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Old 27th February 2009, 08:57 PM   #11
athos56 is offline athos56  United States
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Default Spec sheet

From the looks of the spec sheet, under typical operation at 225v the draw is 5ma at idle. This is why I'm confused. Is there a lot of clipping in this typical application?
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Old 27th February 2009, 09:08 PM   #12
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Quote:
Originally posted by athos56
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.
I use screen voltage regulation in 3 models of my amps, both for 6L6 and GU-50. Also, I sense screen grid currents to control optical compressors. Voltage regulators are designed to supply more of current they want, while sensors are fine tuned by trimpots to start compression as soon as screen grid currents start going up sharply.

6L6 grids I run at 305V with 400V on anodes, while GU-50 screen grids I run at 275V when anode voltage is 800V. Biasing tubes I monitor cathode currents in order to set working points. Screen grid currents at idle don't matter as soon as tubes are alive.

Quote:
Originally posted by athos56
From the looks of the spec sheet, under typical operation at 225v the draw is 5ma at idle. This is why I'm confused. Is there a lot of clipping in this typical application?
Is there a lot of clipping, depends on conditions you are going to use your amp in: desired sound pressure, room conditions, sensitivity of speakers, max power of your amp.

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Old 27th February 2009, 09:20 PM   #13
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Cool, I guess I was just pointing out that if the current is 5ma at idle and with the max dissipation of 8w, at the voltage I was looking at the current was 26ma. It seems like the VR tubes could handle whatever happened current wise. I'm not understanding where the clipping and blown VR tubes are coming from. But I admit I am an amature with only 4 amps completed. There is always the UL taps on my OPT if I can't figure it out.
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Old 27th February 2009, 09:42 PM   #14
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Approaching clipping pentodes start drawing more current so if your regulator can't supply it screen grid voltages drop down abruptly that may cause different character of distortions you (or users of your amps) may not like.

Edit: VR tubes can't draw "whatever current"! And they have non-zero dynamic resistance, so a source follower keep them happy all the time: regulation depends on dynamic resistance of what supplies/draws the current VS their own dynamic resistance.

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Old 27th February 2009, 09:53 PM   #15
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Ok, I'm not against a source follower, I just have to look up how to hook it up. Thanks for the replies.

Here is the page where I got the Idea, I wanted to do something similar.

Post on 807 VR screen regulator
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Old 27th February 2009, 11:22 PM   #16
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Default Ok Ok

I think this is what you are talking about. Let me know if I'm wrong. If so what are the ratings I'm looking for on the Mosfet? (If I barely know whats going on in the Hollowstate world I'm even more lost in the solid state world )

Mosfet + VR tubes thread: Here
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Old 28th February 2009, 01:18 AM   #17
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In your case I would use something rated for 500+ volts.

irf830, for example.
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Old 28th February 2009, 02:09 AM   #18
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Default A start...

Ok here is a literal translation of the post linked above, I'm going to assume that its missing something. Is this close?
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Old 28th February 2009, 02:46 AM   #19
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Default Re: A start...

Quote:
Originally posted by athos56
Ok here is a literal translation of the post linked above, I'm going to assume that its missing something. Is this close?
Yes. You will need to supply current to your VR tubes of course, also a gate stopper and screen grid stoppers will be needed to avoid HF oscillations. I would start with 1K each gradually decreasing screen grid stoppers.
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Old 28th February 2009, 03:06 AM   #20
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Default Thanks for hanging in there with me!

Ok how about this, 1k on gate and screen. Now, for the leg to the VR tubes, would you calculate the resistor like normal to set the current through the tubes?
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