[patrick-turner]

On Wednesday, 27 February 2013 23:11:52 UTC+11, Alex Pogossov wrote:
Thanks Patrick, It is a useful advice to specify AC voltages where applicable. Will try next time. We all are free to publish in any style we like. My general direction was (I thought) to promote relatively simple solutions where adding a transistor dramatically improves tubed circuit without adding complexity.. With this boosted cathode follower, you almost always can solder the bjt into an existing pure cathode follower. Literally, only one extra solder lug -- to support the base of the bjt -- will have to be added. And the output impedance and distortion reduces significantly. And a few quick comments: Using a bjt for boosting is better. It is crucial to use bjt as a current amplifier, not voltage amplifier.

***Don't let me spoil your posting style. And don't let me ever stop you asking questions. If I had a quid for every time someone said to me " Patrick, for heaven's sake man, please be clearer" then I'd be rich. A building site manager once said to me, 24 yo, and with a tendency to bull****, "Son, say what you mean, and mean what you say..." The words are still echoing around in my head, some 43 years later. Now, sometimes ppl might think I'm tactless, no empathy either. Nope, It just means I've left out the waffle&bull****.

From what little I could see, the bjt in your booster does act as a current amp because you have a higher source drive from 33k // 12AX7 Ra to drive the bjt base with grounded emitter, so Rb input is very low. So the 12AX7 anode voltage change is low, probably negligible, thus allowing the 12AX7 to work as a follower.

JFET as a boosting element will give worse results both in distortion and output impedance.

***Well, sure there would be a big difference between bjt and j-fet. J-fet Rin at gate is high like a triode grid. J-fet Gm is much lower than a bjt.

But for me to really assess what you are saying means I have to build your circuit and measure it carefully. I'm time limited, and have a very full list of other interests, and, I've never been keen to do other people's homework when I have such huge amounts of my own.

Besides it will be prone to oscillation, as you correctly mentioned, due to a pole in the hi-Z gate node. Interestingly that a tube with a bjt for boosting gives the best results and the widest range of applications -- can work up to high signal voltages with low distortion and low output impedance. Other combinations are inferior, with only perhaps boosted emitter follower being better on distortion, but things may go wrong if the signal source has large impedance and the "main" following transistor has Early effect (beta modulation).

**It all seems like you are using a tiny tube current change to change a base current in bjt to get a bigger collector current change so bjt acts like an active current source load for triode, and its sort of PCFB which is then regulated by the NFB of the follower connection. But with just a 12AX7 with say idle Ia = 1mA, then class A current change can only ever be +/- 1mA peak, and anything more gives high THD. What you have is a tube white follower, super-charged with a bjt which has high ac current gain and hence strong action in the 0.7mArms region. One has to wonder why more ppl don't use white followers, or your idea of boosting. More than one idea of boosting can and do exist.
Patrick Turner.