> this isn't a cathode follower. What is it called then?
The output stage of the 175/176/177 is a plain old tube stage. Technically a common-cathode stage, plate-loaded, transformer coupled, yada yada....
The reason I asked is: you were asking for a faster rectifier. The 175+ design gets rectifier power from the output stage. If you use something other than the plain old 175+ design, then that makes a difference.
I have seen a private sketch. Right off I don't see a Release resistor. As-drawn, the first time it goes into GR, it will stay there all day (well, for a minute or so). Just to get it working, put 1Meg from where the time-constant caps connect to the input grids. That will be a usable value, exact value to be determined after much musical abuse.
Ah.... you have deviated from the UA time-constant network. Looks a lot like a Fairchild. OK. The Fairchild costs more ($5 of switched caps instead of $1 pots) but I suppose that's unimportant in a box that will be worth $500-$20,000.
You can't just use the Fairchild time-network because the Fairchild's rectifier really needs to be driven from 600Ω, and the 175+'s 12SN7 output is more like 6K per side. Taking from the 600Ω secondary won't give you the voltage. The Fairchild time-net really needs a 10 Watt rectifier driver.
SO: use the Fairchild network BUT raise all the impedances 10 times higher. Bigger resistors, smaller caps. A 2Meg resistor becomes a 22Meg resistor; a 1uFd cap becomes a 0.1uFd cap. Now the output stage and rectifier and time-constant network are all working at good impedance.
Problem now is that the output of the time-constant is a very high impedance, higher than the rated grid resistance of most tubes. It may be OK with selected 6BC8, if you burn-in for a week to getter the gas and keep them hot. But you also have to select balanced 6BC8 to avoid thump (the balance pots won't fix a badly balanced tube). SO you might end up with an awful lot of 6BC8 that can't be used.
FIX: put a unity-gain DC buffer between the time-constant output and the input grids. I think the lame old TL071 is perfect: very high input impedance and plenty fast for sub-milliSecond attack times. However you will need a -30VDC supply for it. (Tip: a pair of 15V 1W Zeners in the center-tap leg of the power transformer.)
Also in private viewing:
You raised the 12AX7 grid resistors to 25 times higher than UA's values. Why? UA used low-value to swamp unbalance in their not-centertapped interstage transformer. Even with a better tranny, I think you need to swamp too. I can't read the tranny ratio but remember the primary sees 10K, so the secondary can probably be loaded with 2*20K (what UA did).
You have two balance pots in the 12AX7 cathode, they both do the same thing: screw-up the gain-balance! Maybe there is a "sound" reason you deviated so far from UA's feedback network, but I just see setup difficulty. You also seem to be set up for lower output stage gain than UA had: maybe this is a change in interstage tranny ratio, maybe you want lower noise at the cost of higher 6BC8 distortion.