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Old 27th February 2009, 04:53 AM   #1
athos56 is offline athos56  United States
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Default Question about a VR screen supply

I have been doing some reading about possible tweeks to my KT-88 amp. I was thinking of trying a VR tube regulated screen. Here are two sketches. Would one of these work better then the other? I was thinking of 2 0D3's for 300v or and 0C3 and an 0D3 for 255v.

I was thinking also of using a resistor and a pot so I could play with different VR tubes.
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Old 27th February 2009, 10:54 AM   #2
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Lots of useful information in this thread.

RCA Voltage Regulator:
RCA voltage regulator

Enjoy,
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Old 27th February 2009, 05:28 PM   #3
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Default Thanks for the link.

Those regulators are cool. There were a few other threads I checked out, including one were fdegrove posted a VR screen supply for an 807 (which is the basis for what I drew). One thing I'm wondering is whether or not it would be worth implementing as shown. How great would the return be by going with something more complex than 2 VR tubes?
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Old 27th February 2009, 06:35 PM   #4
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Both will not work because VR tubes can't survive currents needed to regulate KT88 grids directly. I even don't speak of the 2'nd version where VRs are powered from anode....

Take one cheap MOSFET and use it as a source follower with your VR tubes to get more than plenty of regulation.
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Old 27th February 2009, 08:28 PM   #5
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Ahhh, I was under the impression from my reading of other threads that the screens drew about 5ma idle and around 15 when pushed. Where can I find info on the current draw of screens?

I also read that when you set up a VR regulator that you allow for enough current to supply the load + the midpoint of the VR tubes rated max. So if you ran your tube at 80ma and wanted the VR tube at 20ma, you set up your psu to deliver 100ma at the desired voltage, the tubes would draw their 80ma and the 20ma would go to the VR tubes. I'll try and find the post where this is outlined by someone smarter than myself...
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Old 27th February 2009, 08:37 PM   #6
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Default fdegrove

Here is the drawing by Fdegrove. Which I based #2 on...
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Old 27th February 2009, 08:55 PM   #7
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Here you go:

Click the image to open in full size.
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Old 27th February 2009, 09:05 PM   #8
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Doing more research. The Kt-88 spec sheet states that the max screen dissipation is 8w, if I put that and 300v into a Ohms law calculator it comes out to 26.67ma, which is right in the middle of an 0D3's operating range right? So anything under that Max screen rating should be ok.
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Old 27th February 2009, 09:19 PM   #9
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Quote:
Originally posted by athos56
Doing more research. The Kt-88 spec sheet states that the max screen dissipation is 8w, if I put that and 300v into a Ohms law calculator it comes out to 26.67ma, which is right in the middle of an 0D3's operating range right? So anything under that Max screen rating should be ok.
It is up to you, but speaking of audio I would always think of dynamics, keeping in mind clipping that happens in real life for sure. I would check how it sounds with and without a source follower before drawing a final conclusion.
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Old 27th February 2009, 09:28 PM   #10
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Right, I'm not trying to argue , I'm trying to make sense of what I see and read. What operating point should you run a KT-88 screen at? What voltage and how much current will it draw? I mean I could always just hook up the UL tap and call it good, but I thought a little regulated supply would be fun to try.
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