Quote:
Originally Posted by Patrick Turner
I say all this because anyone asking the original poster's question
needs to begin to ask 100 MORE QUESTIONS, and get out to his workshop
where he should become at least +20dB MORE ACTIVE with soldering iron,
voltmeters, THD measuring gear, and oscilloscope!
Only then will all answers be revealed about what likely outcomes will
be, and whether any outcomes are better or worse than doing it some
other way, and whether the criteria for citing "better way" is valid
or not.
So after several busy days I'd say any moderately intelligent person
should fill an exercise book up with carefully recorded results which
he/she can apply later when building any amp.
My 3 dracma's worth.
Patrick Turner.
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Well I guess it is time to fix my audio signal generator and provide actual circuit measurements here and as much more specific questions.
I would say at this point I have made at least a few hundred pentode circuits and performed tests on them. One thing I did do was what as suggested here, I put a 10K 12watt pot as a reostat on g2, with a 5K OPT load, with an EL84, and ran the pot up and down with another 1K 10 watt pot on the cathode.
What I found was:
1. As I increased the unbypassed resistance value on g2 this lessened the idle current of the circuit, and so had to compensate with Rk to a lower value to keep the idle current.
Obviously g2's unbypassed resistor effected the current, but the question was: Is this because of lower g2 voltage. So I added back voltage on g2 to bring it back up to the original value (without changing the resistor value), this did bring the current back up some, but not to its original value. The unbypassed g2 resistor was effecting the circuit in a static sense, to some degree.
2. As g2 was increased, Rk was reduced to maintain the same current, and voltage on g2 was increased but still within the tubes operational maximum wattage, output wattage on the dummy load resistor would increase. Because the sensitivity of the circuit and tube was effected by the unbypassed resistor more input voltage swing was required, but now the tube would obtain higher output wattages before clipping.
I will set up a circuit and do all these tests again, and measure distortion. My distortion meter only does THD, I do not have a spectrum analyzer to see what order/s are being increased or decreased by the unbypassed g2 resistor.
Also, tubes differ in the effects obtained by an unbypassed g2 resistor. Therefore I will perform measurements on a few different tubes.
Rather than start a new thread on this topic I will post here to this one as it is be relevant to this discussion here.
My findings so far are that the g2 resistor are both dynamic and static in nature, how and why is what I intend to figure out. I guess since this is "unintended" pentode operation, there are no formulas to calculate an unbypassed g2 resistors effect, from a datasheet.
Any suggestions on how to fairly set up my test circuit are welcome.
Terry