7189 in pentode mode?

[Gabevee]

7N7,

I just went over that article. I see what you mean there.

First off... it is somewhat of an op-amp inasmuch as overall voltage gain is somewhere on the order of 10,000 (the 2700 of the first two stages times the driver/power output). The first two stages (the 12AX7) is essentially a preamp (sensitivity of 28mV after NFB. Yikes! One can almost literally hook a magnetic phono cartridge directly to it! Well... save for the RIAA curve).

Most of us make power amps beginning where the 12AU7 is and apply NFB from the secondary to the 12AU7, or driver/inverter stage. As for the reasons the anode feedback is used... seems reasonable to me. I haven't done it myself. I have toyed with the idea, though. Looks to me like 6 of one and half dozen of the other.

As for the larger capacitance... I agree. But of course, I wouldn't put an 80-100µF cap right at the rectifier, but after the choke. Although, I seem to recall that in the back of the RCA tube manual c.1958 there is an amp that uses 80 µF caps in the rectifier circuit, both before and after the choke using a 5U4 (or was it a 5Y3?). So... it seems that it was available at least in the fifties.

Otherwise, that huge gain certainly would be unstable left open loop.

Also, I find it kind of amusing that he says that designing an amp with good response may lose the good qualities by use of NFB, but then goes on to design an amp with good response and low distortion before NFB.

My experience, FWIW, has been that if the design is good, NFB won't help much, if at all, except to reduce gain, increase bandwidth, and level the sweep response (making all frequencies equal in amplitude). But, I haven't gone to extremes with it either.

fdegrove,

Thanks for the link!

Gabe