current feedback gain block

just a quickie… before i forget about it. was meaning to talk about other direct coupled techniques and wanted to come back to a fun arrangement with pentodes and tetrodes…

with all this discussion of “best pentode” and transconductance amplifier stages, i don’t want to give you the idea that there aren’t other wonderful ways to do things! yes, those circuits are exciting and crazy high performance, not to mention new and original work! but lets not forget that pentodes are just amazing, whether they are best, or not!

i have mentioned before that it is very easy to operate multigrid tubes with very low plate voltages and “normal” screen voltages. no it’s not done very often, but maybe it should be?

here’s another look at the current feedback gain block, but with a twist…


please note that the screen dissipation shouldn’t be exceeded. in this case, it just can’t be… the russian 6E5P is damn near indestructible. and super quiet!

with a 300 volt B+ and a 150 volt gas regulator tube, you get a split supply that can be used to assist in the direct coupled stages. it could be easily rearranged with a + 220 volt supply and a 102 volt gas tube (0B2) which is the better reference. with only 25 volts on the plate, U1 can easily be DC coupled to U2. the buffer M3, provides the method to return the feedback current to the first stage. and my favorite part of this circuit is the shared screen supply… i don’t think you will see that very often!

this gives you a gain of + 40dB, which can be connected to another stage, and more feedback returned to the cathode or grid… already, this stage has a great performance:


and the trend in distortion:


i use this circuit in my parametric equalizer design. with an optocoupler “twin T” filter between the following stage and the input… overall a useful building block for vacuum tube opamps.