• C4S loaded 6N1P driving paralleled 6N1P parafeed via C4S into PP output transformer
• C4S loaded 6H30pi driving paralleled 6N1P parafeed via C4S into PP output transformer
• C4S loaded 6H30pi driving 12B4 parafeed via C4S into PP output transformer
• C4S loaded 6N1P driving 12B4 parafeed via C4S into PP output transformer
• C4S loaded 6H30pi cap coupled to 12B4 C4S sunk Cathode Follower OTL
• Resistor loaded 6H30pi cap coupled to 12B4 C4S sunk Cathode Follower OTL

OK, so I've got this little headphone amp (you know, for work...) I've been fiddling with for about 6 months.
 
It uses a 6H30pi as a voltage amplifier (10kohm Rp, 4.3V bias, 15mA) cap coupled via a .47uF Angela tin foil/PP film cap to a 12B4 cathode follower (C4S loaded, 12mA), OTL output, 1kohm load across output jack, 75uF (combo of 3 film/foil cap types). PS is UF4007 FWB - CLCLC 250VDC with 1.3mV hum at last cap. Filaments are DC, Schottky rectifiers, CRC, 6.1V...
 
I previously was using a C4S to load the 6H30pi, but switched to Caddock 816 resistors (big heatsinks, they still get warm) because of problems with my much-transplanted set of C4S boards. When I fired up the thing I got no glow on the CF C4S boards until I touched the probe of my multimeter to the grid pin on each socket (note, CF is cap-coupled on both input and output) - then things worked wonderfully, and the music flowed!
 
After wondering about this odd C4S behavior I realized that the meter was providing a ground reference for current to flow, and then the C4S was kicking in; but this was not acceptable for normal powerup - and yes, it did repeat when I shut down and restarted. Another symptom was a very slow rise over time of the output voltage I attribute to leakage across the coupling cap. So I installed a voltage divider from the B+ to the 12B4 CF grid to the ground using Caddock MK132 (good quality metal films I happened to have in the parts bins in these values: B+:221k:grid:68k:GND) that holds the grid at 60VDC and the thing starts reliably 100% of the time.
 
The problem is this seems to have sucked the life out of the sound - It was much livelier without the voltage reference.
 
So, anyone have an idea for a next step? Grid choke to GND? 1meg ohm R to GND without the B+ reference? Output of the 6H30pi is around 98V which is above what tubeCAD is saying is a good input for the CF, but then tubeCAD doesn't have a constant current source option either.
 
Whatever, this current sound will not do.

What's the Rp of the 6H30pi? Can't you just dispense with the CF altogether and couple the cans right to them? If not, it seems like maybe reworking the PS so you can direct couple the stages might be a neat way to go.

Sometimes the process of posting makes things fall into place in my head (sometimes I just get more confused), but I'm now thinking that at one point before I added the resistor networks to the CF grids I had been running the thing overnight and the leakage had driven the grid up to 117VDC (you have to be quick to even see it as the process of measuring it made the voltage drop immediately). The output voltage was happy at almost 100VDC. Maybe I should just snip out the coupling cap and voltage divider, and let the C4S ride at a higher voltage and be done with it. The 12B4 has plenty of headroom. Simpler is better.
 
I'm afraid to take pictures of the inside of this thing, with about 6 major circuit/tube combos (several iterations attempted parafeed into some cheapy XFMRs) there are signs of wiring and soldering distress all over. At least the outside looks OK.

OK, performed more surgery last night - removed the coupling cap and voltage reference dividers and just directly connected the plate of the 6H30pi driver to the grid of the 12B4 and let the C4S ride and do it's current sinking thang.
 
Sounds much better without the extra complications and the 12B4 is happy with the grid at 97VDC instead of 60VDC. I was shocked at the sacrifice to the music when I put in those 68k resistors - an interesting lesson. The Dowdy-lama once told me to avoid any resistor over 20k in a circuit - they are just bad news. Food for thought.
 
Had not worked with the 12B4 until now, an impressive little tube; as a former TV sweep tube it is durable and will take a huge voltage (550 volt rating) without complaint. They sound good and are relatively cheap.
 
VS, I'm using the 12B4 as a CF in OTL mode for now and it works fine - if at some point I get caught up on my other spendy projects I might take a run at a pair of 'proper' headphone output trannys - and switch the 12B4s to VA drive (or maybe try grounded plate drive?). Thanks for the suggestions.