Tapping dcv from filament supply

[dude]

I'm trying to tap a low dcv from the filament supply.

The PT has no ground, just the artificial 100 ohm R method.

I'm installing a VVR with a PC processor fan under it, heat sink and all. It's a 12v dc fan but I've used the PT's filament supply before and get about 5 or 6 vdc on the fan (runs slower but works fine).

I've changed the acv filament supply to dc with diodes but since the PT has no ground, I'm a little stuck.

Of these two diagrams, can I use one or either one?

thanks, al 

[jjasilli]

Here's how I've done 12VDC cooling fans:  

N.B.  Don't know about tiny 1" CPU fans, but for the 3" 12VDC fans I've used, my understanding is that they want at least 8VDC or they might not start, which may cause an excessive current condition which may burn out the fan & other components.   Also, if running in a voltage starved condition, there might also be excessive current draw in the heater circuit.    You might want to checkout current draw at 5 - 6 VDC.  Also, a CPU fan alone is insufficient for a processor chip; it needs a heatsink.   Heatsinking the VVR might be sufficient without a fan.

[dude]

Here's how I've done 12VDC cooling fans:  

N.B.  Don't know about tiny 1" CPU fans, but for the 3" 12VDC fans I've used, my understanding is that they want at least 8VDC or they might not start, which may cause an excessive current condition which may burn out the fan & other components.   Also, if running in a voltage starved condition, there might also be excessive current draw in the heater circuit.    You might want to checkout current draw at 5 - 6 VDC.  Also, a CPU fan alone is insufficient for a processor chip; it needs a heatsink.   Heatsinking the VVR might be sufficient without a fan.

The fan I'm using is 2" sq and needs 5vdc or more to start, there are three tubes (2) 6V6s, (1) 12ax7. I've done this a few times but in all cases the filaments had a CT, this PT doesn't.

The chassis is steel, and the VVR is located next to the 6V6's and PT, a lot of heat there. I can't get a heat sink inside the amp, no room.

So, I bolted the Mosfet inside the chassis and put the 2" sq heat sink with fan on top - underneath the mosfet on the outside of the chassis. I don't know what the spec's are on the PT as it's from the early 70's and has no CT taps, I'm running a bridge rectifier.

I not sure about the wiring of a rectifier off the a/c filament supply for the 5 or 6 dc volts I need for the fan.

Unfortunately, I can't open the file you sent for some reason...? I appreciate your help.

Maybe you could send it a different way...?     

[Willabe]

You can down load the program that will let you open the file here.

 http://www.el34world.com/Forum/index.php?topic=11457.0


                               Brad     

[jjasilli]

Ditto.  Meanwhile, here's a jpg version.

[PRR]

Full Wave Bridge rectifier.

Put the two AC leads to the 6.3VAC wires.

Put 470uFd across the FWB's + and - leads, correct polarity.

Put fan across the cap. Polarity may matter.

You should wind up near 6VDC. Many 12V fans will run that low, usually. If they fail to start on low voltage, no harm will come to a small fan. (Big motors with constant load, yes, will burn-up when stalled on low voltage.)

Do not ground the fan anywhere.

You don't care how or if the heater winding is grounded.

[dude]

Thanks, all

I got about 7vdc on the fan, works fine.

al