E-Linear: an interesting kind of feedback

Oops, I bought another pair of output transformers. This iron is 60W rated push-pull with a 6k6 primary, so the natural pairing would be 6L6GC.

I’ve used the 6L6GC in the past (see the Luciernaga). Because it is common in guitar amplifiers, it’s an easy to find tube both new and vintage. While it’s no power-house in triode mode, the 6L6GC is quite capable in pentode or ultralinear.

Operating the output tubes as pentodes means higher output impedance and distortion. The Quality Amplifier from a couple weeks ago avoids the issue entirely by operating the outputs as triodes. The other approach, which is actually more common today, is to sacrifice some of the circuit’s gain to bring output impedance and distortion back down. That’s called (negative) feedback.

The question is how do we want to apply it?

This post won’t get into the nitty gritty of feedback (that deserves its own page on the website), but there are generally two prevailing approaches. Global feedback is what we see most often; this takes a signal from the output of the amp and wraps it all the way back to the input. The Williamson amplifier is the quintessential example of this.

The second approach to feedback is to use local loops. These affect a circuit just like global negative feedback, but are isolated to just a stage or two. Local feedback, because it involves fewer stages and phase shift, is more stable than global negative feedback. That means they’re generally simpler to employ.

The circuit above is a variation of what seems to be unofficially referred to as an E-Linear stage. Feedback from the output transformer primary is applied via the ultralinear taps directly to the load resistor for the input stage. This local feedback drastically reduces the output impedance of the 6L6GC.

The input stage is commonly a pentode because the high plate resistance is a benefit here to applying feedback. In this case, the input stage is a hybrid cascode, which still has a high “plate resistance” owning to the MOSFET upper device. That also gives us more options for the lower triode tube.

I like the simplicity of this circuit quite a lot. In Class AB, it looks like a good 25W should also be available with a pretty modest B+ (or a little less in Class A). Seeing as how I’ve got the iron on hand, I hope to give this one a test at some point!