Let's say you were to do a voltage divider from the B+ rail. The problem there is you may temporarily exceed max heater-cathode voltage if the B+ rail comes up faster or slower than the cathodes of the output tubes (for example you used silicon rectification and your B+ supply came up within a few seconds, but it takes the output tubes ~30 seconds to fully warm up). The best approach is to bias up the filament from its own cathode. That way the difference in voltage between heater and cathode will always be zero (in other words it would be just like having the cathodes tied directly to ground with a 6.3V supply driving the filaments), and the filament and cathode will always be at the same voltage potential regardless of the state of warmup of the amp.
If you have a center tapped filament transformer, simply connect the cathode directly to the center tap. If you don't have a center tapped transformer you can connect it to either end of the filament, or you can simulate a center tap by connecting two resistors of the same resistance value to the secondary ends, and connect the other end of those resistors together, and tie the cathode to the junction. Any size resistors from say 100Ω to 1KΩ will work just as well.
Thanks Kward,
Its a all-tube DC SEP amp. So, I always employ tube rectifiers, for their sonics, and, also for start up considerations, typically - a directly-heated 5U4GB.
In a Direct Coupled tube amp, I start-up everything, all-together, at once, NO delays of any kind are used.
So far, from what I get out of this thread, it looks like I won't have to build a voltage divider and use a 10 UF film cap.
Lets see, however, if some EEs offer a different insight.
At this moment, I am all for connecting the center tap of the 6AQ5 Ef XFR to the top of the 5K 50 Watt Rk resistor.
I am very appreciative of you first two posters, sharing your initial thoughts with me. Thanks so much !!
I think this will become an important amp for me to build, for a variety of very good reasons. Mainly, I have " had the dance " already, past four years, with sonic shortcomings in DHT 45s and 46s on the living room's VOTTs, and this inexpensive pentode tube design may possibly.... easily....... whomp em both. A 6AQ5 is a superb sounding tube !!
That surely would be much fun to accomplish !!
Dowto1000