6AS7G Amplifier Design

[jr_tech]

How does V1b function?

jr

[benman94]

Originally Posted by jr_tech

How does V1b function?

jr

Both halves of V1 function as cathode followers as V2 needs to be fed from an extremely low impedance source. It helps to understand the circuit if you actually ignore the presence of V1 and imagine an input transformer in its place.

Now, the Van Scoyoc circuit can be used in one of two ways, the single ended configuration as I have used or with a balanced push pull input. If you tie the grid of either V1A or V1B to ground, the circuit functions as a phase inverter with a tremendous amount of gain and relatively good AC balance, differing only by an approximate factor of u/(u+1) where u is the mu of V2 (70 in the case of the 6SL7 shown). If you use it with a push pull input, you would have two separate grid leaks to ground and apply a signal of some amplitude to V1A, and a signal of the same amplitude but opposite phase to V1B.

In practice, the grid of V1B is often not connected directly to ground, but is connected to a very small resistor (about 100 ohms) which is tied to ground. Then this resistor is used as the bottom leg of the voltage divider formed by the feedback resistor from the secondary winding of the OPT in designs that use global feedback.

It is really a shame that this circuit wasn't used by D.T.N. Williamson in his famous amplifier. Whereas a Williamson has three RC time constants that must be managed to prevent instability with the application of feedback, replacing the first 6SN7 in a Williamson with this circuit would give only two RC time constants to manage. Since the Williamson is on a knife's edge in terms of stability to begin with, this would have likely prevented a lot of damaged loud speakers...

Here's a link to Van Scoyoc's original article from 1948:
https://pearl-hifi.com/06_Lit_Archiv...e_Inv_Comp.pdf

There are some downsides to this circuit. You essentially "waste" a 6SN7 that provides no increase in gain and serves merely to feed the signal to the 6SL7, and it is very, very sensitive to the amplitude of the input so it MUST be used as the very first stage in an amplifier. This isn't necessarily a bad thing however, in that it forces the designer to make the rest of the amplifier push-pull in nature, and that in turn causes the natural consequence of better power supply ripple and noise rejection.