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Old 12th October 2012, 02:52 PM   #51
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Originally Posted by Conrad Hoffman View Post
... IMO again, if bypassing electrolytics has an effect, the correct design solution is really to bypass someplace else, either at the rectifiers to kill RF, or at the circuit where you're not fighting with the inductance of the wiring from the main caps. The correct return point for the bypass is also important, and it may not be where you think!
Above 10KHz electrolytics behave differently, from brand to brand, from type to type. Wouldn't it be a explanation for differences in decoupling a ps?
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Old 12th October 2012, 03:00 PM   #52
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Originally Posted by 20to20 View Post
Surely you realize that the filament is a resistor equal to whatever the other resistor's value is reduced by, to meet the biasing needs.
Not totally sure what you are saying here. Filament is usually one or two ohms added on to the cathode resistor - the DC filament supply goes through both the filament and the cathode resistor. The tube is biased at the voltage on top of the cathode resistor. This may be what you are saying? The total circuit is usually 5-50 ohms depending on the tube. I've been using filament bias for over a year on all input tubes. Resistor is 5 ohms on a 26 and 22.5 ohms on a 4P1L. I'm very happy with it.

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Old 12th October 2012, 03:20 PM   #53
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Originally Posted by andyjevans View Post
Not totally sure what you are saying here. Filament is usually one or two ohms added on to the cathode resistor - the DC filament supply goes through both the filament and the cathode resistor. The tube is biased at the voltage on top of the cathode resistor. This may be what you are saying? The total circuit is usually 5-50 ohms depending on the tube. I've been using filament bias for over a year on all input tubes. Resistor is 5 ohms on a 26 and 22.5 ohms on a 4P1L. I'm very happy with it.

1-2 ohms is when the filament is cold. Depending on the tube type, it rises to anywhere from 20 -80 ohms when the filament is hot.
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Old 12th October 2012, 05:08 PM   #54
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Originally Posted by 20to20 View Post
1-2 ohms is when the filament is cold. Depending on the tube type, it rises to anywhere from 20 -80 ohms when the filament is hot.
I don't think so - we're talking directly heated tubes here. For a 4P1L it's 1.9v at 600mA which is 3 ohms.

Last edited by andyjevans; 12th October 2012 at 05:10 PM.
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Old 12th October 2012, 05:56 PM   #55
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Originally Posted by andyjevans View Post
I don't think so - we're talking directly heated tubes here. For a 4P1L it's 1.9v at 600mA which is 3 ohms.
That's gotta be an interesting diagram. You should post that.
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Old 12th October 2012, 08:42 PM   #56
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You must excuse me but I have no idea what you are talking about. The last time I looked at ohms law 1.9v at 600mA was a resistance of 3.17 ohms. That's for a hot filament because unless I'm hallucinating, my amplifier is actually producing music.
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Old 12th October 2012, 09:04 PM   #57
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Originally Posted by andyjevans View Post
You must excuse me but I have no idea what you are talking about. The last time I looked at ohms law 1.9v at 600mA was a resistance of 3.17 ohms. That's for a hot filament because unless I'm hallucinating, my amplifier is actually producing music.
I have no doubt your amp is singing beautifuly. It's just the filament biasing that is interesting. Using a filament for a bias resistor is uncommon. I've seen it a few times but not with the tubes you are using. Your DHT filament power and biasing circuit would be interesting. When you use the term "filament biasing," I'm taking that to mean you use the filament resistance to bias some other tube's cathode. I've seen it done with 12ax7 type tube heaters to bias output tubes and use their cathode voltage to power the 12ax7 heater. Is your scheme different?
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Old 12th October 2012, 09:23 PM   #58
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Have a look at this:
Attached Images
File Type: png 4P1L fil_bias.png (39.5 KB, 189 views)
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Old 13th October 2012, 12:19 AM   #59
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I thought the Coleman regulator board has an electrolytic cap (C15) from filament positive to ground?

Based on this version of Rod's (apologies if this is not representative of his latest/current design)
New DHT heater

Isn't that cap in effect acting like a cathode bypass?
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Old 13th October 2012, 12:42 AM   #60
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Originally Posted by hirafu_boarder View Post
I thought the Coleman regulator board has an electrolytic cap (C15) from filament positive to ground?

Based on this version of Rod's (apologies if this is not representative of his latest/current design)
New DHT heater

Isn't that cap in effect acting like a cathode bypass?
It's exactly cathode bypass cap in his amp.

The regulator itself is not referenced to the ground at all. It connects between filament legs of the tube.
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