Recently we heard about Mr Pogossov's "boosted cathode follower" using a bjt between the cathode and 0V rail where the bjt has base current from an anode R between anode and B+. Current production of bjt is regulated by Vg-k of the follower tube, and all sort of positive claims are made about lower Rout, lower THD etc, but not one measurement of Vac signals possible or distortion have been offered to prove the claims are valid. In the fullness of time, the voltages and THD measurements may follow, we might hope.
Meanwhile, instead of using a bjt as an active current source in the cathode circuit, we can always use the bjt as a passive current source, as seen in my preamp circuits at Fig4 Schematic at page
http://www.turneraudio.com.au/Line-preamp-2003.htm
One might be delighted to find THD at a volt of output to an average power amp is so remarkably low because the 12AU7 involved does not have to produce a current change to a load carrying DC, and only has to power the output load which is usually the high input resistance of a following amplifier.
But the amp isn't distortionless, it just has about 1/3 the usual THD merely by making the load "easier" to power, ie, higher ohm value, therefore making the internal triode NFB more effective.
The other form of boosted CF is Allen Wright's Super cathode follower where the anode supply to CF is bootstrapped. It means that insead of a fixed B+ for a CF, you have a second CF triode ( or j-fet, mosfet or darlington pair bjt ) arranged so its cathode output is at say +150Vdc, and its gris is biased with 1M to +143Vsc, and the input Vac is applied to BOTH CFs, so that the lower one's cathode produces the wanted output load signal, with cathode current minimized with a CCS cathode load, while the anode voltage of lower CF has Vac applied at just slightly less amplitude by the top follower, so in reality, the Ea of the bottom follower dealing with wanted signal functions with a constant Ea, as well as a load current that is minimized. Wizards amoung you can work out the resultant lowering of THD by careful load line plotting with curves and a ruler, or even perhaps a simulation program, although it seems hardly anyone here bothers with such BS any more.
One might bootstrap the anode voltage of the follower so MORE Vac is applied to anode than is applied to its grid and arrange the circuit so the anode voltage applied is subject to Vgk, and is in effect, a NFB voltage applied to anode. Nobody seems to have tried that anywhere, yet, and I guess its because its all too hard to do and the average CF is already pure as fresh snow so no need to pay the costs or take the time for more unecessary complexity - ( maybe not if you can go skiing instead of soldering....)
You may recall I mentioned using 2 more additional triodes used to boost the function of what may be a "weak" CF, say 1/2 a 12AX7, by means of using the additionals as a cascaded amp with say EL84 CF output with its cathode connected to 12AX7 cathode, so forcing the internal voltage gain to very close to the µ of the 12AX7, with very much reduced Rout and THD, as could be easily forecast and calculated with nornmal gain&NFB equations. But its all so complex, and easier to just use the EL84 without the 12AX7, and maybe one finds its better and boosted "enough".
The other boosted CF involves using a pentode and forcing it to operate as a pentode while in CF mode. Most CF with say EL84 will have screen tied to B+ with anode, so thay are triode, with open loop gain say 18, so CF gain = 18 / ( 18 + 1 ) = 0.947, and open loop THD of say 1% into RL = 20k at 10Vrms is reduced to 0.053%, so that the 1Vac output level its about 0.005%. Not a high need to boost any darn thing eh?
Well, the EL84 could have its B+ anode voltage kept fixed, but have its screen voltage fed by CCS from B+ higher than anode, and then bypassed with C to cathode. This means OLG is raised from 18 to pentode gain of about 120. The OL THD might be 2% at 10V out, and this is reduced by factor of 1 / ( 1 + 120 )
to become 0.0165%, so at 1Vrms it is 0.00165%, rather low enough for most ppl I suspect.
But any THD in the cathode signal is fed to screen, where it does not act to lower THD. So if the screen was fed from a second triode CF whose grid input comes from the INPUT signal for '84 grid, then I would suggest the effect is closer to boostrapping the anode voltage of a triode, but its easier to do because far less dc current is involved, and screen input resistance is high and easily powered by a small triode. Using a triode-pentode like 6U8A would be a good tube to use as a preamp boosted CF with bootstrappped screen voltage.
I better quit now lest ppl complain about infomation overload, if they ain't already bore ****less.
Patrick Turner.