Patrick is correct, we don’t see many audio amplifiers making use of the 6U8. However, I did not have to look far to find one. It is one of Patrick’s very own early efforts. It has two 6U8s. The schematic is attached so that everyone may marvel at it.
NTL an interesting topology as anyone who has tried it will agree. Patrick manages to get about 95% of the signal at the plates of the output tubes all the way back to their grids in the form of NFB. If one looks carefully we can see an anode follower.
Without the MU followers as plate loads one is lucky to get ½ the signal all the way round the loop. In Patrick’s amp the result is the output tubes look very much like very stiff triodes driving the OPT. That will help the LF response very much. But not so much the HF since the signal still has to get past the OPT leakage inductance & winding capacities.
Each of the mu followers present what looks like a constant current load (Hi R) to the pentodes. Under ordinary circumstances that often creates a problem wherein the operating point (Q) is said to be ‘undefined’. Very small changes in either the pentode or mu follower result in large shifting of the Q point. The circuit could be unstable. In Patrick’s circuit the plate 470K NFB resister will keep things under control. Refer to the attached.
The circuit utilizes many parts to accomplish this. Some would say too many. Unfortunately the Law of Diminishing Returns sets in rather quickly. A simpler circuit using one less tube & fewer parts can accomplish the same thing.
I used the 6U8 for the screen resistance tests simply because I had one. But any ordinary small signal pentode would have done just as well to illustrate how the measurement can be made. The data is useful when designing an amplifier that might use the screen of an amplifier input tube to establish an LF step in the frequency response.
And I used a soldering iron as Patrick often suggests. But I also frequently use modern tools such as simulation software to get some idea if a particular circuit is worth pursuing. Before I plug in the iron! And it is better not to build the entire amp to measure only a part of a circuit. Best to look at each section in isolation to better understand what its function will be in the whole.
And I don’t use my slide rule a lot anymore either. The HP calculator running RPN is a hell of a lot faster. For anyone interested in RPN there is a less than one MB download at this link-
https://sourceforge.net/projects/fwcalc/
RPN does the calcs from the inside out, just as we do with pencil & paper. It stores the intermediate results so you won’t need the pencil until the final result. No Drucker!!
Cheers to all, John L Stewart