the relative spacing of the curves  if the spacing is different for each voltage step, then the transfer characteristic is not linear. If the curves are evenly spaced, it's linear. In that region, distortion will be low.

13th December 2006, 07:11 PM  #5 
diyAudio Member
Join Date: Jun 2005
Location: San Diego, CA

OK JoshK.....
As you mentioned "fixed scale" ...lets assume 1v bias spacing for our fixed scale, this way we have a basis for comparison... Now by definition, mu is related to the spacing between the bias curves for a FIXED CURRENT.... This means you draw a HORIZONTAL line across the curves which Yintercepts the current you are testing at... This is a partial differential equation, since you are looking at the change in Plate Voltage for a change in Bias Voltage...with a fixed plate current.... So your original question is about potential AC voltage swing.... Most all small signal triodes have the same swing, they can swing from rail, durring cuttoff, to close to 0 volts, when at 0bias.... I believe what you are really asking is what tube has the most AC swing for the SAME INPUT VOLTAGE SWING....... Lets say your fixed input is 1V AC rms ....... Now looking at a bunch of different tubes...the tube that will give you the greatest plate voltage swing for that 1V AC input will be the tube with the largest mu, or largest bias curve spacing, to our agreed 1V bias spacing as agreed upon earlier...... There are other considerations with a triode voltage gain stage....the plate resistance and plate load are important as well.. The basic first order gain equation for a triode stage is (mu*Rp)/(Rp+rp) ...... Rp= plate Load, rp= plate resistance... What gets interesting is when you replace the plate load resistor with an inductor.... The midband gain becomes mu ..... Chris 
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