Elevated DC heater supply

[johnheath]

Hi...

Sorry for asking this....again. I have searched this forum and tha internet without finding a solid answer so I feel that I need to ask again since I have uncompromising info.

I like DC on the heaters and since I also like SRPP topolgy there is sometimes a need for elevated heater DC

I know how to get this from a voltage divider  and a cap. Lets assume that I place this voltage divider  at the B+ of lets say 250 volts...

I use a 150k and a 100k resistor and achording to the formula it would give me 100 volts for the "elevation"

Now... I have heard and read that this 100V should be connected to the 0 volts of the heater supply...But also heard that if I do so it would be shorted to ground.... But I have also read that the heater won't draw any current from the elevated heater supply so.... I am getting confused here.

What I would like to know, and surely some other people in the sam situation, is ... Where do I connect my divided 100 volts as in the example? On the 0 volts (ground) or on the positive, 12,6 or 6,3 volts, side of the heater supply?

Sincerely

/John

 

[johnheath]

ps. I have been doing this for a while and connected it to the 0 volts(heater ground) and it has been working just fine... but if that is wrong and I should connect it to the positive side of the heater supply I guess that it would be very different values on the heater supply and if not... where on the positive side do I connect the elevated 100volts? After the rectifier bridge or in the end of the rail as after the caps and resistors?

Just asking here

Sincerely

/John

 

[emrr]

The heater '0' does not connect to ground in this case, just to the divider point. Heater is then 100 and 106/etc.  Of course if you connect '0' to ground then the bottom of the divider is shorted out.

 

[johnheath]

Ok - So, even though I use DC Ishould not ground the circuit?

/John

 

[ruffrecords]

Your heater voltage needs to be completely isolated from the circuit  0V. If your heaters are ac this is easy because the transformer heater winding provides this isolation. If the heater supply is dc then it must not be connected to the HT 0V. So in both cases both sides of the heaters are connected directly to the heater supply. Then, one side of the heater supply is connected to the elevation voltage. That is the only other connection you need to make to the heaters.  It does not really matter which side you connect to..

The heaters will at 0V as far as ac is concerned because of the decoupling capacitor you used in the elevation circuit.

Cheers

Ian

 

[johnheath]

Finally… finally an answer that explains it clearly.

Thank you so very much Ian

/John

 

[emrr]

Right, like I said. 

 

[JohnRoberts]

No it looks ground referenced they just drew the cap upside down, but labeled the polarity correctly.

JR

 

[scott2000]

Thanks!

I actually drew it and I realize it's ground but, I was confused about the elevated theory....??? I think it's around 70v from a measurement I took before but I'm kind of hesitant about probing that area again until I understand more about it....

I may be looking at everything wrong though..... :-\

.....

 

[ruffrecords]

I think you need to trace the junction of the 470K and 100K resistors because something is not right about that schematic.

Cheers

Ian

 

[scott2000]

I think you need to trace the junction of the 470K and 100K resistors because something is not right about that schematic.

Cheers

Ian

ok here it is... kind of.....

https://s20.postimg.cc/a82su705p/MYSTERY_BOX_2_B.jpg actually need to re-draw......

..this is only one channel of the two.......



there are also a couple of 100uf caps off the top half 22k plate resistors to ground....just left them off

Thanks!

 

[ruffrecords]

Looks like someone though of using elevated heaters but did not implement them - or someone modded the unit, The preamp stage is a mu follower. Not common in old stuff.

Cheers

Ian