It looks like I didn't make much sense, so I will try to clarify. The design I would like is your basic mu follower with two triodse in parallel in the lower section and single pentode up top. That is starting to become rather difficult so a basic mu follower with a pentode top is probably what will really result.
Take the example I THOUGHT was going to be a real winner:
The blue line is where the load line for the lower triode would be if I can find a way to parallel two of them together in the lower stage. The red load lines are where I would like to operate if I am forced into a single lower triode. The currents are 7.5mA and 11mA respectively. The swing is 130V in both cases. Amplification factor is 20, and rp is the usual 6600 and 7700 Ohms.
Now the upper pentode:
This is the 6JC6A pentode. The lines correspond as before, mostly. The red/pink lines in this case, however, are lower than they should be, I started going lower to see if I might get it to work at a lower current. Neither way worked. The voltage swing of the blue line gets a bit closer to 130V than I would like, but I was willing to take the risk. Also note that the 400V on the pentode would not be possible, that was more for an experiments sake that I will explain bellow.
Now the problem arises in the combination of factors that are joined at the hip, as it usually the case, they conflict with one another. I have been laboring under the possibly false idea that pentodes acting as CCS's still need their linearity considered in order for them to sound their best. I have been trying to find pentodes that were the most linear in the current ranges I have been hoping to operate under, like the one above. The problem is that CCS's benefit from having the flattest load line possible as that will increase the cathode and load resistors (Rk2+RL) which will in turn increase the rp seen by the lower triode/s. rp=(Rk2+RL)/(1-Av'), where Av' is the cathode follower gain of Av/(Av+1). So, I have been working to find a tube that would allow me to have as flat of a load line as possible while offering the current range linearity I also wanted. When I speak of acceptable rp values, I am hoping for at least a megaohm, which is achievable, buuuuut...
Additionally, the screen power dissipation Pg2 benefits from having a low current. Pg2=Ig2^2*Rg2, where Ig2 is the screen current and Rg2 is the screen resistor. Now, lowering the current, would mean a flatter load line, which would increase Rg2, BUT also decrease Ig2 and because Ig2 is squared in that equation, that is the better trade off. The greatest struggle has been keeping this Pg2 down to the .7W that is specified by the pentode shown above. I could accomplish it, but the currents were so low as to make the triode very nonlinear, as well as the pentode itself. The voltage of the pentode could be increased to have the same effect, unfortunately, all pentodes seem to be basically voltage bound at or bellow 300V in these current ranges.
The idea then was to find a pentode with higher current capabilities, this got me researching again. I found a section in the Morgan Jones book talking about constant current SINKS (not source like I am trying to develop here). In the book he talks about using pentodes at unusually low currents when trying to use them as sinks. Well that would solve my problem perfectly! The question though is if linearity is a problem? It would appear that it is not much of a problem in sinks as he recommends it and even offers up pentode options that work well for specific current ranges. Now that has me wondering if the same is true for sources. I have looked far and wide for that information but I cannot find it. That probably means the answer is so obvious that I have overlooked it. From the schematic for a pentode sink given in the book, I would not think that the two are the same, so what may work for the sink may not for the source. This also has me wondering if maybe it would be possible to make a mu follower using a sink instead of a source. I do realize that I would lose the PSRR in that case so it may not be a true mu follower, but maybe a more simple topography to implement, while offering most of the same benefits. As I was planning to go parafeed anyway, the PSRR should be pretty good so this makes this option even more intriguing.
Vp 1 | Vp 2 | Vp max | Vg1 | Ip:Ig2 ratio |
7 | 125 | 300 | 2.75 | 3.81 |
Vg1-1 | Vg1-2 | Ip max | Ivg1 | |
0 | 2.75 | 0.0176 | 0.0104 | |
| | | | |
Av1 | | | | |
42.91 | | | | |
Av1' | | | | |
0.98 | | | | |
Rk1+RL | | | | |
17,045.45 | | | | |
rL2 | | | | |
748,450.41 | | | | |
Rk1 | | | | |
264.42 | | | | |
RL | | | | |
16,781.03 | | | | |
Rg2 | | | | |
63,935.73 | | | | |
Ig2 | | | | |
0.00 | | | | |
Pg2 | | | | |
1.36 | | | | |
(Rg2 in the table is the screen resistor, not the grid to cathode resistor like in the image above)
Hopefully that clears things up a bit.
Now I MIGHT have found a pentode (Z759) that would work. It gives me an rp of 748kOhms with a Pg2 low enough to pass. It even seemed relatively linear, not as good as I would like, but better than I could with any other tube I could find. The values for this tube can be seen above. While the tube isn't terribly expensive, it is really difficult to find. It's the best option I have found so far though, unless I find out that I should be running a pentode source like a pentode sink.