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 Post subject: Protecting 1.4 VDC battery tube filaments
PostPosted: Jul Wed 09, 2014 4:57 am 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2210
Location: Thornhill, Ontario
Hi, Vacuumlanders,
This has the potential (no pun intended!) to save a few 1.4 volt filaments run off a rectifier + regulator "A" supply.
There is always a chance (small, IMHO) that the "A" supply regulator could fail to full output voltage well over 1.4 volts, or a chance (higher, IMHO) that a "civilian" could inadvertently or in ignorance crank up the "A" voltage.

To protect the tubes, wire two, 4 to 5 amp Si diodes in series across the filament line at the radio (under the chassis, not at the power supply as there may be a voltage drop in a long line), plus a WW resistor (see below), so that they are forward biased (total voltage about 2 x 0.7 = 1.4 volts = safe.) These diodes "crowbar the filament supply to no more than, say, 1.6 volts under very high diode current (an unlikely event.) Then feed the "A" supply from a higher voltage source, e.g. 6 volts, through the series resistor; 10 ohms WW, 10 watts, for a 5 tube radio with, say, 3 X 50 mA tubes and 2 Xt 100 mA tubes. Total filament current 250 mA.

Here's what happens: 6 minus 1.4 = 4.6 volts... this is now across 10 ohms, so the "A" p/s is delivering 460 mA. If the tubes take 350 mA, as in our example, then the diodes take the extra 110 mA and are fully on. If the p/s voltage is turned up to say, 10 volts, then there's (10 - 1.4)/10 = 860 mA in the 10 ohm resistor (over 7 watts!), but the tubes remain safe with the diodes taking just over 1/2 amp and still no more than about 1.4 to 1.5 volts on the filaments. Safe! At a lower supply voltage the diodes don't conduct enough to stay at 1.4 volts forward drop, so the filaments would be under-run, in our example this would be at below about 5 volts.

You can work examples for other tube numbers (current) and worst case supply voltages.

I have done this for a Kolster 6D converted to five 1.4 volt tubes (the example above), run off an ARBE-III p/s. "A" is set to 6 volts. Works perfectly. Running the ARBE-III at 1.5 volts is NOT safe as the adjustment could be moved.

One last point. If you build a custom "A" supply for use as above, you must use a regulator to cut out all 120 Hz ripple. I tried using a large 6 volt FW rectified wall-wart; the 1.4 filament voltage was fine but the radio hummed badly... even a large cap across the supply did not fix it.

Roger Jones,
Thornhill, Ontario
Ontario Vintage Radio Assoc.

 Post subject: Re: Protecting 1.4 VDC battery tube filaments
PostPosted: Sep Sun 21, 2014 3:15 pm 

Joined: Dec Sun 02, 2012 2:26 am
Posts: 59

Yes this method for protecting the filaments works very nicely. About 25 years ago I used this design approach for powering my R-174/URR shortwave receiver. The receiver and power supply are still working fine today.

My March 2013 post describing it is in the "Communications Receivers" forum. There is barely noticeable hum. The 44 mH chokes are quite effective, as evidenced by one of the reply posts where the author performed a SPICE simulation of the circuit. For your convenience here is the schematic that I posted.

R174_AC_POWERSUPPLY_SCALE80.png [ 117.21 KiB | Viewed 2367 times ]
 Post subject: Re: Protecting 1.4 VDC battery tube filaments
PostPosted: Oct Sat 04, 2014 12:30 am 

Joined: Aug Sat 31, 2013 11:22 pm
Posts: 2312
Location: Ft.Worth, Texas
Great idea ! I guess the only issue would occur if there was a diode or resistor failure.

Cliff Fitch

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