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previous: GFR Re: MesaBoogie OT's -- 9/18/1998 5:51 AM

Can you explain Simul-Class?

9/22/1998 11:47 PM
Steve A.	Can you explain Simul-Class? GFR: I keep looking at the schematic for the MkIIC+ and I don't understand how V8 & 9 can be Class A since V6 & 7 are Class A/B and they share the same bias feed and input. V8 & 9 appear to be wired as triodes- so how do they get away with omitting the 470 or 1k resistor between the plate and the screen? I can't quite make out the values of the grid resistors used in the IIC+ (or III), but it seems like not much of the signal would be getting to the grids of V8 & 9. And with the cathodes to V6 & 7 not connected to ground for the "Class A" mode, the B+ goes through the 470 ohm screen resistors to the screens of V6 & 7 to the plates of V6 & 7 and onto the ultra-linear taps of the OT, the regular taps of which go to the jumpered screens and plates of V8 & 9. A lot of this goes counter to intuition (or at least my intuition). For starters I'd think that the with the cathodes of V6 & 7 not connected to ground that there would be no current from the screen to the plate (since you can disconnect the cathodes of power tubes as a Stndby switch...) but if that were the case then there would be no B+ going to V8 & 9. I'd also think that the grid resistors to V8 & 9 (820k?) would greatly attentuate the input signal, but I guess it also lowers the bias voltage ~11v which would increase the bias current enough to push it up into quasi-Class A operation? Confused? You bet I am! It's enough of a challenge just figuring out plain old Fender and Marshall designs without having to deal with the curveballs that R. Smith keeps throwing at us... Steve Ahola

9/22/1998 11:47 PM

Steve A.

Can you explain Simul-Class?

GFR:

    I keep looking at the schematic for the MkIIC+ and I don't understand how V8 & 9 can be Class A since V6 & 7 are Class A/B and they share the same bias feed and input. V8 & 9 appear to be wired as triodes- so how do they get away with omitting the 470 or 1k resistor between the plate and the screen?

    I can't quite make out the values of the grid resistors used in the IIC+ (or III), but it seems like not much of the signal would be getting to the grids of V8 & 9. And with the cathodes to V6 & 7 not connected to ground for the "Class A" mode, the B+ goes through the 470 ohm screen resistors to the screens of V6 & 7 to the plates of V6 & 7 and onto the ultra-linear taps of the OT, the regular taps of which go to the jumpered screens and plates of V8 & 9.

    A lot of this goes counter to intuition (or at least my intuition). For starters I'd think that the with the cathodes of V6 & 7 not connected to ground that there would be no current from the screen to the plate (since you can disconnect the cathodes of power tubes as a Stndby switch...) but if that were the case then there would be no B+ going to V8 & 9. I'd also think that the grid resistors to V8 & 9 (820k?) would greatly attentuate the input signal, but I guess it also lowers the bias voltage ~11v which would increase the bias current enough to push it up into quasi-Class A operation?

    Confused? You bet I am! It's enough of a challenge just figuring out plain old Fender and Marshall designs without having to deal with the curveballs that R. Smith keeps throwing at us...

Steve Ahola

Replies:

Dave H. q{I keep looking at the schematic f... -- 9/23/1998 5:51 AM
GVB You can check out the patents that ... -- 9/23/1998 7:37 PM