Ray,
IMO, the important parameter is gm. Since the O/P impedance is approx. 1/gm, high gm is what I look for.
Poor linearity is taken care of by the 100% local NFB. While not particularly satisfactory as a SE voltage amp, the 12AT7 makes an excellent cathode follower. Even variable mu tubes, like the 6ES8, usually regarded as unsuited to HIFI can make acceptable cathode followers.
IMO, the important parameter is gm. Since the O/P impedance is approx. 1/gm, high gm is what I look for.
Poor linearity is taken care of by the 100% local NFB. While not particularly satisfactory as a SE voltage amp, the 12AT7 makes an excellent cathode follower. Even variable mu tubes, like the 6ES8, usually regarded as unsuited to HIFI can make acceptable cathode followers.
EC8010 said:
Of course, but that doesn't imply that all devices are equal. Some show much less variation than others.
agent.5 said:
It's been said that they make a better CF because of their potentially high gm. But I have to admit that I find the problem of keeping the screen at a constant voltage WRT the cathode enough of a complication that I have not tried them.
-- Dave
Dave Cigna said:
There's no more gm than if you strap that same pentode as a triode (usually very slightly less). I assume you're thinking of some of the final valves such as E810F and E55L. As you say, keeping Vsk constant is a pain, and in the few experiments I've made, not really worth the effort beyond novelty value.
I think that is making things unnecessary complicated. As said, we "tolerate" gm variation in normal voltage amplifiers, and a CF is just the same with the input signal connected differently. Because of the inherent NFB is should basically be that much cleaner than the equivalent voltage amplifier, i.e. not the weakest link in the circuit.
I am also worried about the heater-cathode feed-through, though perhaps small. Rf tubes like the ECC81 is not designed for particular good h-k insulation, as in audio standard. I felt more comfortable raising the heaters to above ground potential; more positive than any cathode if possible. I infrequently had a small problem of this nature with r.f. pentode-triodes (e.g. ECF80).
I am also worried about the heater-cathode feed-through, though perhaps small. Rf tubes like the ECC81 is not designed for particular good h-k insulation, as in audio standard. I felt more comfortable raising the heaters to above ground potential; more positive than any cathode if possible. I infrequently had a small problem of this nature with r.f. pentode-triodes (e.g. ECF80).
Gee, thanks for the replies! Some interesting material there. I was wondering if a rather non-linear tube (like a 12AU7) might perform OK as a cf?
On the subject of pentodes as cfs, I thought the use of a suitably sized capacitor from g2 to cathode was supposed to be sufficient to give a constant g2-cathode voltage at audio frequencies, so long as the screen is fed from a sufficiently high impedance to allow the cap to do its job.
On the subject of pentodes as cfs, I thought the use of a suitably sized capacitor from g2 to cathode was supposed to be sufficient to give a constant g2-cathode voltage at audio frequencies, so long as the screen is fed from a sufficiently high impedance to allow the cap to do its job.
ray_moth said:
Yes, it does. You can plot out a loadline, and figure that you'll get a THD of under 1.0% easily. 12AU7s are also supposed to make good SRPPs as well.
It does. This is one of those cases where a single series dropping resistor to supply Vsgsg is OK. Here, you don't want good screen regulation, and one series resistor will have a higher impedance than will a voltage divider. That keeps the required coupling capacitor smaller than would otherwise be required. It's an arrangement you also see quite often in Kimmels.
AndreasS said:
Probably enough for a pre-amp stage. But maybe not if the CF is used as an output stage. Let me introduce, IMHO, a brilliant circuit using pentode CF as an output stage (page 4 of the PDF).
http://circlotron.tripod.com/a50.pdf
Man-oh-man!
Entirely not the subject, but did that EV brochure make me think back!
The Days when designs were good, yet nobody was afraid to include the circuit diagram in the instruction book. Compared to nowadays, when you have to beg and just get the reply "I am afraid we cannot reveal our circuits; others might copy our designs!" But I did get some of those circuits.....
I will censor my remarks myself before Ray_Moth does so officially.
Ray, yes, I think that part might be improved, but that is not my purpose in commenting here.
"Thanks for the memories." (Did Maurice Chevalier sing that?)
Entirely not the subject, but did that EV brochure make me think back!
The Days when designs were good, yet nobody was afraid to include the circuit diagram in the instruction book. Compared to nowadays, when you have to beg and just get the reply "I am afraid we cannot reveal our circuits; others might copy our designs!" But I did get some of those circuits.....
I will censor my remarks myself before Ray_Moth does so officially.
Ray, yes, I think that part might be improved, but that is not my purpose in commenting here.
"Thanks for the memories." (Did Maurice Chevalier sing that?)
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