In Reply to: Re: BATTERY for filament heating posted by stelios on October 26, 2001 at 13:32:14:
man paul... i like the way you think...the two 6B4's in series will work, but the problem comes when you look at the bias of the tubes...
look at the whole string in series... you have:
12.6+.... fil1 ..AC gnd.. fil2.... 12.6-the AC ground will either be ground if you fixed bias, or the R/c combo for cathode bias in ant event the cathode is at AC ground so there shouldn't be the mixing (of course the cap isn't perfect sooo...) the problem shows up when your two tubes have a difference in DC potential between them... so one tube will start with 6V or so different bias than the other...
its "dumb" ideas like this that get us thinking outside the box though... my next thought was to series the 845 and the 2A3 for 12.5V but the current draws would have to be the same... damn that 3.25A filament... but its still thinking outside the box...and thats where all the good stuff is found.
please note that i used quotes around the word "dumb" because i think it was a great idea... but so often thes good ideas mistakenly get filed into the "dumb" category.... at some point or another about 1/2 the things i like in my amps were labeled as something that "won't work" by those that know better...
dave
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