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In Reply to: RE: here we go again....... posted by Palustris on August 19, 2016 at 11:33:16
While I'm not against CCS's as plate loads in general, I can't see the benefit in this application. MOSFET's at such low currents exhibit very low Gm and even though putting them in cascode multiplies their Gm, it's still too low for all practical purposes. BJT's would offer better performance here I think and a gyrator even better. In any case, why not just use a plain ole resistor as the load? Don't get me wrong, I'm not saying that this circuit doesn't sound very good (I'm sure it does), only that it might also sound very good with resistive loading while reducing complexity.
However, the biggest issue here IMO, is that the proposed amp using the 71A as the final, will be very low in power. The output tube will be running at a plate dissipation of .02x180=3.6W and If we assume a plate efficiency of about 20% it will have an output power of 0.7 watt. What's the sensitivity of the OP's speakers?
Follow Ups:
I agree that there are many ways to get the sound one is searching for. While the CCS I chose may not be ideal, the proof is in the pudding. I tried several alternatives and this approach was marginally cleaner and clearer than the alternative approaches. Maybe it is the 6K5, maybe the CCS, I can't really say other than it is slightly and noticeably cleaner than the other topologies I tried. Why would a 6K5, a low transconductance tube from the 1930s, sound cleaner than a state of the tube art high transconductance Mullard E180F (triode connected) from the 1960s?
As an aside: I use the K&K CCS because I tried wiring up MOSFETs on perf board but was frustrated with the perf board, small parts and solder "wiskers" and I really couldn't see well enough to ensure I was soldering what I thought I was soldering. The K&K CCS board and Pete Millet's A2 buffer make using solid state devices easy. I know I can experiment with different devices like the BJTs on these boards, but for now I am using them as designed because they sound excellent. I have plenty of alternative SS devices for my next amp using the 6F5 and will experiment some more.
As always the proof is in the pudding as the saying goes and results trump theory every time. I also don't like to fuss with perf boards and small parts and so I understand the appeal of ready-made SS boards. This convenience along with your stated performance gains, I can see the appeal. Perhaps I'll try it myself in the future.
They are Altec Granadas and are about 91 db I think, maybe a little better. I have run them satisfactorily with quite a few flea power amps. Not window shakers, but then I don't like loud music anyway, just good music.
I'll agree and say that I would take quality over quantity 99% of the time. However, in this situation you might be pushing it a little bit with a low 90's speaker and less than 1 watt of available power. Not saying that it's not going to work at all, but you will be limited IMO.
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