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I've looked at Peter Millett's circuit for this and I have a question. Why not just use a resistor, or a couple, in parallel for the plate load? If you are using this circuit you don't need any gain but you do need current which you get in spades so why the CCS?
Edits: 04/09/16Follow Ups:
The CCS plate load results in a more horizontal load line for the 6as7 which will give less harmonic distortion.
"Unfortunately, at first glance, these tubes don't look linear. In a normal resistor-loaded, grounded-cathode amplifier, they have high distortion, mostly even harmonics.
This distortion decreases with increasing plate load resistance, but a larger resistor means that you need a higher supply voltage. Luckily, there is a way to present a very high load to the plate of the tube without using a big resistor and high voltage power supply, a constant-current source (CCS) load."A large value resistor would be fine but would require a high wattage resistor and a lot more B+.
If we assume that the EL34 based CCS provides a 1meg ohm load to the 6as7's plate,
A 1meg ohm plate resistor with 50ma. of current flowing through it will drop 50,000 volts and that will produce 2500 watts of heat.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/09/16 04/09/16 04/09/16
Thanks. It was a academic question. The 6AS7 has way too much input capacitance for my taste. Just wondering. 25 watt wirewounds are relatively cheap and dropping 90v across them would not be a problem. 90 volts across a 1.2 K plate resistor. 70v plate to cathode for the 6AS7 and 20v across a 250 ohm on the cathode all @ 80 mA. It would make a hell of a buffer but rolloff in the highs would be bad.
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"
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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