F6 Amplifier

[Nelson Pass]

Originally Posted by lhquam

Based on their location it looks like there are two 3 watt source resistors per JFET and one 3 watt resistor for each feedback loop.

You will want to recall in the talk that this photo was taken early in the process...

[flocchini]

I am still trying to understand the effect of the output resistors

I went back to the .1ohm/10ohm pot combo and noticed the following when I run the pot through its 15 turns. The resistance on my DMM starts at .3 ohms rises to 3ohms and then falls to .4 ohms. It does this whether I am measuring from wiper to either side of pot. I understand that my DMM does not measure milliohms but I did not expect this behavior. I am assuming it is some kind of artifact.

I think somebody mentioned earlier in this thread using rotary switches to load the source of the output JFet. More of a pain to put together but for each JFet would a rotary switch with 0 ohms, 2x .025 ohms in parallel,.025 ohms, 1x .025 and 1x .1 ohms in parallel, .05 ohms, .025 ohms +.05 ohms in series, and .1 ohms. This would give a switch with 0, .0125, .025, .033, .05, .075 and 1 ohms.
Overkill? At least I would know load resistance.

Thoughts

Thanks

Bob

[buzzforb]

I will probably botch this, but the pot can only minimize the effect of the Rs, not increase it.

[Antoinel]

Originally Posted by Nelson Pass

You will want to recall in the talk that this photo was taken early in the process...

The pictures of the device in the 6moons article gave the impression it was a prototype. No power switch on the front panel, it sat on an active workbench, and the wiring at the rear panel appeared to be work or testing in progress.

[Zen Mod]

Originally Posted by flocchini

I am still trying to understand the effect of the output resistors

I went back to the .1ohm/10ohm pot combo and noticed the following when I run the pot through its 15 turns......

Bob

that's logical

but - do not forget that ZAP is voltage divider there , so , you're having 0-100% voltage for degeneration purpose , of whatever source resistance it is

[flocchini]

from buzzforb
I will probably botch this, but the pot can only minimize the effect of the Rs, not increase it.

from ZenMod
but - do not forget that ZAP is voltage divider there , so , you're having 0-100% voltage for degeneration purpose , of whatever source resistance it is

You guys are, of course, right. For .1 ohms it should be 0 to .099xx or .1ohms.
Probably should just accept I can't measure it with my equipment. Maybe its time to bight the bullet and learn how to measure distortion. A number of you have mentioned Electroacoustics Toolbox and they do have a demo version.

Best

[Tea-Bag]

Originally Posted by lhquam

Here is an LTSpice simulation of the F6 using IRFP240 fets, and "tuning the Zen pot to keep the ratio H2/H3 approximately constant. I estimate the transconductance of the IRFP at 1.4A bias to be 3.5S.

This looks pretty good. I need to build it and see how it measures and sounds.

Has anyone thought out the value of R1 to keep relative gain at 15db if using IRFP240's? I am prototyping this, but uncertain of a formula that can help.

My apologies if this has been mentioned already. I assume tweaking this and the degeneration resistors will get this working on point. I plan on using for source resistors 1R to start with the 10R Zen Pots.

[flocchini]

Can the IRF240s and the FQA16n25C)s be interchanged?
Thanks

[buzzforb]

They will need degeneration. Like th eIRF 240's. This assumes that the bias circuit is setup to provide enough voltage. From there it is all on you tweaking the circuit.

[Antoinel]

lhquam: You may wish to operate Teaser-6 in Class AB , characterize its performance, and compare it with Class A operation. You'll have the making of an article for publication which will augment the knowledge in those by Mr. Pass.