kambo said:
messed with AC heater wire, in every possible way, even new grounding etc.. not a chance...
my only guess is : since i have wired 5687 for 6.3V heaters, pin 4+5 - 8
pin 8 is next to grid pin 7, and heater wire need to cross from pin 4+5 junction to pin 8...
may be if i wire for 12VAC, i wont have hum !?
in case of DC heater supply :
would be a good idea to auto shut B+ down when heater supply fails ?
You shouldn't have all this problems with AC heaters if everything is right... Have you checked if power transformer really has enough current, does it have datasheet, can you find someone else online who have used it before? Or try it yourself?
Another thing could be trying artificial center tap with low value resistors, or "hum pot". Haven't tried it myself on transformer with CT, guess it should work anyway. Heater circuit has to be elevated for srpp in Slow Blow if you are not building MILA. Not sure what you are doing.
Wires comming from heater psu should go to the sides of chasis and then to tubes. So from psu to the side, from there to output tube, from there to the side and again to input tube. I don't follow this exactly when doing AC heaters, many people report it helps...
If wires are too thick you might have problems twisting/dressing them properly. Solid wire is easier to work with, although not the best solution as it could snap if very thin. Maybe use a piece of thin, well twisted solid wires near sockets if you think this is your problem, they can connect to stranded from there...
5687 is a little more difficult tube in this regard, i would bring tightly twisted wires near pin 9 to the center of tube, connect, bend it back to pin 9 and go to another tube. Both tubes have to be in phase, heater psu first connected to output tube and then to input tube.
Not sure if HT only is damaging to this kind of tubes if problem doesn't last long time. We already wrote about it here.
I'm sure you know what i wrote, hope it helps anyway. Regulator might work with you transformer when using LDOs, low drop diodes and heater voltages at around 5,5V. Slow Blow needs two even if they are LDO. I'm just experimenting with two on one heatsink from computer psu (fat Al L profile with fins on the top) where calculations show one is enough, but i want to see the difference in practice.