What is the driver circuit here?

[mskl99]

I came across this old schematic, I think from a Williamson type amplifier. Can someone tell me what the name of that S6N7 driver stage feeding the KT-66 grids is. It looks like a push-pull configuration, but the bottom of R12 just seems to be hanging there. Does it have a name?
Thanks,
Mike

 

[mskl99]

Coincidentally, I looked a couple of posts down the page at SandyG's quad kt88 project, and he has an almost identical driver stage, but there is a connection at the bottom of the cathode resistors to ground. Could there be a mistake on my schematic?
Mike

 

[Tubelab_com]

The circuit will not work as drawn. The junction of R10, R11 and R12 must be grounded.

 

[Tubologic]

The name is: drawing error. The node joining R10-R11-R12 should be connected to ground.

 

[mskl99]

That's what I figured. So if the junction of R10,11,12 is tied to ground, what does this stage add, beside some more amplification, I guess. R12 is 560 ohms, and R13,14 are both 47K.
Mike

 

[kevinkr]

V2 is a short tailed pair (R12 in this design is probably 390 ohms or so) a form of differential amplifier. The Williamson employs large amounts of global feedback to reduce overall distortion, this stage provides a substantial % of this gain. The design looks a lot like the later version of the musicians amp with a UL output stage.

 

[DF96]

Yes, it is a short tail pair (with the missing ground). This will be similar to a pair of antiphase common cathode amplifiers with bypassed cathode resistors, but will have slightly less even-order distortion and (probably) slightly more odd-order distortion. It saves one resistor and two electrolytics.

 

[SY]

It looks like the connection to the grid of V2b is not correct as well.


edit: never mind, my error. Tough to read those old fuzzy things.

 

[DF96]

Old fuzzy things find it tough to read? That is certainly my experience!