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Old 5th October 2008, 03:51 PM   #21
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Quote:
Each transformer should have something like a 24V 5A - 6A rating for this application.
Why such a high voltage to start with? Two 100VA transformers aren't that small.
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Old 5th October 2008, 03:57 PM   #22
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Quote:
Originally posted by astouffer


Why such a high voltage to start with? Two 100VA transformers aren't that small.

Available off the shelf voltage, easily acquired new or surplus, and the GM-70 uses 20V @ 3A so this is a reasonable choice.

An 18V to 20V toroid could be a better choice provided winding dcr is low and some margin for low line conditions is taken into account. You should probably have no less than 4.2V of headroom for optimum regulator performance at low line. (Includes IR drop across current sense resistor.) I know the LT1084 is a low drop out device, but it performs substantially better when not operating on the edge of drop out. (Internal loop gain is dependent on voltage margin.)

Given full wave bridge into large electrolytic cap you need to take into account power factor which could be as low as 0.6 - 0.7.. Hence 100Va rating..
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Old 5th October 2008, 04:04 PM   #23
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Hi Kevin

Thanks for your input! I remember reading an older post from you where you describe the problems with using AC. I had already decided to go with the hassle of DC, but then I came across the website from Linn Olson, who employes AC for his Karna amplifier...and a sprinkle of hope returned...

I am thinking about getting a 24VDC switched supply, use a LM338 as CCS (though I have 2x pieces of LT1083 and 2x of LT1084) and include some capacitance and CMC's for HF filtering. That would be the most compact construction form, I think...

Erik
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Old 5th October 2008, 04:08 PM   #24
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Quote:
Originally posted by ErikdeBest
Hi Kevin

Thanks for your input! I remember reading an older post from you where you describe the problems with using AC. I had already decided to go with the hassle of DC, but then I came across the website from Linn Olson, who employes AC for his Karna amplifier...and a sprinkle of hope returned...

I am thinking about getting a 24VDC switched supply, use a LM338 as CCS (though I have 2x pieces of LT1083 and 2x of LT1084) and include some capacitance and CMC's for HF filtering. That would be the most compact construction form, I think...

Erik
Sounds good to me and much more efficient than using a linear supply.
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Old 5th October 2008, 10:00 PM   #25
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kevinkr: Ah the confusion was my fault. You were talking about the GM-70 and I thought you meant 300B.
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Old 5th October 2008, 10:09 PM   #26
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Originally posted by astouffer
kevinkr: Ah the confusion was my fault. You were talking about the GM-70 and I thought you meant 300B.
I probably should have made it clearer, on reflection I can see what was confusing about my post. I used the 300B as anecdotal evidence as to why this might not work so well with the GM-70's 20V filament, but did not spell that out clearly at all. So it's really my fault - without the clarification of my next post it probably wasn't that clear to anyone what I was on about. Some of my posts today have been kind of wonky, wonder what is up with that.
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Old 5th October 2008, 10:24 PM   #27
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Quote:
Originally posted by ErikdeBest
Hi Kevin

Thanks for your input! I remember reading an older post from you where you describe the problems with using AC. I had already decided to go with the hassle of DC, but then I came across the website from Linn Olson, who employes AC for his Karna amplifier...and a sprinkle of hope returned...

I am thinking about getting a 24VDC switched supply, use a LM338 as CCS (though I have 2x pieces of LT1083 and 2x of LT1084) and include some capacitance and CMC's for HF filtering. That would be the most compact construction form, I think...

Erik
Hi Erik,
I think you will right on the hairy edge (as it were) with the LM338 in terms of drop out voltage with a standard 24V switcher, IIRC this type has a drop out voltage of around 3V, so add in the additional drop in the current sense which is about 1.25V you will require at least 4.25V of margin..

Try to find a switcher that has an adjustable output range of at least +10% ideally being adjustable to 26V or so.. Make sure it is rated well in excess of 60W continuous, particularly if you intend boost the output voltage a little. I would get a couple more LT1084 if you can't boost the voltage enough.

The current setting resistor needs to be rated for at least 3W for stability and could be made up of several resistors in parallel to get both the required value of around 0.41 ohms and (to fine trim to) the proper voltage across the filament terminals once the tube has fully warmed up. Use precision resistors to make sure you do not get excessive drift in the filament current as the resistors heat up.
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Old 6th October 2008, 03:47 PM   #28
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Hi Kevin

Thanks again! You are right, 24VDC could not be enough when using the LM338. If I remember well most of the 24VDC supplies I have seen have the adjustment possibilities of +/-10%, so I will surely keep that in mind when buying one.

I just remembered of another smart implementation I saw mentioned in a datasheet, the diagram is attached. It could save me a volt or so, and make a 24VDC output suitable...it could even be used to adjust for small current differences between GM70's to attain 20V across the filament. I will at least have to try it out

Erik
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Old 6th October 2008, 05:15 PM   #29
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Quote:
Originally posted by ErikdeBest
Hi Kevin

Thanks again! You are right, 24VDC could not be enough when using the LM338. If I remember well most of the 24VDC supplies I have seen have the adjustment possibilities of +/-10%, so I will surely keep that in mind when buying one.

I just remembered of another smart implementation I saw mentioned in a datasheet, the diagram is attached. It could save me a volt or so, and make a 24VDC output suitable...it could even be used to adjust for small current differences between GM70's to attain 20V across the filament. I will at least have to try it out

Erik
Hi Erik,
Needless complicated, you can do the same thing with a single chip and it doesn't address the drop out issue at all.. It just allows you to adjust to 0 current - a feature you actually do not need.. You can accomplish much the same thing by connecting a small pot across the current sensing resistor. (Say 100 ohms or so) Just be sure to place a load across the output otherwise you will get the supply voltage on the input minus the device's internal unloaded voltage drop..
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Old 9th January 2009, 02:22 PM   #30
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Can be used a 6c33c-b or 813 socket ?
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