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In Reply to: RE: RH84 test amp: multiple problems found ... posted by DAK on September 24, 2016 at 11:03:24
I probably shouldn't say anything but elblanko just posted a link to a great article by Corwhurst where he explains that if the grid line spacing is not even along the load line, to the left and to the right from the idle point, harmonic distortion is the result.Read the "Load line conclusions" starting at the bottom of page 2.
With the 12at7 I can not see where a operating point and load line can be placed where the plate voltage swing will be even to the left vs. to the right of the operating point. (P.S. 450vdc at 5ma. and 350vdc at 7.5ma. look OK but the tube is only rated to 300vdc plate so we can't use those operating points. We have to stay below 300vdc plate)
No matter where you operate the tube and no matter what the angle of the load line, the plate voltage will change much more when the tube is driven along the load line to the left of the idle point vs. to the right.
A linear tube has even spacing between the grid lines and it's easy to fine a operating point and a load impedance that causes the tube's plate voltage to move in a equal and symmetric way, to the right vs. to the left.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/24/16 09/24/16Follow Ups:
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.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
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'
Have Fun and Enjoy the Music
"Still Working the Problem"
In the original rh circuit one could also use tubes of similar characteristics, like 5965, or 7062, and 12ay7, but microphonics is not a problem in this circuit. I have tried all those tube types and my current favorite driver tube is the 7062. I have not check the load lines of thee tubes so i am not sure how they stack up against the 12at7.
The 12ay7 is by far the most linear tube of the bunch.Pick a spot. The datasheet says 250vdc plate at 3ma. and -4vdc bias.
Load it high for a near horizontal load line. Nice even spacing between grid lines and the plate voltage moves very close to the same going to the left vs. going to the right.
With a horizontal load line I see (very close to) the plate voltage moving 87.5 volts from -4 to -2 volts grid and (very close to) the plate voltage moving 87.5 volts from -4 to -6 volts grid. If true in practice, that means that there is (very close to) no harmonic distortion.
The plate resistance is high but I don't think that would be a problem driving a pentode (pentodes don't have much Miller capacitance to worry about driving).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/24/16
I am breaking in a RH 6l6 amp with Norelco outputs from Switzerland. They sound very nice with the 7062 and 6L6 metal tubes. very clean sound on my Altec. Although i think it is mostly the NOS 6L6 that i just started listening to today. I will plug in some 12ay7 and let you know how that fares. cheers.
I'm not sure you can just plug the 12ay7 in a circuit designed for a 7062.
But even if you could, to get the full benefit of the 12ay7 you would need to optimize the circuit for the 12ay7.
Plate voltage, plate load resistor, cathode bias resistor.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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