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In Reply to: RE: 6as7 "preamp". posted by gavinhaley on April 09, 2016 at 13:22:43
A 6as7 with a 1.2k plate resistor at 70 volts plate to cathode?
The harmonic distortion would be so high, I'm not sure at that point the roll off would be noticed. :-)
As Pete pointed out in the article, these are not linear tubes. The only way to get the HD down to something acceptable is to load them with a very high impedance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
Im not really all that good with AC analysis. I know how to bias tubes and to avoid high capacitance. Just enough to be dangerous. If I were building a 6AS7 amplifier isn't 1.2K the load impedance I would choose for the output transformer? Where is the difference?
If you're building a power amplifier the load impedance has to be lower than in a voltage amplifier, otherwise no work would be done (the amp wouldn't have much power).Again, the 6as7 is not a very linear tube. It will not give much useful power no matter how you load it unless you're willing to accept a high amount of harmonic distortion.
Pete is saying that it will give some useful voltage (at a low output impedance with plenty of current capability) for use as a preamp but only if you load it into a really high impedance.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/09/16
I wonder how a plate choke would work? You would have to cut back the current substantially so you would be in a even more non- linear territory. Probably not worth the money.
A plate choke would split the difference - it would be a higher Z than a resistor, but a lower Z than a CCS.
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