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In Reply to: RE: RH84 test amp: multiple problems found ... posted by Tre' on September 24, 2016 at 11:50:44
What you are seeing is 2nd order. The 12at7 family does have different gain at different grid voltages. In a push pull amp this can actually be used to give a reduction in higher order distortion. You will end up will a little higher overall THD but a better sounding amp.
The 7026 is kinda in the 12at7 family.
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FWIW, the 3rd harmonic in the driver stage is 12 - 15dB below the 2nd harmonic when the overall power output is measured at 1.2W, there is some variation between 12AT7 triode sections.The feedback is probably cancelling out some of the 2nd harmonic.
The level of 3rd harmonic at the output of the circuit is only 2 -5dB below the 2nd harmonic.
Edit: with power output of the circuit @ 1.2W, the driver stage THD measured between 7.5 - 8.25% (there was a difference between 12AT7 triode sections).
THD varies a lot with signal level.
Edits: 09/27/16
I was not intending to speak to the distortion of the amplifier as a whole.
I was just trying to point out that a 12at7 is not a very linear tube.
BTW Keep up the good work and reporting. It's very interesting and informative.
Tre'
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