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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...
Since one can get more than 12V with a 12V lead acid battery then maybe the 2A3 and the 845 filaments can be connected in series. To maintain 3.25A through the battery a resistor of 2.5/0.75 = 3.333R, > 2W could be used in parallel with the 2A3 fil.
That is if we can make sure the battery provides constantly 12.5V across it (or 3.25A through it). I think that some 12V batteries can go up to 13-14V when fully charged. So some adaptive solution is needed i.e. regulators or maybe a trimpot and a meter.
Seems complicated and if cathode bias is used it gets worse defeating my original purpose of avoiding resistors in the cathode.
But if using RC coupling and fixed bias then it seems worthwhile.
If I do use batteries in the end I think I'll go for either one of two options:
a) work out the max time I'll be likely to be listening to music and get an appropriate battery
b) have two batteries which can be connected alternately and have enough capacity so that one can keep playing for as long as the other takes to get charged. The problem would be how to make the transition.
yes "dumb" but I am doing it, each valve is adjusted accordingly in my amp, that is to say the bias controlled through the driver is altered to get a satisfactory average for each tube the only thing you can't alter unless you monoblock is the B+ to each channel, but working with the bias alone you can get both tubes even.
Now I have Anrej's 6a5g's in place though I'm going to try ac heating and this will no longer be a problem. Though I love the sound of lead acid batteries they are an inconvenience. It's all that charging and disconnecting from charger for playing beloney that wears you down in the end, and the fact that only the electrically competent can use the amp. In a house where the 4 year old son has to show the 43 year old mother how to get sound out of the video this is an issue.
I'll keep chipping away for "dumb" ideas!
well we have to remember at some point not to long ago a group of folks shunned SET's as dumb for PP pentodes... then those PP pentodes got dumped into the dumb category for solid state...
its funny how the whole "dumb" concept is brought to life by engineers, and then the marketing division gets ahold of it and they get their cut and the only people who suffer are those convinced by someone else that everything they spent on their sysrem was right.
some fool once said... two wrongs do not a right make... well i have more than two wrongs and guess what... i think my sound is pretty right...
someday those marketers will realize that three lefts actualy make a right, and that means they can sell 3X the stuff... which will of course require 3X the engineers and everyone will be happy.
oh yeah... i don't care how far apart my tubes are operating... as long as they are not totally screwing up and the sound is good... why worry...
on the 6A5G... i would love to see the whole story on that tube... i have heard autopsy reports that many are just plain old DHT's and the books are wrong... who knows maybe the relabel of the 6B4 was just a marketing ploy to keep the tube base diagram drawers happy :-)
they have the V shaped filament (ok I should call it heater but like I say looks and glows same amount as PX4 V filaments). The cathode is connected at the midpoint and they sound pretty good!!! Whether that cathode connection makes them the same characteristics as a dht but with an equal tap I don't know.
I haven't heard any other idht output valves except the 6EM7 power stage. The 6EM7 is far more clinical, and gets distorted at high output, as a look at the curves would tell (know you don't bother much about curves but they do guide and in practice yes though I prefer 6EM7 to multi grids hooked up as triodes it ain't a dht.) I would like to get hold of some R120's they would be an inetersting listen compared to the 2a3 type and these 6a5g's.
when you say the cathode is connected in the center of the VV it is just a CT DHT filament with the CT labeled cathode... the tube is still directly heated, it just has a cathode tap...or thats how i see it.. the tube drawing shows the filament CT connected to a cathode, but i have yet to see the cathode element in any of the tubes... has anyone else???
for it to be an IDT like most people call it it would need a cathode sleeve... i suspect you could ignore the cathode connection altogether and wire it with a traditional hum-balance or DC and the tube would function the same... but given that the CT is there you would be foolish not to use it!
That makes sense to me, as I said it sure looks like a filament. I see the centre tap as a gain, though an inconvenienceas it means it's not a plug in replacement for 6B4G. Having said that not a problem to guys like us, I very much like the sound of these so I'll use them until destruction.
I have been following this post partially because I would like to heat my 26s with nicads and partially because I am at the mercy of batteries everyday. Most of our Betacams run off of a BP-90 style pack. They are rated at 12 volts but if the meters in our veiwfinders are at all accurate, a fully charged battery will read well over 12 volts...sometimes over 13! Most use high capacity (either 4 or 5 Ah) Sanyo cells in series. I'm curious if these cells are purposely under-rated or if this might be the result of the chargers that we are using. At any rate, I'm hesitant to try these for fear of putting undue stress on my tubes.
FWIW-you can find these 12(13.2) volt nicad packs on ebay for under $100 with a 3 year warranty from time to time. Look for Anton Bauer brand or Pro Battery. Check out the link below- I called a few months ago and they were selling them outright for $80.
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