4in1 tube mic preamp SE/PP TRI/PEN

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solkatten

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The clear winner in the THD department is triode input plus PP output. The PP does a good job on canceling the H2 which dominated the distortion in SE mode...

The pentode plus SE has lower THD than triode plus SE, which I thought was a suprise. But in combination with PP the pentode gave higher THD... is there some cancelation mechanism? What do you guys make out of this?

THD at 1K 0dBu

TRI PP: 0,3%
PEN SE: 0,6%
TRI SE: 1,4%
PEN PP: 1,7%
 
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solkatten

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The lowest THD is actually "UL" 3/4 towards pentode mode, plus PP at 0,15%. The plot thickens...
 

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solkatten

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You live and you learn...

Increasing the output towards +15db didnt increase THD much, but a hotter input signal did, so it was the input stage that generated the distortion. The CF was at half the supply voltage a 5ma so it should be in a good place, the 6j32p anode was anemic at 0,9 ma and a supply voltage of 160v. Now supply voltage 200v bias at 3ma. I set it up as Morgon Jones says it should be done... I also realized I hooked up the screens pin 2 and 7 to the grid instead of the cathode... And pin 8 is now connected to cathode instead of pin 1. And now the THD looks better... and the measurments makes more sense...

The slightly hotter PP is 0.18% at 1K
SE is at 0,35%
 

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solkatten

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Because this is a coloured pre, how much THD can it crank? Pentode/SE/no feedback/+15db its 2,7% of delicious valve harmonics 😉
 

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solkatten

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Maybe we dont need the gain switch. Triode mode is clean enough and got a lot of headroom. And you have good controll over gain by the PP/SE and TRI/PEN controll. PP has 11db more gain than SE and PEN having 10db more gain than TRI... And you have no longer got interaction between gain switch and tri/pen controll...

..and I really like the openess of the sound in this preamp, whitout any global feedback..
 
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solkatten

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Now I think I am satisfied with the design. It sounds and measures good. I will remove the gain switch... I will record a couple of samples, then the preamp will do a studio tour and do some real life work... Thanxs for the feedback (y)
 

solkatten

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Updated schematics input stage
 

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