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Old 2nd June 2004, 07:47 PM   #1
zobsky is offline zobsky  India
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Default cathode battery bias questions

will be modifying my 12B4A preamp ( yet another 12b4a ) over the next week or so (stage 1 mods)

stage 1: (need to use a 2 - 3.3 uF coupling cap, battery bias, drill some more vent holes in the chassis), .. cure some start up buzzing / popping , chassis finishing

stage 2: plate loaded choke, CCS (on back burner for the time being)

stage 3: boutique components, steped attentuator, output transformers (parafeed??) other black arts, etc .. (tentative ...)


regarding the battery bias
1. will using a NiCad / NiMh battery to bias the cathode drain it eventially? I think not but wanted to be sure before I mod the circuit. I have heard enough arguments for battery bias, .. but are there any against? No need to use a bypass cap in parallel with a battery, right????

trying to determine the correct value of battery to use . frank (post #10) has posted the final schematic of the preamp as it is today http://www.diyaudio.com/forums/showt...609#post400609 ) with 12k plate resistor / 220R grid stopper (metal film for now) and 500R cathode resistor..

i measured the cathode at 30V and plate at 70-75V for the 2 channels.

from my figure, .. what is the correct value of battery (voltage / current ) to chose (the green / blue lines signify the actual / theoretical cathode bias points right now, ..not too far off)

i'm a bit confused about whether to use 15 or 20mA as the current rating or 10 or 30V as the voltage rating?.. i'm thinking (-) 10V would be the point to look for, .. but current rating?? still learning, as you can see

also, .. besides the point, ..but is it normal for the output coupling cap to become warm / toasty during operation (could be because i just had 0.1 uF in there blocking all the low freq. signals??)


thanks, V
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Old 2nd June 2004, 07:51 PM   #2
zobsky is offline zobsky  India
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pic,

any particular battery type / rating to use / avoid??
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File Type: jpg bb-12b4.jpg (76.6 KB, 317 views)
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Old 2nd June 2004, 07:54 PM   #3
EC8010 is offline EC8010  United Kingdom
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You are going to need seven AA size NiCd or NiMH to give roughly 10V on charge and to withstand the continuous trickle charge of 20mA. That's going to take up a sizeable amount of room, and you need two of them for stereo. How about using a single PP3 lithium (about 10V) and grid bias instead? You will need to add a capacitor between the grid and the volume control, but the whole thing will be a lot more manageable.
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Old 2nd June 2004, 08:06 PM   #4
zobsky is offline zobsky  India
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Quote:
Originally posted by EC8010
You are going to need seven AA size NiCd or NiMH to give roughly 10V on charge and to withstand the continuous trickle charge of 20mA. That's going to take up a sizeable amount of room, and you need two of them for stereo. How about using a single PP3 lithium (about 10V) and grid bias instead? You will need to add a capacitor between the grid and the volume control, but the whole thing will be a lot more manageable.

thanks for the speedy reply
what about a single 9V cell? that would be much smaller physically? ...hmm, they aren't rechargeable, so that might be an issue

any current draw factors to consider?


thanks
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Old 2nd June 2004, 09:40 PM   #5
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Rayovac, for 1, offers 7 cell rechargeable (NiMH) "9" V. snap connector batteries. You have to make sure you acquire 7 cell batteries. The "gotcha" is the tubes' idle current. I don't know if the small cells used in "9" V. rechargeables can stand the 20 mA. "trickle".

BTW, the bias batteries will require occasional replacement. I'm guessing on a 2 year service life.

1 more thing, you could use a stack of 5X red LEDs instead of a battery for the 10 V. of bias.
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Old 2nd June 2004, 09:47 PM   #6
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Cathode or grid bias with batteries sound very different. You should give it a listen before comitting. The lithium battery on the grid will have the advantage of proving a more stable and much longer lasting bias.
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Old 3rd June 2004, 01:47 AM   #7
zobsky is offline zobsky  India
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Quote:
Originally posted by analog_sa
Cathode or grid bias with batteries sound very different. You should give it a listen before comitting. The lithium battery on the grid will have the advantage of proving a more stable and much longer lasting bias.
grid bias sounds like a worthwhile consideration, ... I was looking at NiMh cells this afternoon after initiating this thread, .. and saw stacks of 3.6V batteries intended for cordless phone batteries @ $4 each occupying about 1.5 square inch.. For a stereo preamp with 10V bias, I'd need 6 of these suckers totalling about $25, .. making quite a lot of demands on the real estate left in my chassis,

what are the sonic differences in grid battery bias vs. cathode bias? operations pros / cons

thanks
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