An illustrated guide to building an F5

[AndrewT]

Those two diagrams show different styles of cascodes.
the upper it titled:
FET - Bipolaire
The lower is not titled but to me seems like jFET - jFET

But your titles read as the same, jfet/transistor & jfet/jfet.

[triode_al]

Of course the text should be "The jfet+bipolar transistor cascode on the top" [of the illustration].
for me a transistor is a bipolar NPN or PNP device; I would never call a jfet a transistor. Just my nomenclature.

This is the input of the described amplifier of Hiraga [without the feedback], just to see the overview of the input of that reference:
monster27fig8b input.gif
albert

[Eric]

6L6 : I guess I'm a bit late to the party, but you have an EXCELLENT build and this is a GREAT thread! Thank you!

I also love the Musical Fidelity chassis re-do! I was wondering where those great sinks came from!

All of this makes me want to build an F5, but I'm not sure what I'd do with it once it was completed. I already have a trio of Aleph-X amps and an A40. I use the A40 in my office at work and just love it! I have contemplated building an F5 and a DCB1 into the same chassis for kicks...

Great work, all!
Eric

[AudioSan]

i'm starting a F5 build now to

[AndrewT]

Originally Posted by triode_al

....the text should be "The jfet+bipolar transistor

Then relabel the diagram jfet/bjt.
This way no one should be confused.

[triode_al]

Originally Posted by AndrewT

Then relabel the diagram jfet/bjt.
This way no one should be confused.

Done; thanks

[tpark]

triode_al
Do you have any data about the distortion when you bias the cascode from the source of the fet rather than ground as you mentioned earlier. If this voltage can move from +/-4 volts as you shown, biased at the source pin of the fet would keep the drain voltage at a fixed level presumably linearizing the gain. Perhaps some would remember how Borbely did it in his later all-fet line amp ( AudioXpress, May 2002 ) with K246 and J103 cascoding the K170 and J74 using self-bias. I don't know how that sounded as I never built that amp. I could never get his dual differential designs to equal a good triode stage.

[triode_al]

Originally Posted by tpark

triode_al
Do you have any data about the distortion when you bias the cascode from the source of the fet rather than ground as you mentioned earlier. If this voltage can move from +/-4 volts as you shown, biased at the source pin of the fet would keep the drain voltage at a fixed level presumably linearizing the gain.

Perhaps some would remember how Borbely did it in his later all-fet line amp ( AudioXpress, May 2002 ) with K246 and J103 cascoding the K170 and J74 using self-bias. I don't know how that sounded as I never built that amp. I could never get his dual differential designs to equal a good triode stage.

tpark, This is getting quite off track probably.
- sorry I only have the degradation curves shown earlier and not with a floating Vsource of 8 V pp. Would be interesting. However, it does not work to measure with a closed loop (a running amp). But looking at the curves we see the bad effect of the floating source:

2sk170 unlineair region.gif
The jfet is 'forced to bump around'.

Well of course we keep 15-20 dB away from the worst case situation, so in practice . . . we are all happy anyway; and some like the ensuing distortion when driven hard.

I remember Borelby connecting the sources together, so Vds of the bottom unit stays constant.
albert

[tpark]

I am sorry to have started this topic and I realize the cascode thing was off-topic but we can always learn something. On rechecking my wiring I found I had the diode on the plus side in backwards and therefore fortunately for me this blocked the zener chain and basically shut off the pass device therefore the 4 volts. My bad. The channel is cooking as I type this and seems stable. I have however gone to attaching the base of my reference zener to the source of the bottom fet and will see how it sounds when I finish the amp.

[biju55]

Congratulations !!! Very nice amp!