This month I'd like to present a few ideas for making substitution "eye" tubes work for the relatively pricey 6U5 and 6E5 tubes. I want to thank Barney Wooters for much of the information contained in this article. Barney developed the circuit values, and has used and proven the circuit in his own restorations projects.
There is a relatively inexpensive "eye tube available known as the 1629. Although this tube would work in most replacement situations for the 6XX tubes, in terms of operating characteristics particularly for the 6U5, it does have a couple of major exceptions. One exception is that the 1629 uses a 12 volt filament while the 6XX tubes use a 6 volt filament. The other exception is that the 1629 uses an 8 pin base while the 6XX's use a 6 pin base.
The filament issue is really the show stopper problem. However, Barney worked out a very clever, inexpensive and unobtrusive solution for providing the required voltage. His solution is to use a voltage doubler that converts the 6 volt AC filament voltage to 12 volts DC.
Refer to fig. 1 to see how this circuit works. The voltage doubling circuit consists of diodes D1, D2 and capacitors C1, C2. Since the supplied filament voltage is AC the individual diodes, in this configuration, conduct current on alternate half cycles of the input voltage. That is, first the voltage passes through D1 and charges C1. Then, on the other half cycle of the AC voltage, the voltage passes through D2 and in turn charges C2. Since the capacitors are wired in series, they effectively present twice the charging voltage all at once to the load - the 1629 filament.
As in most things, nothing is perfect. This circuit is no different. The charging voltage, on each capacitor, is something less than the supplied filament voltage. In addition, the 1629 filament drain of 150 Ma. is enough to keep the caps from coming up to full charge. So, the output voltage will be something less than the optimum value of 12.6 volts. However, we can make lemonade from this lemon by considering that although this circuit certainly works as advertised, it could also be used to re-invigorate an otherwise unusable 6XX tube because of it's depleted and/or darkened green iris - if you chose not to replace it. That is, use this circuit to increase the filament voltage to the 6XX "eye" tube. You might want to try experimenting with lower values for the capacitors in order to arrive at a supplied voltage that's perhaps only a few volts higher than the 6 volt supply. What do you have to lose? Heck, the tube is no good anyway!
Another idea for keeping the old weak 6XX tube in place and not tearing into the filament circuit is to use a source of B+ supply for the tube that is higher than what is normally supplied. Look for this higher voltage at the audio output screen grid, or even directly from the cathode of the power supply rectifier. In the case of an "eye" tube the B+ supply does not have to be very smooth.
The circuit itself can be mounted anywhere - presumably under the chassis. These parts are very small and can be mounted by perf-board, by tie-point bracket or even left "hanging in the air" (if properly insulated). Simply cut the filament voltage lines where they go ONLY to the "eye" tube. The cut lines that come from the tube are then connected to the (+) and (-) points in the doubler, and the cut lines that come from the filament winding of the power transformer go to the AC points on this circuit. By-the-way, it's assumed that the filament supply that you're working with IS derived from a transformer.
What happens if your "eye" tube is in a series filament string? Well, you can get darn close. The required filament current for the 1629 is half that of the 6XX tubes. However, Ohm's Law will tell you that current is the same, no matter what, for all components in a series circuit. So, we need to allow 150 ma. of current to "go around" the 1629 by adding a resistor in parallel with the 1629 filament that is equal to the resistance of the 1629 filament. The resistor value should be V(1629 fil.)/I(1629 fil.) = 12.6/.150 = 84 ohms (at just under 2 watts - so use at least a 5 watt'er). The next problem is that you now have a filament that will take an additional 6 volts away from the rest of the tubes in the string. Take heart as this voltage loss, distributed among the rest of the tubes, will probably not have any adverse effect. Oh well, my information says that these kinds of radio sets are very few and far between anyway.
The socket problem is taken care of by substituting an 8-pin bakelite ring mounted socket for the old 6-pin socket. In most cases the original socket is fitted with a hood that hides and protects the lead wires as well as providing a mounting point for the "eye" tube. Therefore, once your sockets are interchanged, the above chassis look will be exactly the same as the original. Once again, refer to fig. 1 for socket pin-outs.
If exact look is not an issue you might also consider making an adapter with an 8-pin socket and a 6-pin plug (perhaps from an old tube). If you do this you could put the doubler in the adapter and not cut any wires at all. Just make sure you have room in the cabinet.
Another idea comes from Dick Hagrman. He has substituted a 2E5 for a 6E5. In this case, since the required filament voltage is now lower, you only need to install a dropping resistor in series with one of the filament lines in order to provide the correct voltage. Ohms Law will tell you that for a 2.5 V. load at .8 A. (this is the 6E5's filament requirements), coming from a 6.3 V. source, you would need a 4.75 ohm resistor. If you can't find one a 5 ohm'er will work. Although the resistor will dissipate a little more than 3 watts, it should be closer to a 10 watt unit for safety and for better under chassis heat dissipation.
Now for the lazy restorer's idea. Antique Electronic Supply (and perhaps others) sells a ready made adapter that goes between a 1629 and the existing tube socket for a 6U5. AES's goes for $15.95 plus shipping. As above, make sure you've got the room if you go with this solution.
Good luck in keeping those wonderful "magic eyes" staring out into your living room.