I've heard people talk about the Baxandall tone stack but I've never boned up on it myself. Is that the kind of tone stack in the B15's that are mounted on the back of the pots?
The ultra- linear tap is for the screen supply b/c it's @ 40%? It's another concept that I haven't investigated.
Inre OT's I read that comment of Jess Oliver's about reconing 8 ohm speakers as 4 ohm in order to drive the power tubes to saturation sooner. Seems like you could get the same effect by running an 8 ohm speaker off the 16 ohm tap.
The tone circuit is often referred to as a Baxandall. When you put a tone control in an amp, some of the signal is lost. The baxandall control network is an active circuit that compensates somewhat for insertion loss of the tone network. A passive baxandall circuit is called a James circuit. This is what you find in the NF schematic. The purpose of the Mosfet is to help compensate for the insertion loss. The advantage of the Baxandall/James circuit is that the insertion loss is not as great as that found in other tone circuits such as the Fender stack. Because of this you probably don't need the Mosfet. On the other hand, it costs you almost nothing to try it and see what happens. That's the great thing about building a testbed amp. Once you have the basic circuit in place you can try all kinds of interesting things and see what happens. When I'm designing an amp and I'm at the stage of voicing it, I'll sit there all day clipping in and out all kind of different component values to get just the tone that I want.
The module that Ampeg used in their B15's was made by P.E.C. Six components encapsulated in plastic. It was a common component in TV's, Hi-Fi's and radio's at the time. You see them on ebay sometimes but it would be better to build your own mounted on the pots. I've heard reports that replacing the PEC module with discrete components makes the amps sound better. The component values within the old modules sometimes drift with time. I suspect that most of the time the differences are subtle. But sometimes that's all you need to make the amp sound better. The PEC modules used ceramic capacitors. These capacitors tend to have a relatively lower inductance than other types of capacitors. I don't think that this would affect the tone but you never know.
The ultralinear output configurations were used in a lot of amps. More so in hi-fi where a clean undistorted output was desired. This is a good thing in a bass amp if you don't want distortion. I used to have a 135W Fender Twin Reverb made in the 70's. I liked it a lot for guitar and just as a head for bass. Although it did distort when pushed, it was very clean and cut through well. You can see the Beatles using this model in the Let it Be roof top concert.
The output transformer acts like a multiplier that matches the impedance of the tube output to the impedance of the speaker. If the tube output impedance is 6K ohms and the speaker is 8 ohms, you would use a 6K:8 ohm transformer to properly match the impedances. If you connect a 4 ohm speaker to the 8 ohm tap (or an 8 ohm speaker to a 16 ohm tap which is the same thing) the primary side of the transformer will be cut in half to 3K. The tubes want to see 6K:8, they are presented with 3K:4. A mismatch. The amp will strain the tubes making them not last as long in putting out more power, it will distort more, and the transformer will run hotter because more current is going through it. With some amps this isn't a problem and you can get away with doing it. On other amps it leads to problems. It depends if the transformer can take the higher current. It used to work fine on the older Fender amps. In general, it isn't a good thing to do. If you try it and it works and you like the sound, no problem. That's the great thing about these old tube amps.
David
