The inspiration came from Matthew Reidsma's Experimenter 45.
Now here comes my part.
I want to do the same as Matthew, using the 1626 in the same configuration. However, there were a few problems.
Of all the best laid plans of men and mice...
Initial plan was to discard the 5842 and use a 5965 in its place. I built a 5965-1626 Darling before. Read the Darling Club page. The 5965 has too high a plate resistance here (6-7kohm), so was discarded. Then an idea came to use the lot of 5687s which I have acquired quite a number of. Too bad their mu of 18 isn't high enough. Unless, of course, I use a preamp. Never mind... So, back to the 5842.
And then, I realize I can't bias the 1626 at -45V. I want to bias it at -25V ala the original Darling. After all, it sounds so sweet at that point, so why tinker a winning formula?
If I bias it at -25V, that means I have only 25V drop across the 5842 plate resistor. Now here lies the problem. I can't use the 4.7kohm resistor if I want to use the 10mA through the 5842. I have to go down to the region of ~2kohm. Not a good idea as you want to load the 5842 plate at least twice the plate resistance of 1-2kohm. I can still use the 4.7kohm if I let 5mA flow through the 5842. It'll work but the 5842 prefers higher current! As you can see, I'm stuck here, more current, not enough plate resistance; enough plate resistance, not enough current... Oh my....
I wrote to Matthew and one of his recommendations was to use a 6922 or 6DJ8. Looking at the tube data, it could actually work at 5mA/4.7k but I don't want to spend more $$ on tubes. Hmm...
One day, while discussing my predicament with tube buddy YH, he said "why not use a current source?"
That's right! This will solve all my problems. The current source will provide an AC impedance of tens of kilo ohms and I can use a variable resistor to find the correct resistance to drop off the desired 25V. So this should replace the 4.7k.
Cool... Now what? I could use the C4S or some other constant current source but purists might balk at seeing silicon in the circuit. What? Listen to sand?
Let the hemoglobin flow!
Okay, okay... more thinking and then it dawned on me that a choke is the answer! Yes, iron!
[By now you should realize how dumb I have been. All the stuff presented here has been done by others before me. They have evolved beyond homo sapiens and I'm still Neanderthal...]
I looked through the Hammond catalog and was grinning ear to ear when I saw the specs of the 154E.
Do you see that? If I run the 5842 at 15mA, then 15mA*1666ohms is... 25V! Yes! That's all to it! The choke drops the desired 25V while still presenting a high impedance to the 5842.
At 20Hz, Z = 2*pi*f*L + DCR = 2*3.14*20*20 + 1666 = 4178 ohms
Good enough right?
*Note that you need to use a 1.3-1.5V battery to get the 15mA.
The Hammond 125E output transformer should be 5.6k or higher. You can take this further by replacing the input 100kohm resistor with an input transformer and you now have a "No R, No C amp"!
Okay, now that the above is done, what else can we do?
Perhaps I want to present a higher plate impedance to the 5842. I could use the Hammond 157G which is 30H and has a DCR of 595 ohms. I will need a resistance of 1kohm in series to reach that magic number of 25 again. Better still, I can bias the 5842 at different currents and place a suitable resistor there.
Or perhaps I want to parafeed the output? Use a Hammond 157G as plate choke?
*Handmade Electronics has some hefty chokes with 60H inductance. You can definitely use them in place of the Hammond 157G. Or any choke for the matter, as long as it could withstand 25mA of DC current through it.
I haven't (and don't know how) calculated the capacitor value there but I guess a 5uF will work.
There you have it! Experimental 1626 or Experimental Darling! Simple right?
Now how does it sound?
I'm embarrassed to say that I haven't built this amp yet. *blush*blush* I have almost everything here except those Hammond chokes. Shipping from US is too expensive for the chokes. :(
*Please note that all iron above were chosen for cost factor ONLY. Any other equivalent exotic make will work.