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Old 9th November 2003, 07:21 PM   #1
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Default 'Battery under cathode' bias - long-term users?

Hi,

I spent some time today reading up on battery bias where the battery is placed between the cathode and ground (as opposed to being in the grid circuit). I want to try it this way because it doesn't need an input transformer or cap. The issue is, of course, the constant current that flows through the battery (in my case, 20mA for a CCS-loaded ECC99). Here's what I've learnt so far:

* Use NiMH. They're designed to handle recharge currents, they have lower internal impedance than NiCd, they don't suffer from memory effects, and they're better for the environment when you have to throw them away.

* Typical chargers use 10% of the Ampere-Hour rating, so pick a battery that has an AH rating of 10x the bias current. Then I read that charger currents are spec'd on temperature rise, and since this battery will be in a hot place (inside the amp), pick one that's 20x the idle current draw.

* Watch your voltages, batteries are spec'd for output voltage under load, a battery in a cathode bias circuit won't be loaded.

That gives me pretty much all the information I need to pick a battery. Radio Shack has these - NiMH 3.6V 580mAH batteries which should work fine (someone tried these and they charge up to 4.2V with no load, which is close enough to my 4V bias). They also have a 220mAH version.

Questions:

* Has anyone used a battery under a cathode for a reasonable length of time (say, several months or better, years)? Any undesirable side effects? Any failures or performance drifts that I should know about?

* The AH rating gives the total amount of energy/charge the battery can store. It doesn't say anything about how that is distributed across the A and H axes, i.e., as you raise A and lower H, how far can you go. Is it safe to assume that a 220mAH battery can in fact supply 220mA for 1 hour? And from that, does it follow that a 20mA charging current applied for several hours won't do any damage?

* Would there be any disadvantages to picking the 580mAH one over the 220mAH one? The charging current will be 1/30th the nominal AH rating - is that too low?

* Does anyone think that this could/would sound worse than the R/C I have in there now, and the whole thing is a bad idea?

* Anything else that I missed out, or need to think about?

Thanks,
Saurav
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Old 9th November 2003, 07:44 PM   #2
JoeBob is offline JoeBob  Canada
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I've used NiMH batteries in such a place for a little while now (months, under a year) and have had no problem. When I first looked into the possibility of using batteries here I was told that they would last several years. They were used to bias a small signal tube in a preamp, although that preamp is longer in use today (still functional, just a better one has taken it's place). I find the battery sounds better than a resistor/cap combo, but that's just my opinion. But usually those caps are big and almost everyone here would agree that eliminating an electrolytic is almost never a bad thing.

Well, that's just my experience...
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Old 9th November 2003, 08:22 PM   #3
Zen Mod is offline Zen Mod  Serbia
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I use Varta 1V2 4Ah under EC8010 in my WOT preamp for few years already.
current is around 20mA-you don't realy need more for preserving voltage ,'couse accu is not really in role of current source for anything.
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Old 9th November 2003, 11:24 PM   #4
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Quote:
I find the battery sounds better than a resistor/cap combo, but that's just my opinion.
Very possibly. I still find a battery in the grid to be less audible if the cap is already there.
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Old 9th November 2003, 11:33 PM   #5
JoeBob is offline JoeBob  Canada
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I never actually tried battery grid bias, I first tried on the cathode and the sound was great, then time passed and I thought why mess with it... Maybe I should give it a try someitme.
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Old 10th November 2003, 01:24 AM   #6
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JoeBob,

How much current are you running through the batteries, and what is the AH rating of the batteries you used (if you remember)?

Thanks,
Saurav
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