dc regulated filament supply

[scottdarling1221]

I have a dc regulated supply to light my filament on my big ceramic triode tube. Any ideas on how i can bias the tube since theres no center tap.

Both filament leads are above ground.
Can i hook up a string of diodes to the negative side of the supply and the other end to a 10k resistor and short the resistor out when transmitting.

[KD6VXI]

Take a couple resistors and run one end from each filament lead and short the other ends (Y config).

That becomes your CT.

As to the resistance values, I'm sure someone else can chime in and give you a formula for finding it....  The transformers I use all have CT's on them.


--Shane

[scottdarling1221]

Thanks
I have 6.3 volts total so 3.15 per leg
Any more info would be greatly apreciated

[KA2QFX]

What tube is this? 
What is the design of the filament supply? Full wave bridge? Series pass regulator? 

Some tubes, like the GS35 have an indirectly heated cathode tied to one side of the filament, so the DC cathode current can flow on only one pin of the filament.  But directly heated cathodes should have their cathode surrent evenly distributed on all filament connections.  This make DC fed filaments more complicated for obvious reason.

We need more info>

But offhand I would say if you're regulating a directly heated cathode and the DC supply uses a series pass element to regulate the positive supply output, I would place the cathode bias device (Zener, regulator, cut-off resistor ...) off of the positive side of the filament. This should accommodate cathode current flow from either filament terminal evenly since this doesn't oppose normal current flow of the filament supply....

But like I said, we need more info. 

You don't necessarily like things simple, do you?

Mark

[AB2EZ]

Scott

See the figure attached below

I believe your original proposal is correct (if I am interpreting it properly).

Notes:

If the cathode is separate, and attached to 1 side of the filament, then you should attach the biasing circuitry to the same side of the filament. This will avoid heating the filament (beyond its optimal level) with cathode current flowing through it.

If your filament power supply has significant capacitive or inductive coupling to ground... you might want to include a filament choke to keep r.f. out of the filament supply.

If the r.f. stage is grid driven, you should include r.f. bypass capacitors to ground on both sides of the filament/cathode.

Best regards
Stu

[WD5JKO]

If the tube is the same as under the screen supply thread, it is a mule!

see 4cx15000a/8281:
http://www.g8wrb.org/tetrodes/

Filament is 6.3v @ 160 amps. Eimac says no more than 1 power cycle of the filament per day, and to regulate the filament IF running at reduced voltage to increase tube life. The data sheet seems to show that this is a directly heated tetrode, i.e. no separate cathode.

Jim
WD5JKO

[scottdarling1221]

Its a 15000 triode.The amplifier in the other thread is a tetrode 8281 and i already have a acdc  200 amp regulated power supply lighting the filament in hat amp.Eimac suggests powering it on only once a day do to the inrush current since the tube filament looks like a short when cold.

Acording to reid brandon at eimac its always better to light any filament tube or just a regular light bulb with dc then ac.

The regulated supply provides over voltage protection over current protection and softstart.This supply has been in service for over 8 years without a single problem.I put the peter dahl transformer up for sale on ebay.Plus i was able to rip out the hum bucking circuit.

Im building a multiband triode version since i have all the parts already so figured why not put them to use.The problem is no center tap.

The soft start takes about 8 seconds so the filament will have a nice easy life.

[scottdarling1221]

If the tube is the same as under the screen supply thread, it is a mule!

see 4cx15000a/8281:
http://www.g8wrb.org/tetrodes/

Filament is 6.3v @ 160 amps. Eimac says no more than 1 power cycle of the filament per day, and to regulate the filament IF running at reduced voltage to increase tube life. The data sheet seems to show that this is a directly heated tetrode, i.e. no separate cathode.

Only true if theres no soft start.

Jim
WD5JKO