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Old 29th October 2006, 05:01 AM   #11
Tweeker is offline Tweeker  United States
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I would think that G2 loads are quite small and a choke with an inductance of ca. 100 Hy is necessary to achieve a genuine choke input power supply and all of the benefits it entails.
Just use a bleeder resistor to get required inductance down.
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Old 29th October 2006, 05:26 AM   #12
jlsem is offline jlsem  United States
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I would think 5 Hy would be enough for most purposes.
For screen voltage of 300vdc @ 5-10ma, critical inductance is on the order of 25-50hy. Twice that is optimal for a true choke-input filter.

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BTW, Jeff always despised voltage regulation for screen supplies, claiming that he had a lot of experience with it and had never found one that was free from noise. He stated that the screen is very sensitive to a noisy supply.
So how would a brute-force regulated screen supply with proper first choke inductance values be noisier than what he's got? I'd like to listen to these amps to see if they are really that good.

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Just use a bleeder resistor to get required inductance down.
You are correct, sir. A 15 ohm, 600 watt resistor would draw enough current (2 amps) to make a 12mH choke work.

I've heard plenty of DIY wonder projects, jumped up by the builder, that were actually pretty poor.

Nothing personal against Mr. Medwin, as he treated a friend of mine kindly during correspondence some years ago.

John
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Old 29th October 2006, 06:23 AM   #13
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For screen voltage of 300vdc @ 5-10ma, critical inductance is on the order of 25-50hy. Twice that is optimal for a true choke-input filter.
If we choose in this case a bleed resistor of 15K ohms, the resistor would pull 20ma, for a total load of ~25-30ma. Regulation would be much improved as a lower resistance choke could be used, and the bleeder would swamp out the effects of changes in the screen current.

No longer would the going from 5ma to 10ma current double the load, it would instead only raise the overall load to 20%. It would also maintain critical inductance regardless of the load.
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Old 29th October 2006, 07:37 AM   #14
billr is offline billr  New Zealand
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Originally posted by sorenj07



what is that thing?

that thing is a 3 phase mercury rectifier used to feed arc lamps and the rest.

my dad managed a cinema which had two peerless carbon arc projectors, each burned 100A at about 100V when on full warp, one of those suckers would drive both of them. they looked like something out of frankensteins attic when going. purple, with lots of sparks bouncing around.

the amps were are all RCA rack things with pairs of 6l6s in them. behind he screen there were altec lansing bass horns about 5 foot tall and about 12 foot wide, sitting on top of that were pairs of multicell HF horns.

helluva set up, and with the efficiency of the drivers, bloody loud.

thought that you may be interested.

kind regards

bill
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Old 29th October 2006, 07:43 AM   #15
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For screen voltage of 300vdc @ 5-10ma, critical inductance is on the order of 25-50hy. Twice that is optimal for a true choke-input filter.
I was thinking of the screen supply for a PP EL34 stereo amp in AB1. Screen volts at about 400v, with screen current varying from 5mA per tube at no signal to 25mA per tube at full signal. Thus the total screen current for a stereo PP amp would vary from 20mA to 100mA.
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Old 29th October 2006, 12:17 PM   #16
jlsem is offline jlsem  United States
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I was thinking of the screen supply for a PP EL34 stereo amp in AB1. Screen volts at about 400v, with screen current varying from 5mA per tube at no signal to 25mA per tube at full signal. Thus the total screen current for a stereo PP amp would vary from 20mA to 100mA.
Yes, that would be a normal way of doing it. I am only referring to Medwin's method of a separate power supply for each screen with a 12mH input choke.

John
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Old 29th October 2006, 12:58 PM   #17
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12mH input choke
I've never seen that. That's weird! Is that what he calls a choke input filter? Maybe he made a typo?
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Old 29th October 2006, 08:00 PM   #18
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You are correct, sir. A 15 ohm, 600 watt resistor would draw enough current (2 amps) to make a 12mH choke work.
A 15ohm resistor would need to be 6000 watts and draws 20 amps at 300V, I suggest parallel oven elements and Hammond 195J20 in series with 195E20.

150ohm is 600watts @ 2A. Hammond 159ZE or 159ZG inductor.
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Old 29th October 2006, 08:24 PM   #19
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What's a few watts among friends?
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Old 30th October 2006, 04:16 AM   #20
jlsem is offline jlsem  United States
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A 15ohm resistor would need to be 6000 watts and draws 20 amps at 300V, I suggest parallel oven elements and Hammond 195J20 in series with 195E20.
You're right. It was late at night and I thought 600 watts was was plenty enough unreasonable and neglected to double check.


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I've never seen that. That's weird! Is that what he calls a choke input filter? Maybe he made a typo?
No typo. He is insists the value be 12 millihenries. If he would just say that he uses a capacitor-input filter on his screens and a very large "RF filter" in front of it, then that's fine. But like I said in my second post, he has a problem with nomenclature, and stubbornly defends himself, in that regard.
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