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Old 9th July 2009, 02:09 AM   #1
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Default DHT bias and filament supply in one?

I was looking for a way to supply current to some 01-A's and bias them without adding any caps or batteries, and this occurred to me. It runs the filament current (250mA) through the bias resistor in addition to the plate current (~3mA) which keeps the resistor small (18R for a 4.5V bias instead of 1K5) meaning that it won't have much impact on rp ... But, I've never seen this done before which makes me think there might be issues. Any thoughts (other than the fact that the battery is upside down)?
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Old 9th July 2009, 02:42 AM   #2
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Default Filament bias

Yes, it's been done before. It works quite well. Another variation on
this technique is to energize the filament separately, as usual, and
supply extra current to the smaller-than usual cathode resistor in order
to achieve the proper bias voltage. This provides an extra degree of
freedom.
Getting rid of the bypass cap without the usual penalty is quite a bonus.
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Old 9th July 2009, 05:09 AM   #3
mach1 is offline mach1  Australia
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If you search the Audioasylum you will find quite a few posts on this method of dht biasing.
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Old 9th July 2009, 09:08 AM   #4
316a is offline 316a  England
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Default Re: DHT bias and filament supply in one?

Quote:
Originally posted by dsavitsk
I was looking for a way to supply current to some 01-A's and bias them without adding any caps or batteries, and this occurred to me. It runs the filament current (250mA) through the bias resistor in addition to the plate current (~3mA) which keeps the resistor small (18R for a 4.5V bias instead of 1K5) meaning that it won't have much impact on rp ... But, I've never seen this done before which makes me think there might be issues. Any thoughts (other than the fact that the battery is upside down)?
The main issue with filament bias is getting the filament supply quiet enough as any ripple will be amplified by the grid . This is more of an issue with higher mu , high filament current valves such as 3C24 . 01A will probably not need a cathode resistor if using as a linestage , there should be enough filament voltage to provide bias on it's own .

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Old 9th July 2009, 11:21 AM   #5
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See this:

http://www.vinylsavor.de/line01a.gif

Apparently it works very well.
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Old 9th July 2009, 10:02 PM   #6
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Quote:
Originally posted by jeff mai
[Click the image to open in full size.

Really? I think I like my way better.

This is going to burn off on the order of 40W of heat and require a monster of a power supply in order to do away with a tiny filament supply.
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Old 9th July 2009, 10:58 PM   #7
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Default Thomas Mayer's supply

I first saw that schematic a few years ago. My impression was that Thomas was trying to do something more than just lower the cathode resistor. Wasteful? Indeed. But I suspect there is something to his idea. This is one I've always wanted to breadboard just to find out how it sounds.
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Old 10th July 2009, 04:50 AM   #8
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Quote:
Originally posted by dsavitsk



Really? I think I like my way better.

This is going to burn off on the order of 40W of heat and require a monster of a power supply in order to do away with a tiny filament supply.
I get what you're saying. I was just trying to point out that what you are doing has been tried with success. I've also seen it done with heater tubes like the 6SN7.
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