Some things for D/V owners to think about - last night I set about removing the pot and then removed the pot PCB.
Removing the pot - I had already desoldered the signal wiring but not the earth wire - I use the Stereo Coffee volume control/preamp. Please use SWIN's excellent photos on the preceding page/118 to better understand what follows. So, the only other desoldering is (1) the signal earth wire (2) the earth from the jack socket and (3) the hard wire after (R) 1K resistor.
You would logically believe that all 3 of these soldered connections would be to earth - NONE ARE. Only the incoming power is earthed to chassis - see photo (2). The RCA input PCB has no earth, which means that the RCA chassis sockets are'nt earthed and that the signal wire earth wire is'nt connected to earth at either end. The jack chassis socket has an earth tang which is soldered to the pot PCB but not to earth - all this is very weird and I'm wondering if all these hum problems stem from this lack of earthing.
I have created a chassis earth form the input PCB to the adjacent chassis, to which I shall run an earth wire from the jack socket and from the hard wire r/hand side after the r/h 1K resistor, effectively creating a star earth.
SWIN - where are those voltage figures, others have measured the voltage at the output caps @ 70V - there can never be enough factual info. The supplied Chinese o/put caps are 250V? but only 10uF. Maybe the reason those using low Ohm h/phones have no problem is related to the fact that the D/V has AFAIK only one small 'sweet spot' below which no volume or way too loud. If these Chinese caps are foil then at higher values the cost of foil caps escalates hugely - this could explain this volume anomaly.
Sorry about the delay, got the voltages, but got stuck trying to find a CAD program with tube symbols - found a few free CAD programs, but a bit steep learning curve.
The output capacitors are the only thing that separates the B+ or drive voltage from your headphones, so a very important to select caps that can stand the whole B+, in case of a drive tube failure.
That said, the orange blobs that are the output caps on this thing, are totally unmarked, and so nobody knows what they are, and what voltage they are rated at.
One cheap and fast way to increase the value of the caps at the output, is of cause to add a electrolytic cap parallel with them.
Just check how the acclaimed Bottlehead Crack does it, a very tiny high voltage electrolytic cap at the output.
There are of cause som not too expensive high voltage foil capacitors suited as output caps in this application, but they are nearly as large as the 336, and would need an outboard case.
The voltages so far - my unit is a 220 Volt 336, and I got B+ 134 Volt DC, 80 Volt DC across the 1 K 20 Watt "golden" large resistors, and that means 80 Volt DC across the output capacitors.
One thing that stands out so far, is that the heater voltage is 6,9 Volt AC - a bit too high for prolonged valve life, standard is 6,3 Volt.
All voltages measured with tubes in, and live working unit that was powered on 1 hour.
As soon as I am back from work, I will try to find a easy to use CAD program, with tube symbols.