6CA4/EZ81 heater bias

[dsavitsk]

I am getting near the end of a project, so I am posting lots of dumb little questions. I apologize in advance for the pollution.

I have a 6.3VCT winding to my power transformer connected to the heater on an EZ81. I am not sure what to do with the center tap, however. Should it go to ground, to B+, or to somewhere else? The B+ is only about 140V here, and the PS filter is C (50uF), L (7H), C (350uF), R (100R), C (50uF).

[anatech]

Most people would tie it to ground. If it's the same string as the other heaters, I would tend to create a +30 ~ +50 VDC potential and tie it to that. 10uF to ground from that point.

-Chris

[Zen Mod]

if EZ heater winding is solo-meaning that no other tube filament is connected to it,just leave it untied to any potential;it is best for EZ this way-not provoking in any way heater-cathode insulation

old tube radios are best proof for that-I have some EZ pulled from 30+years old radios,with at least 20 yrs of everyday service,still testing strong
besides -in some of old radios cathode and one leg of filament are tied together ditto on socket of EZ,so it's pretty obvious where filament winding is referenced

if you have any other tube on same winding-follow anatech's advice


besides that-just from memory-50uF is max for EZ,as first cap;

be sure that your xformer have enough series impedance (or reflected impedance-if you wish) to be on safe side, meaning that your EZ will not be burned promptly...

all that is not so criticall if you use smaller first cap; I urge you to do exactly this ,if your xformer is toroid or similar;

in other way-few or severall ohms on each anode can be good thing

[dsavitsk]

The heater has its own winding that is not shared, so maybe I'll try floating it.

I think you are correct about the 50uF limit.

The transformer is a hammond 369ax. I don't know how much series resistance it has. However, I do have an extra Mills 2R/12W resistor I could put between the transformer and the anode, or somewhere else if that would be a good idea?

These transformer also seems to have a 50VAC bias tap (i've been wondering what to do with the purpe wire) that I could perhaps use for biasing?

[Zen Mod]

just leave it floating

search for EZ datasheet and you'll find needed info about adequate xformer impedance
I believe that you can also find datasheet for that hammond iron

then you can see what amount of resistance you maybe need to add in each anode

I have somewhere mentioned info for EZ,but deep buried in pile of paper;I also-always use originall xformers from mentioned old tube radios,so I'm not worried about any discrepancy between tube and xformer;from same reason I don't know where that papers are ;(

go to my "links" page,look for Geeks page,then look for RDH4 download page,then read ;

if you don't already have paper copy

regarding bias winding-just leave it for some later purpose;
when you need to reference something (as filament to some voltage potential) your reference is always -ground .
counting on that- your tool for voltage reference are nothing less and nothing more than already existing ground and -what else than- anode voltage (Ua ) ;
just make resistance divider with some 1mA through it,divide it in adequate two resistor values,then bypass lower one resistor via good 10uF cap,and job is done.

[anatech]

Hi dsavitsk,
Peak heater to cathode voltage rating is 500 V pk. You should, as choky recommended, let it float. You could reference to ground with no ill effects. The max input capacitor is 50 uF. Given tolerances, I would use a 40 or 33 uF as your input cap. This will extend tube life, although these are robust tubes.

As for the other winding. Save it for something else, again as choky recommended.

-Chris